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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.

Plants can retain either physiologically active green leaves or inactive brown leaves over winter. Research has suggested that leaf retention incurs a cost due to higher herbivore load in the following year; however, no distinction has, thus, far been made between retention of green and brown leaves. We surveyed the over-winter retention of physiologically active green and inactive brown leaves of valley oaks (Quercus lobata) and examined their relationship with the diversity and density of 15 gall-making cynipid wasp species. Cynipid diversity in the spring was 8.2-fold greater on larger trees with more green leaf retention. Brown leaf retention was not associated with spring cynipid diversity, but was related to a substantial 20-fold reduction in spring cynipid densities on large trees. Retention of either leaf type was generally a poor predictor of summer cynipid diversity and density. Overall, green leaf retention better explained cynipid diversity, but brown leaf retention better explained cynipid densities. These differing effects may be explained by the fact that green leaves provide a common cue used by herbivores to find a suitable host. Retained brown leaves, however, may be an ecological trap for over-winter gall wasps that normally drop to the ground.

Infectious disease is an important potential driver of population cycles, but this must occur through delayed density-dependent infection and resulting fitness effects. Delayed density-dependent infection by baculoviruses can be caused by environmental persistence of viral occlusion bodies, which can be influenced by environmental factors. In particular, ultraviolet radiation is potentially important in reducing the environmental persistence of viruses by inactivating viral occlusion bodies.Delayed density-dependent viral infection has rarely been observed empirically at the population level although theory predicts that it is necessary for these pathogens to drive population cycles. Similarly, field studies have not examined the potential effects of ultraviolet radiation on viral infection rates in natural animal populations. We tested if viral infection is delayed density-dependent with the potential to drive cyclic dynamics and if ultraviolet radiation influences viral infection levels.We censused 18 Ranchman’s tiger moth (Arctia virginalis) populations across nearly 9° of latitude over two years and quantified the effects of direct and delayed density and ultraviolet radiation on baculovirus infection rates, infection severity, and survival to adulthood. Caterpillars were collected from each population in the field and reared in the laboratory. Baculovirus has not previously been described infecting Arctia virginalis, and we used genetic methods to confirm the identity of the virus.We found that infection rate, infection severity, and survival to adulthood exhibited delayed density-dependence. Ultraviolet radiation in the previous summer decreased infection severity, and increased survival probability of the virus. Structural equation modelling indicated that the effect of lagged density on moth survival was mediated through infection rate and infection severity, and was 2.5 fold stronger than the effect of ultraviolet radiation on survival through infection severity. We successfully amplified polh, lef-8, and lef-9 viral genes from caterpillar samples, and BLAST search results confirmed that the virus was a nucleopolyhedrovirus.Our findings provide clear evidence that delayed density dependence can arise through viral infection rate and severity in insects, which supports the role of viral disease as a potential mechanism, among others, that may drive insect population cycles. Furthermore, our findings support predictions that ultraviolet radiation can modify viral disease dynamics in insect populations, most likely through attenuating viral persistence in the environment.

Insect declines have been reported worldwide, although the particular causes of the declines may be complex and are poorly understood. Meadow spittlebugs were one of the most abundant insects in the coastal prairie along the California coast 40 yr ago but have largely disappeared. Evidence links this decline to changing climatic conditions, which have reduced survival of eggs and neonates. We identified several refugia where meadow spittlebug populations have persisted amidst unfavorable conditions. Protection from desiccating winds was the common attribute of these refugia. Following a wet year, adult meadow spittlebugs were able to disperse from one refuge that we studied to recolonize coastal prairie habitats, although populations declined over the next two drier years. Because of their previous high abundance, loss of meadow spittlebugs is likely to affect the functioning of this widespread habitat, including energy transfer, their host plants, and their predators. In addition, meadow spittlebugs are unusual in having been the subject of extensive physiological and long‐term ecological data, so they can serve as a bellwether species, indicating the effects of climate change.

