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Plants are facing increased risk of heat stress with global climate change. Reproductive tissues are particularly sensitive which can result in lower plant fitness. Floral shading and closure are possible mechanisms to limit heat stress although most previous work on petal orientation has considered adaptations to raise temperatures. We hypothesized that floral shading could reduce temperature and increase reproductive success. Methods We measured floral temperatures of four species that exhibited intraspecific variation in flower closure (Opuntia ficus-indica, Oenothera elata, Convolvulus arvensis, and Romneya coulteri). We also wired newly opened R. coulteri flowers so that they were either permanently open, permanently closed, or unencumbered controls. Results Individual flowers of all four species that shaded their pistils experienced temperatures 3 - 8

PREMISE Ecologists have an incomplete understanding of the factors that select for deciduous, evergreen, and marcescent leaf habits. Evergreens have more opportunities for photosynthesis but may experience costs when abiotic conditions are unfavorable such as during ice and windstorms. METHODS We documented branch loss for species of oaks (Quercus spp.) in a common garden in California during an unusual windstorm. RESULTS Branches of marcescent trees were more likely to break during the storm, and this pattern had a negligible phylogenetic signature. Branches of evergreen and marcescent species were mostly alive before breaking, which likely accrued a fitness cost, while those of deciduous species were mostly already dead. One explanation for the overrepresentation of broken branches from marcescent species is that their petioles are inflexible and have greater wind loading compared to the flexible petioles of evergreen leaves and the leafless condition of deciduous branches. CONCLUSIONS These results suggest that branch loss during unusual winter storms may be an important cost of a marcescent leaf habit.

The interaction between endogenous dynamics and exogenous environmental variation is central to population dynamics. Although investigations into the effects of changing mean climate are widespread, changing patterns of variation in environmental forcing also affect dynamics in complex ways. Using wavelet and time series analyses, we identify a regime shift in the dynamics of a moth species in California from shorter to longer period oscillations over a 34-year census, and contemporaneous changes in regional precipitation dynamics. Simulations support the hypothesis that shifting precipitation dynamics drove changes in moth dynamics, possibly due to stochastic resonance with delayed density-dependence. The observed shift in climate dynamics and the interaction with endogenous dynamics mean that predicting future population dynamics will require information on both climatic shifts and their interaction with endogenous density-dependence, a combination that is rarely available. Consequently, models based on historical data may be unable to predict future population dynamics.

© 2017 by the Ecological Society of America Climate is widely recognized as an important factor that affects temporal and spatial patterns of occurrence and abundance of herbivorous insects, although the ecological mechanisms responsible are poorly understood. We found that precipitation and standing water were positively correlated with locations and years of high abundance of caterpillars of the ranchman's tiger moth, Platyprepia virginalis. We analyzed 30 years of survey data and found that the number of large rainfall events was a better predictor of caterpillar abundance than total annual accumulation. We considered three ecological mechanisms that could drive this relationship and conducted observations and manipulative experiments to evaluate these mechanisms. (1) Rainfall facilitates more plant growth, although we found no evidence that increased food quality or quantity was causing the positive association between precipitation and caterpillar abundance. (2) Large rainfall events cause predatory ground-nesting ants to be less abundant and we found that the number of ants that recruited to local sites was negatively associated with survival and abundance of caterpillars. (3) We found that litter from wet sites provided a refuge from ant predation; litter from wet sites was not beneficial to caterpillars in the absence of ants. Both abiotic factors (precipitation) and biotic factors (predatory ants) affected the temporal and spatial abundance of caterpillars directly and interactively. Climate models predict that rainfall will become more variable, suggesting that populations of this caterpillar may also become more variable in the future.

Seed mucilage, a coating on seeds or fruit that becomes slimy and sticky when wet, has evolved convergently many times across plants. One common consequence of having seed mucilage is that sand and dirt particles stick to wet seeds and remain tightly bound to the seed surface after the mucilage dries. Here, we test the hypothesis that a mucilage-bound sand coating protects the seed from seed predators; either as a physical barrier or by reducing apparency of the seed (i.e., camouflage). We experimentally manipulated the sand coating on seeds of 53 plant species of 13 families and assayed the defensive benefit of the sand coating in feeding "depots" near harvester ant nests in California's Central Valley. Consistent with a defensive function, sand coating reduced ant predation on seeds in 48 of the 53 species examined. To test whether this striking benefit was due to reduced apparency, we conducted an addition experiment using flax seeds in which we factorially manipulated the color of both the background substrate and the sand coating, creating visually apparent and unapparent seeds. Our results did not support the reduced apparency hypothesis; seeds coated in background-matched sand were removed at the same rate as seeds coated in unmatched sand. The defensive benefit of a sand coating was not well-predicted by seed mass, entrapped sand mass, or sand mass scaled by seed mass. Together, our results demonstrate that seed mucilage is a phylogenetically widespread and effective seed defensive trait and point to the physical barrier, not reduced apparency, as a mechanism.

Habitat‐forming species provide refuges for a variety of associating species; these refuges may mediate interactions between species differently depending on the functional traits of the habitat‐forming species. We investigated refuge provisioning by plants with different functional traits for dragonfly and damselfly (Odonata: Anisoptera and Zygoptera) nymphs emerging from water bodies to molt into their adult stage. During this period, nymphs experience high levels of predation by birds. On the shores of a small pond, plants with mechanical defenses (e.g., thorns and prickles) and high structural complexity had higher abundances of odonate exuviae than nearby plants which lacked mechanical defenses and exhibited low structural complexity. To disentangle the relative effects of these two potentially important functional traits on nymph emergence‐site preference and survival, we conducted two fully crossed factorial field experiments using artificial plants. Nymphs showed a strong preference for artificial plants with high structural complexity and to a lesser extent, mechanical defenses. Both functional traits increased nymph survival but through different mechanisms. We suggest that future investigations attempt to experimentally separate the elements contributing to structural complexity to elucidate the mechanistic underpinnings of refuge provisioning.

Sand entrapment on plant surfaces, termed psammophory or sand armor, is a phylogenetically and geographically widespread trait. The functional significance of this phenomenon has been poorly investigated. Sand and soil are nonnutritive and difficult for herbivores to process, as well as visually identical to the background. We experimentally investigated whether this sand coating physically protected the plant from herbivores or increased crypsis (e.g., decreased apparency to herbivores). We tested the former hypothesis by removing entrapped sand from stems, petioles, and leaves of the sand verbena Abronia latifolia and by supplementing natural sand levels in the honeyscented pincushion plant Navarretia mellita. Consistent with a physical defensive function, leaves with sand present or supplemented suffered less chewing herbivory than those with sand removed or left as is. To test a possible crypsis effect, we coated some sand verbena stems with green sand, matching the stem color, as well as others with brown sand to match the background color. Both suffered less chewing herbivory than controls with no sand and herbivory did not significantly differ between the colors, suggesting crypsis was not the driving resistance mechanism. Strong tests of plant apparency are rare; this experimental approach may be possible in other systems and represents one of few manipulative tests of this long-standing hypothesis.