Plant-to-plant volatile-mediated communication and subsequent induced resistance to insect herbivores is common. Less clear is the adaptive significance of these interactions; what selective mechanisms favour plant communication and what conditions allow individuals to benefit by both emitting and responding to cues? We explored the predictions of two non-exclusive hypotheses to explain why plants might emit cues, the kin selection hypothesis (KSH) and the mutual benefit hypothesis (MBH). We examined 15 populations of sagebrush that experience a range of naturally occurring herbivory along a 300 km latitudinal transect. As predicted by the KSH, we found several uncommon chemotypes with some chemotypes occurring only within a single population. Consistent with the MBH, chemotypic diversity was negatively correlated with herbivore pressure; sites with higher levels of herbivory were associated with a few common cues broadly recognized by most individuals. These cues varied among different populations. Our results are similar to those reported for anti-predator signalling in vertebrates.

Apart from model organisms, 13- and 17-year periodical cicadas (Hemiptera: Cicadidae: Magicicada) are among the most studied insects in evolution and ecology. They are attractive subjects because they predictably emerge in large numbers; have a complex biogeography shaped by both spatial and temporal isolation; and include three largely sympatric, parallel species groups that are, in a sense, evolutionary replicates. Magicicada are also relatively easy to capture and manipulate, and their spectacular, synchronized mass emergences facilitate outreach and citizen science opportunities. Since the last major review, studies of Magicicada have revealed insights into reproductive character displacement and the nature of species boundaries, provided additional examples of allochronic speciation, found evidence for repeated and parallel (but noncontemporaneous) evolution of 13- and 17-year life cycles, quantified the amount and direction of gene flow through time, revealed phylogeographic patterning resulting from paleoclimate change, examined the timing of juvenile development, and created hypotheses for the evolution of life-cycle control and the future effects of climate changeon Magicicada life cycles. New ecological studies have supported and questioned the role of prime numbers in Magicicada ecology and evolution, found bidirectional shifts in population size over generations, quantified the contribution of Magicicada to nutrient flow in forest ecosystems, and examined behavioral and biochemical interactions between Magicicada and their fungal parasites and bacterial endosymbionts.

Background and Aims Pendulous flowers (due to a flexible pedicel) are a common, convergent trait of hummingbird-pollinated flowers. However, the role of flexible pedicels remains uncertain despite several functional hypotheses. Here we present and test the ‘lever action hypothesis’: flexible pedicels allow pendulous flowers to move upwards from all sides, pushing the stigma and anthers against the underside of the feeding hummingbird regardless of which nectary is being visited. Methods To test whether this lever action increased pollination success, we wired emasculated flowers of serpentine columbine, Aquilegia eximia, to prevent levering and compared pollination success of immobilized flowers with emasculated unwired and wire controls. Key Results Seed set was significantly lower in wire-immobilized flowers than unwired control and wire control flowers. Video analysis of visits to wire-immobilized and unwired flowers demonstrated that birds contacted the stigmas and anthers of immobilized flowers less often than those of flowers with flexible pedicels. Conclusions We conclude that flexible pedicels permit the levering of reproductive structures onto a hovering bird. Hummingbirds, as uniquely large, hovering pollinators, differ from flies or bees which are too small to cause levering of flowers while hovering. Thus, flexible pedicels may be an adaptation to hummingbird pollination, in particular due to hummingbird size. We further speculate that this mechanism is effective only in radially symmetric flowers; in contrast, zygomorphic hummingbird-pollinated flowers are usually more or less horizontally oriented rather than having pendulous flowers and flexible pedicels.

Climate change can affect biotic interactions, and the impacts of climate on biotic interactions may vary across climate gradients. Climate affects biotic interactions through multiple drivers, although few studies have investigated multiple climate drivers in experiments. We examined the effects of experimental watering, warming, and predator access on leaf water content and herbivory rates of woolly bear caterpillars (Arctia virginalis) on a native perennial plant, pacific silverweed (Argentina anserina ssp. pacifica), at two sites across a gradient of precipitation in coastal California. Based on theory, we predicted that watering should increase herbivory at the drier end of the gradient, predation should decrease herbivory, and watering and warming should have positive interacting effects on herbivory. Consistent with our predictions, we found that watering only increased herbivory under drier conditions. However, watering increased leaf water content at both wetter and drier sites. Warming increased herbivory irrespective of local climate and did not interact with watering. Predation did not affect herbivory rates. Given predictions that the study locales will become warmer and drier with climate change, our results suggest that the effects of future warming and drying on herbivory may counteract each other in drier regions of the range of Argentina anserina. Our findings suggest a useful role for range‐limit theory and the stress‐gradient hypothesis in predicting climate change effects on herbivory across stress gradients. Specifically, if climate change decreases stress, herbivory may increase, and vice versa for increasing stress. In addition, our work supports previous suggestions that multiple climate drivers are likely to have dampening effects on biotic interactions due to effects in different directions, though this is context‐dependent., Multiple interacting climate drivers are expected to affect ecological interactions and to vary across climate gradients. In this study, we manipulate temperature and moisture a two sites across a gradient of precipitation and find that warming increases herbivory at both sites, but watering only increases herbivory at the drier site. These findings are consistent with predictions drawn from range‐limit theory and the stress‐gradient hypothesis.