Author(s): Karban, R; Grof-Tisza, P; Mcmunn, M; Kharouba, H; Huntzinger, M | Abstract: 1. Climate and, therefore, abiotic conditions, are changing rapidly, and many ecological interactions depend on them. In this study, how abiotic conditions mediate a predator-prey interaction were examined. 2. Caterpillars of Platyprepia virginalis (Boisduval) (Arctiidae) were found previously to be more abundant in wet habitats and thick litter cover compared with drier habitats and little or no litter. We hypothesised that wet litter provided caterpillars with refuges from an important ant predator, Formica lasioides. It was further hypothesised that caterpillars would be able to move at lower temperatures than ants, thus providing them with a thermal refuge. 3. In the lab, caterpillars were more likely to escape ant predation and survive on wet litter and at lower temperatures. At all temperatures, ant recruitment was lower in wet litter than dry litter although ants were more active on litter than bare soil. Thus, wet litter may serve as a habitat refuge for caterpillars from ants. 4. Caterpillars were able to maintain activity at temperatures 8-14°C lower than F. lasioides. Thus colder temperatures may serve as a thermal refuge for caterpillars from ants. 5. It was hypothesised that caterpillars can escape ant predation when precipitation causes wet litter and at temperatures that they experience commonly in the field. This mismatch between caterpillars and their predators in ability to tolerate wet litter and low temperatures may affect their field distribution and abundance. Expected future warmer and drier conditions may not provide these refuges.

1. Plant–plant communication has been found to affect interactions between herbivores and plants in several model systems. In these systems, herbivore-induced volatile chemical cues are emitted and perceived by other plants (receivers), which subsequently change their defensive phenotypes. Most studies have focused on how the effects of volatile cues affect plant damage, whereas herbivore performance has rarely been examined. 2. In this study, it is shown that plant–plant communication between willows reduced the growth rate, feeding rate, and conversion efficiency of some individuals but not others of a generalist caterpillar, Orgyia vetusta. 3. Using a paired, no-choice trial design, there was substantial variation between caterpillar individuals in their response to willows that had been induced with a volatile plant–plant cue. This variation was explained by feeding parameters of the individual herbivores. Individuals behaved similarly when fed induced and non-induced willow leaves. Specifically, growth rates of caterpillars that grew rapidly on non-induced willow leaves were negatively affected by plant–plant cues, but growth rates of caterpillars that grew slowly on non-induced willow leaves were not affected by the responses to volatiles from neighbouring willows. 4. Induction by volatile plant–plant cues reduced the growth rates of those individual herbivores that caused the greatest damage to willow, but had little effect on weak growers.

© 2014 The Authors. New Phytologist © 2014 New Phytologist Trust. Volatile communication between sagebrush (Artemisia tridentata) individuals has been found previously to reduce herbivory and to be more effective between individuals that are genetically identical or related relative to between strangers. The chemical nature of the cues involved in volatile communication remains unknown for this and other systems. We collected headspace volatiles from sagebrush plants in the field and analyzed these using GC-MS. Volatile profiles were highly variable among individuals, but most individuals could be characterized as belonging to one of two chemotypes, dominated by either thujone or camphor. Analyses of parents and offspring revealed that chemotypes were highly heritable. The ecological significance of chemotypes and the genetic mechanisms that control them remain poorly understood. However, we found that individuals of the same chemotype communicated more effectively and experienced less herbivory than individuals of differing chemotypes. Plants may use chemotypes to distinguish relatives from strangers.

Trichomes are an important physical resistance mechanism of plants, as they reduce insect herbivore movement, feeding, and digestion. However, we know little about how trichomes influence herbivore distributions and populations. We conducted laboratory and field experiments to evaluate the preferences of Platyprepia virginalis (Boisduval) (Lepidoptera: Arctiidae) caterpillars to natural and manipulated densities of trichomes on their primary food, Lupinus arboreus Sims (Fabaceae). We then conducted field surveys to determine whether variation in trichome density among lupine bushes affected caterpillar spatial distribution on the landscape. Platyprepia virginalis caterpillars preferred lupine leaves with fewer trichomes in choice and no-choice experiments. In the field, caterpillar feeding damage was found more often on leaves with fewer trichomes. These preferences scaled up to the level of bushes in the landscape such that more caterpillars were found on bushes with lower trichome densities than on bushes with higher trichome densities. This is one of few studies to show the potential for trichome density to influence herbivore population size and distribution in a natural system at a landscape level. The results are consistent with trichomes functioning as a resistance mechanism with consequences for herbivore choice, performance, and distribution.

Summary There is substantial evidence that the rate of litter decomposition is affected by the match between the litter substrate and the soil matrix (decomposer community). We introduce and test the phenology–substrate-match hypothesis, which predicts that both litter composition and soil matrix will change over the course of the year and that a lagged match between litter type and soil matrix will result in an optimal decomposition environment. We conducted a decomposition experiment in a Mediterranean mixed deciduous–evergreen oak savanna in California. We initiated litter decomposition of both a deciduous oak (whose leaves fall in autumn) and an evergreen oak (whose leaves fall in spring) in both autumn and spring. Consistent with the phenology–substrate-match hypothesis, we found that decomposition of deciduous oak litter was accelerated compared to evergreen oak litter when decomposition was initiated in spring, while evergreen litter was accelerated compared to deciduous litter when decomposition was initiated in autumn. We also found a small effect of microsite on leaf decomposition, where both evergreen and deciduous oak leaves decomposed faster under the canopy of a conspecific. Synthesis. Our study extends theory of litter quality and the decomposer community into a temporal context, which may be an important source of variation in decomposition rates when species with different litterfall phenologies co-occur.

As human-aided range expansions and climate change alter the distributions of plants and their herbivores, predicting and addressing novel species interactions will become increasingly pressing for community ecologists. In this context, a key, surprisingly understudied question is: when an exotic plant is introduced, which herbivores will adopt this new potential host? Whether the plant is a weed, an ornamental, or a crop, the development versus non-development of a novel plant – insect interaction can have profound eff ects for both economic and conservation applications. In this paper, we sketch mechanistic and statistical frameworks for predicting these interactions, based on how plant and herbivore traits as well as shared evolutionary history can infl uence detection, recognition, and digestion of novel plants. By emphasizing mechanisms at each of these steps, we hope to clarify diff erent aspects of novel interactions and why they may or may not occur. We also emphasize prediction and forecasting, as a major goal is to know in advance which interactions will develop from the many plant or insect introductions that occur in natural and man-made systems.

The tritrophic model featuring plants consumed by herbivores consumed by parasitoids or predators has become the primary paradigm used to describe herbivore dynamics. However, interactions involving herbivores can be habitat- specific and plants often provide habitat, as well as food. Structural complexity of the habitat may favor predators or may allow herbivore prey to escape detection and capture. This study considered the spatial and temporal dynamics of an arctiid caterpillar, Platyprepia virginalis. The tritrophic model that includes only a tachinid parasitoid that attacks P. virginalis and the caterpillars’ primary host-plant, Lupinus arboreus, has failed to provide much insight into this system. Instead, we found that ants killed and removed many small caterpillars. Protecting caterpillars from ants increased their survival three-fold and five-fold in assays conducted during two years. Caterpillars were more likely to survive in short-term assays at sites that naturally had a deeper cover of dead and living plant material. Experiments with baits showed that ant recruitment declined as litter depth increased on average. These survey results indicated that ant predation was an important source of mortality for young caterpillars and that the presence of thick litter reduced this mortality. These results were corroborated in an experiment that manipulated litter depth and ant access to caterpillars. Previous findings that other defoliating caterpillars increased litter depth and benefitted P. virginalis are also consistent with this hypothesis. Litter acts as an important non-trophic resource, allowing caterpillars to avoid predation by ants such that wet sites with deep litter act as source populations for caterpillars. Our results show strong effects of both trophic and non-trophic interactions since plants indirectly provided limiting habitat and this heterogeneous habitat strongly affected risk of predation and ultimately caterpillar abundance and distribution.

Summary 1. There are now approximately 10 examples of plants that use volatile cues emitted by damaged neighbours to adjust their defences against herbivores. For two of these examples, preliminary evidence suggested that plants may experience net benefits from such eavesdropping, although eavesdropping was uncommon in one case and estimates of plant fitness were ambiguous in the other case. 2. In the current study, we examined the long-term consequences of exposure to cues emitted by experimentally clipped sagebrush neighbours. In this sagebrush system we have repeatedly found that sagebrush plants that have experimentally clipped neighbours experience less herbivore damage over the season than plants with unclipped control neighbours under field conditions. We followed a cohort of young sagebrush plants from emergence in 1999 for 12 years. Neighbours of half of these plants were artificially clipped every spring from 2004–08 and survival and flowering was measured in each autumn from 1999–2011. 3. Survival of marked branches of young plants was not consistently affected by whether its neighbour was clipped. Plants near clipped neighbours produced more branches during this period than those near unclipped neighbours. There were no measurable treatment effects on plant survival over the 12 years. Branches near clipped neighbours produced more inflorescences than branches near unclipped neighbours. 4. Seedlings were more likely to survive to the end of their first dry season in two different years near clipped neighbours compared to unclipped neighbours. 5. Synthesis. The results suggest different effects of clipped neighbours that depend on plant age. Responding to the cues of experimental clipping may provide a slight net benefit, considering these results and other published studies, even though these cues provided little predictive value about actual risk of herbivory. Responding to reliable cues may be even more beneficial and may favour plants that eavesdrop on neighbours.

1. Ecologists have argued about the commonness and strength of interspecific competition between insect herbivores, but facilitation between herbivores has received much less consideration. We previously found that when two species of folivorous caterpillars co-occurred on a shared host plant, feeding by early season tiger moth caterpillars reduced the growth and reproduction of later season tussock caterpillars. However, densities of tussock caterpillars in summer were positively correlated with densities of tiger moth caterpillars the following spring. 2. In this study, we experimentally manipulated numbers of feeding tussock caterpillars and found that they facilitated tiger moth caterpillars. 3. The depth of the litter layer beneath host lupine bushes was positively correlated with the number of tussock caterpillars feeding on each bush. Experimental additions of litter beneath lupine canopies during summer resulted in increased numbers of tiger moth caterpillars in the following spring, indicating a causal role of litter. Litter potentially provides food, habitat and protection from desiccation and predation. We failed to find evidence that tussock caterpillars facilitated tiger moth caterpillars by mechanisms independent of litter. 4. Our study demonstrates that facilitation may operate between insect herbivores, across life-stages through indirect interactions that are non-trophic. Facilitation operated by a novel mechanism, the accumulation of litter which was a by-product of feeding by one species was valuable to a second species. Facilitation persisted in time and space far beyond the creation of litter by tussock caterpillars which should be considered important ecosystem engineers from the point of view of tiger moths. Facilitations that involve habitat modification may generally connect species that do not interact directly or trophically, and have not previously been considered to affect one another.

Entomopathogenic nematodes (EPNs) can kill and regulate populations of soil-inhabiting insects, but studies evaluating these interactions in native ecosystems are rare. The objective of this study was to examine the effects of EPNs on a non-agricultural caterpillar, Platyprepia virginalis (Boisduval) (Lepidoptera: Arctiidae), under natural conditions. Platyprepia virginalis caterpillars live in litter on the soil surface feeding beneath bush lupine during summer, autumn, and winter. Initial laboratory assays revealed that the caterpillars were vulnerable to at least two species of EPNs with which they co-occur in the coastal prairie in northern California (USA). In contrast to laboratory assays, caterpillars survived exposure to prairie soil containing EPNs under natural conditions in field assays. To better understand the divergence between laboratory and field results for this native caterpillar, we used sentinel insects [Galleria mellonella L. (Lepidoptera: Pyralidae)] to identify particular locations where EPNs were present in the field. Platyprepia virginalis caterpillars were caged at these sites but again showed no evidence of susceptibility to EPNs. Platyprepia virginalis caterpillars reduce their exposure to EPNs by spending their time in and above the litter rather than contacting the soil when given the choice in nature. We conclude that P. virginalis is unlikely to serve as a reservoir for EPNs and that nematodes are unlikely to be important mortality factors for P. virginalis in this natural system.