Induced plant responses to herbivory are common, and we have learned a lot about the mechanisms of induced resistance and their effects on herbivore performance. We know less about their effects on herbivore behaviour and especially on spatial patterns of damage. Theoretical models predict that induced responses can cause patterns of damage to become aggregated, random or even. A recent model predicted that informed herbivore movement coupled with communication between plants would make damage more even within individual plants. We tested these predictions in the field using a specialist beetle Trirhabda pilosa that feeds on sagebrush Artemisia tridentata. Both the beetle and the plant are well-documented to respond to damage-induced volatile cues. Beetle larvae were more likely to move from damaged leaves and leaves that had been exposed to volatiles from nearby damaged leaves compared to undamaged control leaves. Previous laboratory results indicated that beetles were more likely to choose undamaged leaves compared to damaged leaves or those exposed to volatile cues of damage. A comparison of damage patterns early in the season and after completion of beetle feeding revealed that variance in damage among branches decreased as the season progressed; that is, damage became more evenly distributed among the branches within a plant. Larvae damaged many leaves on a plant but removed relatively little tissue from each leaf. Herbivore movement and the spatial patterns of damage that it creates can be important in determining effects on plant fitness and other population processes. Dispersion of damage deserves more consideration in plant-herbivore studies.

Plants respond to damage by herbivores or to reliable cues of damage by changing in ways that provide greater resistance and increase their fitness. Sagebrush has been a model system for understanding induced resistance, although resistance in this system is commonly assessed by quantifying damage at the end of the season; this measure is slow and lacks accuracy and interpretability because so many other factors also affect levels of damage. Herbivore choice offers a potentially rapid and accurate alternative assay of induced resistance. Here we evaluate whether feeding behavior of a specialized Chrysomelid beetle, Trirhabda pilosa, could be used to assay induced changes in plant quality. Beetle larvae were offered the choice between two leaves in Petri dish arenas. We found that beetles avoided leaves that were naturally damaged by herbivores, experimentally clipped with scissors, and exposed to the volatiles from naturally or experimentally damaged neighboring leaves compared to control leaves. Experiments varied the source of the damage, the duration of the feeding test, and how damage was measured, still, beetles consistently preferred uninduced controls by a 2:1 ratio over leaves exposed to cues of damage. These results suggest that behavioral assays using T. pilosa larvae can be used to rapidly evaluate induced resistance in this system. More generally, movement and feeding behaviors of herbivores are an important and underappreciated component of induced plant responses.

‘Hilltopping’ is a common mate-locating behaviour exhibited by numerous insect taxa; individuals aggregate on summits, ridges and other topographic features, and thereby increase their likelihood of mating. Recently, hilltopping has gained interest as a model system to study nonrandom dispersal. We tested four predictions from the hilltopping literature regarding individual movement behaviour and the resulting spatial distribution of summit aggregations. Through observations and capture–mark–recapture studies using the day-flying tiger moth, Arctia (formerly Platyprepia ) virginalis , we found evidence for all predictions. The highest densities of moths were associated with a few, high-elevation summits and were recaptured over multiple days. No individuals were found to move between summit aggregations and mated females had shorter residency times than males. We discuss our results in the context of the predictions, the behaviour of other hilltopping species, implications for population structure and spatial 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.