Summary 1. Induced resistance is now widely accepted as a potent and widespread ecological force although several pieces of the story remain very poorly known. Theory predicts that induced defences should be favoured in variable environments especially when plants can use cues to reliably predict future conditions; however, this idea has not been seriously evaluated for plants. 2. Theory also predicts that plastic, induced defences should be favoured over permanent, constitutive defences if defences are costly and not always needed. This hypothesis has received considerable attention and limited support; resource allocation costs have been difficult to detect although ecological costs of defence may be more common. Recently, priming has emerged as a mechanism that may further reduce costs. Primed plants do not immediately produce the gene products associated with induced resistance but later respond more rapidly and strongly to severe or repeated attacks. It remains to be determined how common priming is. 3. Much of what we know about induced resistance is from the herbivore’s point of view. Induced resistance will be beneficial from the plant’s point of view if herbivores avoid induced plants, but the behavioural responses of herbivores to induced plant variation are still poorly studied. 4. Recent progress in understanding the detailed spatial and temporal extent of induced resistance has improved our appreciation of the phenomenon. Although some induced responses are systemic throughout entire plant individuals, many others have been found to be localized to damaged tissues. Plant vascular architecture and transpiration rates greatly constrain the distribution of vascular cues. Some plants rely on volatile cues that are active over relatively short distances and may be subject to eavesdropping by other plants, herbivores, and carnivores. Similarly, the temporal duration of induced responses may have important consequences on effectiveness although limited information is available concerning lag times before induction and relaxation times following induction. 5. Limited spatial and temporal scales of induced responses make plants more variable from the herbivore’s point of view. Recent work suggests that plant heterogeneity may be difficult for herbivores to cope with although this hypothesis awaits further empirical testing.

1. Population dynamics of insect host-parasitoid systems are important in many natural and managed ecosystems and have inspired much ecological theory. However, ecologists have a limited knowledge about the relative strengths of species interactions, abiotic effects and density dependence in natural host-parasitoid dynamics. Statistical time-series analyses would be more informative by incorporating multiple factors, measurement error and noisy dynamics. 2. We use a novel maximum likelihood and model-selection analysis of a state-space model for host-parasitoid dynamics to examine 21 years of annual census data for woolly bear caterpillars (Platyprepia virginalis) and their locally host-specific tachinid parasitoids (Thelaira americana). 3. Caterpillar densities varied by three orders of magnitude and were driven by density dependence and precipitation from the previous March but not detectably by parasitoids, despite variable and sometimes high (>50%) parasitism. 4. Fly fluctuations, as estimated from per cent parasitism, were affected by density dependence and precipitation from the previous July. There was marginal evidence that host abundance drives fly fluctuations as a generic linear effect but no evidence for classical Nicholson-Bailey coupling. 5. The state-space model analysis includes new methods for likelihood calculation and allows a balanced consideration of effect magnitude and statistical significance in a nonlinear model with multiple alternative explanatory variables.

1. Platyprepia virginalis caterpillars are dietary generalists and feed on multiple host species within a single day. We conducted field experiments to evaluate their performance on diets consisting of only their primary food, Lupinus arboreus, or diets consisting of L. arboreus plus other acceptable host species. 2. We found that relative growth rates and rates of survival were higher when they fed on mixed diets compared to lupine only. These results were consistent with hypotheses that mixed diets provided balanced nutrition, diluted toxins, and/or allowed recovery from parasitoids, although our data did not allow us to separate these non-exclusive explanations. 3. We assayed alkaloids in their host foliage, in the caterpillars themselves, in parasitoids within caterpillars, in food boluses passing through their guts, and in frass that they excreted. We consistently found positive assays for alkaloids in foliage and in frass but negative assays in caterpillars, parasitoids, and food boluses. This suggests that the alkaloids that they ingest are metabolised or rendered non-reactive by unknown means during passage through the gut. We found no support for the hypothesis that mixed diets prevented caterpillars from exhausting food supplies or allowed them to sequester chemicals from their alkaloid-containing hosts. 4. Behavioural observations revealed that previous experience influenced a caterpillar's likelihood of moving to a different host. Caterpillars that had previously fed on other hosts were more likely to move to lupine while caterpillars previously collected on lupine were equally likely to choose more lupine or a different host. 5. These results are unusual in providing a clear and consistent benefit of diet mixing in a natural field setting where multiple ecological factors act upon the caterpillars.

Animals have the ability to distinguish self from non-self, which has allowed them to evolve immune systems and, in some instances, to act preferentially towards individuals that are genetically identical or related. Self-recognition is less well known for plants, although recent work indicates that physically connected roots recognize self and reduce competitive interactions. Sagebrush uses volatile cues emitted by clipped branches of self or different neighbours to increase resistance to herbivory. Here, we show that plants that received volatile cues from genetically identical cuttings accumulated less natural damage than plants that received cues from non-self cuttings. Volatile communication is required to coordinate systemic processes such as induced resistance and plants respond more effectively to self than non-self cues. This self/non-self discrimination did not require physical contact and is a necessary first step towards possible kin recognition and kin selection.

Although competition has been a major focus in ecology for the past century, most empirical and theoretical studies in this area have emphasized interactions between closely related species. However, there is growing evidence that negative interactions among distantly related taxa also occur and may be far more important than previously thought. In this study, we took advantage of an 11-year-old replicated vertebrate-exclosure experiment in a coastal dune community in northern California, USA, to examine the effects of the two most common vertebrate herbivores ( jackrabbits and black-tailed deer) on the abundance of the three most visible invertebrate herbivores (two snail, a moth, and a grasshopper species). Our results indicate that four of the six possible pairwise interactions were significantly negative for the invertebrates. Jackrabbits reduced the abundances of snails by 44-75%, tiger moth caterpillars by 36%, and grasshoppers by 62%. Deer reduced the abundances of snails by 32%, increased the abundances of caterpillars by 31%, and had no measurable effect on grasshopper abundance. Our data also revealed that jackrabbits significantly decreased the volume of forbs and common shrubs and the flowering by grasses in our study plots. We were unable to detect an effect of deer on these measures of vegetation. These results suggest that by changing vegetation, jackrabbits may reduce invertebrate populations that are limited by food, protective structures, or microclimate provided by plants. Of these three mechanisms, only shade was strongly supported as limiting snail numbers in smaller-scale manipulations. In most systems, as in this one, the number of pairs of distantly related herbivores far exceeds the number of pairs of congeners. Since interactions among distantly related herbivores may be common in many cases, these interactions are likely to be important and should receive far more attention from ecologists.

Emissions of volatiles increase following herbivory from many plant species and volatiles may serve multiple functions. Herbivore-induced volatiles attract predators and parasitoids of herbivores and are often assumed to benefit plants by facilitating top-down control of herbivores; this benefit of induced emissions has been tested only a few times. Volatile compounds released by experimentally clipped sagebrush shoots have been shown to reduce levels of chewing damage experienced by other shoots on the same plant and on neighboring sagebrush plants. In this study, I asked whether experimental clipping attracted predators of herbivorous insects to sagebrush shoots. I also evaluated aphid populations and chewing damage on clipped and unclipped shoots and whether predators were likely to have caused differences in aphids and chewing damage. Shoots that had been clipped recruited more generalist predators, particularly coccinellids and Geocoris spp. in visual surveys conducted during two seasons. Clipping also caused increased numbers of parasitized aphids in one season. Ants were common tending aphids but were not significantly affected by clipping. Despite the increase in generalist predators, clipped plants were more likely to support populations of aphids that increased during both seasons compared to aphids on unclipped control plants. Clipped shoots suffered less damage by chewing herbivores in the 1-year in which this was measured. Chewing damage was not correlated with numbers of predators. These results suggest that predators and parasitoids were attracted to experimentally clipped sagebrush plants but that these predators were not effective at reducing net damage to the plant. This conclusion is not surprising as much of the herbivory is inflicted by grasshoppers and deer, herbivores that are not vulnerable to the predators attracted to sagebrush volatiles. More generally, it should not be assumed that predators that are attracted by herbivore-induced volatiles necessarily benefit the plant without testing this hypothesis under field conditions.

Wild tobacco growing in a field near experimentally clipped sagebrush became more resistant to herbivores than tobacco plants near unclipped sagebrush. Here, we asked whether eavesdropping on sagebrush is a common phenomenon. Do three other forb species that grow near sagebrush experience less herbivory when their sagebrush neighbors are clipped experimentally? Similarly, we asked if sagebrush becomes more resistant when conspecific neighbors are clipped. We found that levels of damage to leaves of lomatium, lupine, and valerian were unaffected by experimental clipping of neighboring sagebrush. These three forbs share many generalist herbivores with sagebrush, as does wild tobacco. Sagebrush individuals near clipped neighbors experienced approximately half the leaf loss to herbivores over the summer as plants with unclipped neighbors. Most of this herbivory was caused by generalist grasshoppers and mammals. Success of a chrysomelid beetle that specializes on sagebrush was unaffected by clipping neighboring sagebrush. These results indicate that communication between sagebrush and tobacco is not a unique phenomenon since sagebrush also responded when neighbors were clipped. In contrast, other plant species generally respond less strongly to cues than does tobacco, indicating that not all species respond to relatively high levels of jasmonate in their environments. Eavesdropping may be restricted to those species, like tobacco, that are strongly inducible and to those species, like sagebrush, that release high concentrations of cues.

1. Considerable evidence indicates that the Earth's climate has warmed over the past 20 years and most models predict that this process will continue or accelerate. Previous studies involving butterflies and other species have shown that ranges have shifted towards the poles although the mechanisms responsible for these range shifts have not been demonstrated conclusively. 2. Here it is reported that the range of the meadow spittlebug has moved northward along the California coast since 1988. 3. Survival and reproduction of this species in previous laboratory experiments were very sensitive to humidity and temperature. Small deviations from optimal conditions resulted in high mortality. 4. These laboratory results were corroborated by annual field censuses from one location since 1983 in which population densities of spittlebug nymphs were positively correlated with summer humidity and negatively correlated with temperature deviations from the published optimum. Thus it is shown that there are strong links between physiological tolerance and a geographic range shift associated with climatic change.

Many plants offer rewards to attract ants that provide indirect defense against herbivores. These rewards include nectar and swollen thorn domatia, and their production can be inducible. Theory predicts that costly rewards should be relaxed if the risk of herbivory is reduced, although this has not been previously demonstrated. Acacia drepan- olobium trees in East Africa produce two ant rewards, extrafloral nectar and swollen thorns. We compared the rewards offered by trees experimentally protected from mammalian brows- ing for seven years with those offered by unprotected trees. Protected trees produced 25% fewer nectaries per leaf and 25% fewer swollen thorns than did trees in browsed plots. Relaxation of indirect defenses when trees are protected from herbivores is consistent with the dual hypotheses that inducible defenses are dynamic and that plants can save costs by relaxing these defenses when they are not needed. Among trees exposed to mammalian herbivores, trees that were browsed within roughly the past year provided rewards for ants similar to those provided by trees exposed to herbivores but not browsed within the past year. This result suggests that relaxation of indirect defenses only occurs following more than one year of protection from mammalian herbivory.

Whether plants respond to cues produced by neighbors has been a topic of much debate. Recent evidence suggests that wild tobacco plants transplanted near experimentally clipped sagebrush neighbors suffer less leaf herbivory than tobacco controls with unclipped neighbors. Here we expand these results by showing evidence for induced resistance in naturally rooted tobacco when sagebrush neighbors are clipped either with scissors or damaged with actual herbivores. Tobacco plants with sagebrush neighbors clipped in both ways had enhanced activity levels of polyphenol oxidase (PPO), a chemical marker of induced resistance in many solanaceous plants. Eavesdropping was found for plants that were naturally rooted, although only when sagebrush and tobacco grew within 10 cm of each other. Although tobacco with clipped neighbors experienced reduced herbivory, tobacco that grew close to sagebrush had lower production of capsules than plants that grew far from sagebrush. When neighboring tobacco rather than sagebrush was clipped, neither levels of PPO nor levels of leaf damage to tobacco were affected. Eavesdropping on neighboring sagebrush, but not neighboring tobacco, may result from plants using a jasmonate signaling system. These results indicate that plants eavesdrop in nature and that this behavior can increase resistance to herbivory although it does not necessarily increase plant fitness.

Although many ecologists have discounted the possibility of communication between plants, recent work demonstrates that wild tobacco plants with experimentally clipped sagebrush neighbors suffer less leaf herbivory than tobacco controls with unclipped neighbors. In this report, we examine the fitness consequences of resistance induced by eavesdropping. Annual tobacco plants with clipped sagebrush neighbors produced more flowers and seed-bearing capsules than plants with unclipped neighbors although these performance measures varied considerably over the five years of the study. Tobacco plants with clipped neighbors also suffered more frost damage than controls in one year. There was no indication that eavesdropping was more beneficial to tobacco in years with high risk of herbivore damage. The potential adaptive benefits of eavesdropping remain unclear based on five years of data. However, the fact that eavesdropping had strong effects on herbivory and plant performance suggests that interactions between plant species may be richer than we previously suspected.

Herbivores and pollinators can simultaneously exert selective pressures on plant traits via direct and indirect effects. Net selection on plant traits, such as defensive chemistry, may be difficult to predict from studying either of these interactions in isolation. In this study, alkaloids were manipulated experimentally in the hemiparasitic annual plant Castilleja indivisa (Scrophulariaceae; Indian paintbrush) by growing these parasites with bitter (high-alkaloid) or sweet (low-alkaloid) near-isogenic lines of the host Lupinus albus (Fabaceae) in the field. To evaluate the effect of herbivores, half of the Indian paintbrush plants were randomly assigned to a reduced-herbivory treatment using insecticide, and the other half to a natural-herbivory treatment. Floral traits, bud and fruit herbivory, pollination, alkaloids, and plant performance were measured. These variables were used in a path analysis to dissect the direct and indirect effects of herbivory and pollination on lifetime seed set, and the direct and indirect effects of alkaloids on seed set via herbivory and pollination. Bud herbivory and fruit herbivory directly decreased seed production, whereas polli- nation had a direct positive effect. In addition, bud herbivory had negative indirect effects on seed set by reducing the number of open flowers, which reduced pollinator visits. Alkaloids directly reduced bud herbivory but did not significantly affect pollination or fruit herbivory directly. However, because bud herbivory indirectly reduced seed set by reducing pollinator visits to flowers, alkaloids also had additional indirect benefits for plants by increasing pollination. Overall, the net benefit of alkaloid uptake was due to both reduction in herbivory and an increase in pollinator visits to flowers. This study demonstrates the importance of considering multiple interactions simultaneously when attempting to under- stand the mechanisms underlying correlations between plant traits and fitness.