Author(s): Karban, R | Abstract: 1. Plants perceive herbivore damage or increased risk and respond. These changes may increase plant fitness, although effects on fitness have often been assumed without supporting evidence. 2. Three models have been proposed to explain induced rather than constitutive defence. The optimal defence model posits that induction allow plants to reduce allocation costs; it predicts demonstrably lower costs when defences are not needed. The moving target model posits that induction increases spatial and temporal variability; it predicts that variability will be difficult for herbivores and will provide defence. The information transfer model posits that induced responses provide cues to other tissues on that individual plant and to other organisms in the community; it predicts that induced cues will provide systemic resistance, deter herbivores, and/or attract enemies of herbivores, thereby benefiting the induced plant. 3. All three models predict that cues must be reliable to be useful. In some cases, cues provide specific information about the damaged plant tissue and the herbivore and this specific information may allow plants to fine-tune responses. Recent theory posits that selection should favour plants that minimise recognition errors and reduce fitness costs associated with errors. 4. Future research should focus on exploring different modalities used by plants to perceive herbivore risk, the benefits and costs of perceiving cues and inducing resistance, and the basic natural history of these phenomena. Induced responses have great unrealised potential in agriculture, and research should focus on host plant resistance rather than attempting to involve other trophic levels.

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.

© 2018, Springer Nature B.V. Grasses frequently have unidirectional hairs, prickles, and spines; these leaf features have been hypothesized to move herbivores and their chewing damage away from grass meristems, which are located basally. Observations of chewing damage to two grasses, Andropogon virginicus and Phragmites australis, were consistent with this hypothesis as leaf tips received 10 × and 2 × more damage than bases. Grasshoppers were no more likely to land on leaf tips than bases although they oriented towards the tips after landing. Leaves of A. virginicus that were damaged by chewing herbivores had fewer spines than leaves on the same or neighboring plants that lacked damage. This suggests that herbivores chose less spiny leaves. At a larger spatial scale, plants in neighborhoods favorable for grasshoppers had more spines than plants in less favorable neighborhoods. We found no evidence that marginal spines allowed leaves to shed water more rapidly, a potential alternative benefit. The density of spines on new leaves increased following cues of damage. A. virginicus leaves produced after an adjacent leaf had been clipped with scissors had 13% more spines than new leaves on unclipped plants. Clipping with scissors failed to increase spine density for new P. australis leaves although experimental chewing by caterpillars led to the production of new leaves with 24% more spines than controls. Unchewed new leaves within 20 cm of a chewed neighbor had 13% more spines than controls. Grasses are capable of responding to cues of tissue damage to their own and neighboring leaves, potentially reducing herbivory to meristems.

Although many studies have investigated plant growth in the context of episodic herbivory and pressed resource availability, relatively few have examined how plant growth is affected by pulsed resources and chronic herbivory. Periodical cicada (Magicicada spp.) adults represent a pulsed detrital subsidy that fertilizes plants, and live cicada nymphs are long-lived root-feeding herbivores. Previous studies of cicada herbivory effects have been inconclusive, and previous studies of cicada-mediated fertilization did not examine effects on trees, or on a multiyear timescale. Here, we describe the results of a 3-yr experiment that factorially manipulated the presence and absence of cicada fertilization and herbivory in a population of 100 American sycamore (Platanus occidentalis) trees. We found that cicada fertilization strongly increased tree growth in the year of emergence, creating differences in tree size that persisted at least 2yr later. By comparison, we did not detect reductions in tree growth associated with cicada herbivory in any year of this experiment. However, cicada herbivory reduced the densities of, and damage from, other aboveground herbivores. These results suggest that cicadas affect the size structure of forests over multiple years, and raise questions about how cicada-mediated fertilization and herbivory will affect tree growth over longer timescales.

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.

Many plants engage in protective mutualisms, offering resources such as extrafloral nectar and shelters to predatory arthropods in exchange for protection against herbivores. Recent work indicates that sticky plants catch small insects and provide this carrion to predators who defend the plants against herbivores. In this study, we investigated whether wild tobacco, Nicotiana attenuata, fits this sticky plant defense syndrome that has been described for other sticky plants. We developed a bioassay for stickiness involving the number of flies that adhered to flowers, the stickiest tissues. In surveys conducted over three field seasons at four sites, we found that the number of carrion that adhered to a plant was positively correlated with the number of predators that we observed foraging over its surfaces. The number of predators was positively correlated with the number of seed capsules that the plant produced, a measure of lifetime female reproductive success. Structural equationmodeling indicated strong support for the causal path linking carrion numbers to predator numbers to capsule production. We investigated whether stickiness was an inducible trait and examined two potential cues. We found that experimental clipping of rosette leaves induced greater stickiness, although clipping of neighboring sagebrush leaves did not. Damage to leaf tissue is likely to be a more reliable predictor of risk than is damage to a neighboring plant. The sticky plant defense syndrome is a widespread protective mutualism; its strength and ecological relevance can adjust as risk of herbivory changes.