Summary 1. The negative effect of induced plant resistance on the preference and performance of herbivores is a well-documented ecological phenomenon that is thought to be important for both plants and herbivores. This study links the well-developed mechanistic understanding of the biochemistry of induced plant resistance in the tomato system with an examination of how these mechanisms affect the community of herbivores in the field. 2. Several proteins that are induced in tomato foliage following herbivore damage have been linked causally to reductions in herbivore performance under laboratory conditions. Application of jasmonic acid, a natural elicitor of these defensive proteins, to tomato foliage stimulates induced responses to herbivory. 3. Jasmonic acid was sprayed on plants in three doses to generate plants with varying levels of induced responses, which were measured as increases in the activities of proteinase inhibitors and polyphenol oxidase. 4. Field experiments conducted over 3 years indicated that induction of these defensive proteins is associated with decreases in the abundance of all four naturally abundant herbivores, including insects in three feeding guilds, caterpillars, flea beetles, aphids, and thrips. Induced resistance killed early instars of noctuid caterpillars. Adult flea beetles strongly preferred control plants over induced plants, and this effect on host plant preference probably contributed to differences in the natural abundance of flea beetles. 5. The general nature of the effects observed in this study suggests that induced resistance will suppress many members of the herbivore community. By linking plant biochemistry, insect preference, performance, and abundance, tools can be developed to manipulate plant resistance sensibly and to predict its outcome under field conditions.

Key words: Density-dependent induction, genotype-by-environment interaction, pigment glands, herbivory,induced plant defense, plant-insect interactions, plasticityAbstractCotton plants contain suites of phytochemicals thought to be important in defense against herbivores, some ofwhich are localized in pigment glands which contain gossypol and other terpenoid aldehydes. The simple geneticbasis for the expression of these glands has led to the development of near-isogenic glanded and glandless geno-types. Glands may also be phenotypically induced by herbivory. We determined the consequences of constitutiveandinducedgland expressionontwo types ofherbivores, spidermites (cell contentfeeders)andnoctuidcaterpillars(leaf chewers).Induction of glands was strongly dependent on the density of attackers. Spider mite herbivory on cotyledons(1) increased the density (but not total number) of glands on cotyledons linearly, (2) increased the density andtotal number of glands on the first true leaf linearly, and (3) affected the density and total number of glands on thesecond true leaf non-linearly, compared to controls. Neither constitutive nor induced expression of glands affectedmite population growth. An equal reductionof mite population size on induced glanded and glandless plants (50%)relative to uninduced controls indicated that factors other than glands were associated with induced resistance tomites. Constitutive gland expression had a strong negative impact on caterpillar performance, reducing growth by45%. Induced resistance to caterpillars was three times stronger in glanded genotypes than in glandless genotypes,indicating that factors associated with induced resistance to caterpillars are strongly associated with glands. Threecotton varieties were highly variable in their constitutive and induced resistance to mites and caterpillars.Thus, defense of cotton plants against herbivores can be roughly categorized as constitutive and induciblefactors associated with terpenoid aldehyde containing pigment glands that are effective against caterpillars, andfactors not associated with glands that are effective against mites.IntroductionPlants are the basal resource for organisms in manyfood webs. As a consequence, most plants are con-sumed by numerous herbivores. Where herbivoresreduce plant fitness, natural selection has presum-ably favored the evolution of defensive mechanismsin plants. Plant defenses can be expressed constitu-tively (all of the time) and facultatively (induced bysome environmental cue) (Karban & Baldwin, 1997;Agrawal, 1998). Genotypically determined constitu-tive and induced expression of phytochemicals canmediate either specific or general resistance to at-tackers. Furthermore, different resistance mechanismshave the potential to interact with each other to in-crease (or decrease) defense (Berenbaum & Zangerl,1996). Some plant defenses such as the tetracyclictriterpenoid cucurbitacins of the cucurbitaceae are ef-fective against many attackers including spider mites,roaches, beetles, lepidopteran larvae, mice and ver-tebrate grazers (Da Costa & Jones, 1971; Metcalf L Tallamy et al., 1997; Agrawal et al.

Seventeen-year periodical cicadas (Magicicada spp.) require 17 years to develop underground and all individuals at any location emerge synchronously within several days. The mechanisms that animals use to keep track of time are poorly understood and nothing is known about how cicada nymphs emerge after precisely 17 years. We altered the seasonal cycles of trees supporting cicada nymphs and thereby induced premature metamorphosis of the associated cicadas. This indicates that cicadas accomplish a consistently accurate 17-year preadult development time by counting host seasonal cycles and not either by the passage of real time or by the accumulation of degree days.

Predators and plant resistance may act together to control herbivorous arthropod populations or antagonistically, which would reduce the control of pest populations. In a field experiment we enhanced predation by adding simulated leaf domatia to plants. Leaf domatia are small structures that often harbor predaceous arthropods that are potentially beneficial to the plant. We also manipulated host plant quality by inducing resistance with controlled, early season exposure of seedlings to spider mite herbivory. Our manipulations had profound consequences for the natural community of arthropods that inhabited the plants. Leaf domatia had a direct positive effect on abundances of two species of bugs and one species of thrips, all of which are largely predators of herbivores. On leaves with domatia, each of the predators was found inside the domatia two to three times more often than outside the domatia. Eggs of predaceous bugs inside leaf domatia were protected from parasitism compared to eggs outside the domatia. The positive effects of leaf domatia on predator abundances were associated with reduced populations of herbivorous spider mites, aphids, and whiteflies. Plants with experimental leaf domatia showed significantly enhanced reproductive performance. Induced resistance also affected the community of arthropods. Of the abundant predators, all of which also fed on the plant, only minute pirate bugs were negatively affected by induced resistance. Populations of herbivorous spider mites and whiteflies were directly and negatively affected by induction. In contrast, aphid populations were higher on plants with induced resistance compared to uninduced plants. Effects of induced resistance and domatia were additive for each of the predators and for aphids. However, spider mite and whitefly populations were not suppressed further by employing both induced resistance and domatia compared to each strategy alone. Our manipulations suggest that plant defense strategies can have positive effects on some species and negative effects on others. Negative effects of ‘‘resistance traits’’ on predators and positive effects on some herbivores may reduce the benefits of constitutive expression of resistance traits and may favor inducible defense strategies. Multiple plant strategies such as inducible resistance and morphological traits that aid in the recruitment of predators of herbivores may act together to maximize plant defenses, although they may also be redundant and not act additively.

All plants pass through a series of predictable developmental stages during their lives, called phase changes. The phase change from juvenile to adult leaves is known to be associated with changes in resistance against plant pathogens and herbivores in several species. Virtually nothing is known about changes in resistance associated with the transition from embryonic tissue to autotrophic tissue in seedlings. We studied the consequences of transitions from cotyledons to juvenile true leaves to adult true leaves in cotton seedlings (Gossypium hirsutum) for their resistance to spider mites (Tetranychus urticae). Mite populations grew much more rapidly on cotyledons than on true leaves. However, there was no detectable difference in the population growth of mites on juvenile vs. adult true leaves. We suggest that population growth of mites is positively affected by the high rates of photosynthesis of cotyledons relative to true leaves, or by some process or attribute correlated with photosynthesis. Conditions that caused increased rates of photosynthesis (exposure to light and elevated concentrations of CO2) caused mite populations to increase. Greater mite population growth on cotyledons was not associated with stored reserves in the cotyledons, as the mites did poorly on cotyledons kept in the dark. This study indicates that phase changes can have profound effects on plant resistance to herbivores. Because the seedling stage is so vulnerable to herbivory and so critical to understanding plant population dynamics, a broader consideration of phase changes associated with seedlings is warranted.

1. It is generally assumed that growing in proximity to sources of nectar will benefit plants by attracting predators and parasites that reduce herbivore loads. 2. This paper documents patterns of hornworm Manduca quinquemaculata abundance on wild tobacco plants Nicotiana attenuata. Observations suggested that several plant traits were associated with hornworm eggs and larvae. 3. Hornworms were more likely to be found on large tobacco plants. 4. Hornworms were more likely to be found on tobacco plants with flowering Eriastrum densifolium neighbours. 5. Experimental removal of neighbouring Eriastrum densifolium flowers or whole plants reduced hornworm damage to tobacco plants in neighbourhoods with E. densifolium. The effects of these manipulations were not found to increase reproductive success of neighbouring tobacco plants, although tobacco reproduction was reduced where M. quinquemaculata was abundant. 6. These results suggest that close proximity to nectar resources could decrease plant fitness.

Previous explanations for the evolution of induced resistance of plants to herbivory emphasized arguments based on saving costs when allocations to defense were not needed; these models met with limited empirical support. We offer a novel explanation based on induced resistance providing increased variability in defense. As long as maximal levels of defense are constrained, variability will increase the effectiveness of a given level of investment in defense. We show that variability can decrease herbivore performance if herbivore performance is a concave function of the level of resistance. In particular, if herbivores can choose among different plants and plant tissues, then variability created by induced resistance may benefit plants under attack and hence may be favored by selection. The key assumptions of this model are broadly supported by empirical data from many plant-herbivore systems.

Current models of parasite–host interactions generally emphasize considerations of parasite virulence and parasite transmission rather than host responses to attack. We describe a situation in which parasitism causes a host to change its feeding behavior and this change improves the expected fitness of the host and probably the fitness of its parasites. We have found that a tachinid parasitoid (Thelaira americana) often emerges from its caterpillar host (Platyprepia virginalis) without killing the host. Whether the host caterpillar survives its parasites depends in part on what it has been eating. Unparasitized caterpillars were more likely to survive to adulthood when feeding on lupine, whereas parasitized caterpillars were more likely to survive on poison hemlock. Development time and pupal masses of caterpillars (both parasitized and unparasitized) were not found to be affected by the host plants that they fed on. Survival of fly larvae in caterpillars that we determined were parasitized using ultrasound was not affected by host plant. However, fly pupae that emerged from caterpillars that had been reared on hemlock were heavier than those emerging from lupine-fed caterpillars. This was due primarily to the direct effect of diet on the flies and less so to the indirect benefit to flies whose host caterpillars survived their parasites by feeding on hemlock. Parasitized caterpillars were more likely to select hemlock, and unparasitized caterpillars were more likely to select lupine when offered both host plants in field tests. These results were consistent for the two years that the choice experiments were conducted. These results were also consistent with the hypothesis that caterpillars change their food plant choices so as to increase their conditional success. Conditional food choices that increase success depending upon parasite load are well accepted for humans, controversial for other primates, and unknown for insects. If caterpillars alter their host plant choices as a result of their parasite load, then this phenomenon could help to explain the evolution of host plant choices that have defied explanation in the past. Such a suggestion assumes that nonlethal parasitism is a common phenomenon. We believe that this may be the case since we did not detect the nonlethal nature of this interaction until we began the unconventional practice of rearing in the field. Other workers have described nonlethal parasitism for several tachinid–host systems, and many families of flies are similar to tachinids except that their hosts are vertebrates; these interactions are nonlethal. Future empirical work, as well as models of parasite–host interactions, should consider the possibility that hosts alter their plant choices depending upon their parasite loads.

We have successfully released phytophagous Willamette mites into Zinfandel vineyards that have had chronic problems with the economically more damaging Pacific spider mites. These releases induced resistance in the grapevines, causing smaller populations of Pacific mites and higher berry sugar concentrations. This study addresses the sustainability of our techniques. We define a sustainable technique as one that (1) is effective, (2) requires low inputs of resources, (3) is self-perpetuating, (4) produces minimal pollution, (5) produces minimal effects on non-target organisms, and (6) is compatible with other agronomic practices. Most of these criteria are met by our inoculation techniques; here we seek to determine the persistence of Willamette mites from one season to the next, and to evaluate effects of several commonly used agronomic practices on our vaccinations. Our data suggest that Willamette mites can be introduced a single time into a vineyard; they will reduce populations of Pacific mites and persist from year to year in the absence of miticides or other disruptions. We observed Willamette mites overwintering commonly on the grapevines and on box elder (Acer negundo) foliage adjacent to vineyards. Pacific mites were not observed overwintering in vineyards, and we surmise that they generally blow into vineyards during each growing season. We did not observe Willamette mites overwintering on cover crops although other phytophagous mites and predatory mites were found. In preliminary observations, we found no strong effects (positive or negative) associated with cover-cropping in terms of mite management. Vineyards with Pacific mites are often treated with miticides, either dicofol or propargite. We found that dicofol (but not propargite) was extremely disruptive to mite populations; after an initial kill, applications of dicofol were consistently associated with resurgences of mites that exceeded numbers found in unsprayed controls. Unlike applications of miticides, introductions of Willamette mites to control Pacific mites satisfy our criteria for sustainable agriculture. Here we demonstrated that they are self-perpetuating at low numbers from one season to the next. Furthermore, releasing Willamette mites for control of Pacific mites is compatible with some, but not all, management practices.

Systems of induced resistance for both vertebrates and invertebrates possess a memory; individuals that are attacked by a particular pathogen are able to respond more quickly and more effectively the second time they encounter that pathogen. Many plants have been found to be more induced by repeated herbivory than by a single bout of herbivory. In these experiments the frequency and magnitude of damage were confounded so that it was not clear if memory was involved or if more overall damage amplified the induced response. We conducted an experiment to test the memory of induced resistance in cotton to repeated attacks by spider mites. We held the overall amount of damage (herbivore-days) constant but varied the number of times that plants were exposed to damage. We did not find that plants damaged a second time responded more effectively than those damaged only once. Power analysis indicated that a large effect of memory was unlikely to have been overlooked in these experiments if it existed. Following observations that different types of damage caused different responses in mountain birch (Haukioja et al. 1990), we damaged either cotyledonary leaves or apical buds of young cotton plants and challenged them with spider mites. We found that damage to cotyledons induced resistance but damage to apical buds induced susceptibility. Damage to cotyledons accelerated the rate of abscission of cotyledons, a favored tissue for spider mites; damage to buds delayed abscission of cotyledons. Cotyledon abscission was sufficient to induce resistance but was not a necessary condition since induced resistance has been observed when cotyledon abscission was not found. Our results were consistent with the hypothesis that damage to these two tissues differentially affected hormonal regulation as well as the alternative hypothesis that these forms of damage had opposite effects on plant ontogeny.

Sexual reproduction may be advantageous for hosts that are preyed on or parasitized by enemies that are highly adapted to them. Sexual reproduction can create rare genotypes that may escape predation by virtue of rarity and can create variable progeny that may escape predation if enemies are specialized to only one genotype of host. Populations of the herbivorous thrips, Apterothrips apteris, have been shown to be adapted to individual Erigeron glaucus clones. Here, we show that thrips adapted to the parental clone could better use plant progeny of the "home" clone produced through selfing than progeny derived from selfing of other clones. Thus, despite recombination, progeny produced by selfing presented a resource that was similar to the parental phenotype with respect to use by adapted thrips. We also show that E. glaucus susceptibility to thrips has a genetic basis and then ask whether outcrossing provides a means for E. glaucus clones to escape attack by adapted thrips. When we compared the success of thrips on progeny produced by selfing or outcrossing of the home clone, we found that the merits or disadvantages associated with outcrossing were dependent on the susceptibility to infestation of the parental clones. Selfing by clones characterized by low infestations of thrips appeared to preserve resistant genotypes; all outcrossed progeny had, on average, higher infestation levels than selfed progeny. In contrast, outcrossed progeny of clones characterized by high infestations of thrips had either the same thrips density as progeny from selfing, when the pollen donor was a highly infested clone, or lower density, when the pollen donor was a low infestation clone. The advantages of outcrossing were caused by the alleles contributed to progeny rather than to progeny variability or rarity.

The thrips herbivore Apterothrips apteris reproduces both sexually and parthenogenetically and exhibits fine-scale local adaptation to individual phenotypes of its host plant, Erigeron glaucus. We ask whether we can disrupt the ability of thrips to use progeny of their "home"-plant clone by outcrossing females with males from other plant clones. We compare the performance of sexually produced thrips to that of parthenogenetically produced thrips on plant progeny of the home clone. Because we use thrips from plant clones experiencing both high and low infestations by thrips, we also ask whether the relative performance of "sexuals" versus "parthenogens" differs with the infestation level of the home clone. Plant progeny of 10 E. glaucus clones were either the product of selfing or of outcrossing with the other 9 clones. We have shown in previous work that selfing preserved the parental phenotype with respect to attack by thrips. Because of this result, we predicted that parthenogens should fare better than sexuals on the selfed progeny of the home-plant clone. Our results, however, showed the contrary: sexuals outperformed parthenogens on these selfed plants. We also found that plant characters appear to influence thrips performance more than the mating system of thrips. We found no evidence for outbreeding depression in this system.

This study considered the dispersal and subsequent reproduction of Apterothrips apteris, a wingless, folivorous thrips species that is divided into local populations, each adapted to its individual host plant. We placed host plants with no thrips into the field and observed the rate at which thrips colonized these plants. Colonization events were rare; on average each pot received 1.47 immigrants during the year. Immature thrips were found associated with 64.5% of these events, indicating that reproduction had occurred. Colonization was most likely to occur during the summer. Most colonists walked onto the new host and airborne movements were very rare. Plants that were located far from near neighbors were less likely to be colonized although the size of nearby source populations did not influence the rate of colonization. These results suggest that limited movement is associated with division of this population into isolated demes. Limited movement appeared to be more important as a barrier to successful colonization than was demographic success following arrival at a

Experiments were conducted over several years to assess the effects of the three most abundant herbivores (a thrips, a spittlebug, and a plume moth) on the growth and reproduction of their long-lived, iteroparous host plant. In a greenhouse experiment, thrips reduced root biomass of seedlings, but not shoot biomass. No correlation was found between density of thrips and change in aboveground area of plants after monitoring for 3 yr in the field. Thrips fed on the phyllaries and corollas of flower heads, as well as on leaves, and were particularly damaging to ray petals. Placed in artificial arrays, flower heads with rays damaged by thrips were less likely to be visited by pollinators than were undam- aged flower heads. Plants in a common garden were subjected to three seasons of feeding by either spit- tlebugs, plume moth caterpillars, both, or neither. Survival, growth, and flower production were measured for those 3 yr and for an additional 3 yr following the herbivory treatments. Plant survival was unaffected by these insect herbivores, although gophers (Thomomys bottae) were a major source of mortality. Plants with caterpillars produced more rosettes than those without moths. By consuming apical buds the caterpillars released axillary buds, thereby causing plants to produce more side rosettes. This difference in architecture did not result in an increase in flower production. Plants that had experienced 3 yr of spittlebug or caterpillar attack produced fewer flower heads than controls during their 3rd yr. One year later, and after the herbivory treatments were discontinued, plants that had been damaged by spittlebugs (but not plume moth caterpillars) still produced fewer flowers than those in other treatments. Thereafter, no long-term effects of herbivory could be detected. All three herbivores were found to adversely affect some components of plant growth and reproduction. Because they are consistently abundant at Bodega Bay, these herbivores may reduce the fitness of their host plant.