The risk of consumption is a pervasive aspect of ecology and recent work has focused on synthesis of consumer-resource interactions (e.g., enemy-victim ecology). Despite this, theories pertaining to the timing and magnitude of defenses in animals and plants have largely developed independently. However, both animals and plants share the common dilemma of uncertainty of attack, can gather information from the environment to predict future attacks and alter their defensive investment accordingly. Here, we present a novel, unifying framework based on the way an organism's ability to defend itself during an attack can shape their pre-attack investment in defense. This framework provides a useful perspective on the nature of information use and variation in defensive investment across the sequence of attack-related events, both within and among species. It predicts that organisms with greater proportional fitness loss if attacked will gather and respond to risk information earlier in the attack sequence, while those that have lower proportional fitness loss may wait until attack is underway. This framework offers a common platform to compare and discuss consumer effects and provides novel insights into the way risk information can propagate through populations, communities, and ecosystems.

Grasses are major agricultural products worldwide and they are critical to ecosystem function in many terrestrial habitats. Despite their global importance, we know relatively little about their defenses against herbivory. Grasses tend to be tolerant of leaf loss because their valuable meristems are located underground, out of reach for above ground herbivores. Many grasses have unidirectional leaf hairs, prickles, and spines that make moving up the leaf blade easy, but make moving down, toward the meristem, difficult. We tested the hypothesis that unidirectional grass hairs direct small arthropod herbivores away from the meristems. In a field survey of the distribution of herbivore damage, we found that leaf tips received five times more damage than leaf bases for Avena barbata. Early-instar grasshoppers fed three times as often on leaf tops as on leaf bases of pubescent individuals in a common garden laboratory experiment. This effect was not observed for glabrous individuals where grasshoppers damaged leaf bases as often as leaf tops. A common generalist caterpillar, Heliothus virescens, was more than twice as likely to turn in the direction of the hairs, away from the meristems, when it encountered pubescent leaves of A. barbata. However, larger caterpillars of the generalist feeder Arctia virginalis showed no directional bias when they encountered pubescent leaves. In common garden experiments, selection on pubescence was weak and inconsistent over space and time. Under some circumstances, individuals of A. barbata with pubescent leaves were more likely to produce seeds than were individuals with fewer hairs. The surveys, behavioral experiments with small insects, and estimates of lifetime reproduction all support the hypothesis that unidirectional leaf hairs on A. barbata, and perhaps other grasses, serve as an unstudied defense that direct small herbivores away from the meristems.

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.

The performance and population dynamics of insect herbivores depend on the nutritive and defensive traits of their host plants. The literature on plant-herbivore interactions focuses on plant trait mean values, but recent studies showing the importance of plant genetic diversity for herbivores suggest that plant trait variance may be equally important. The consequences of plant trait variance for herbivore performance, however, have been largely overlooked. Here we report an extensive assessment of the effects of within-population plant trait variance on herbivore performance using 457 performance datasets from 53 species of insect herbivores. We show that variance in plant nutritive traits substantially reduces mean herbivore performance via non-linear averaging of performance relationships that were overwhelmingly concave down. By contrast, relationships between herbivore performance and plant defence levels were typically linear, with variance in plant defence not affecting herbivore performance via non-linear averaging. Our results demonstrate that plants contribute to the suppression of herbivore populations through variable nutrient levels, not just by having low average quality as is typically thought. We propose that this phenomenon could play a key role in the suppression of herbivore populations in natural systems, and that increased nutrient heterogeneity within agricultural crops could contribute to the sustainable control of insect pests in agroecosystems.

© 2018 by the Ecological Society of America Climate change can have strong effects on species interactions and community structure. Temperature-dependent effects on predator–prey interactions are a major mechanism through which these effects occur. To understand the net effects of predator attack rates and dynamic windows of prey vulnerability, we examined the impacts of temperature on the interaction of a caterpillar (Arctia virginalis) and its ant predator (Formica lasioides). We conducted field experiments to examine attack rates on caterpillars relative to temperature, ant abundance, and body size, and laboratory experiments to determine the effects of temperature on caterpillar growth. We modeled temperature-dependent survival based on the integrated effects of temperature-dependent growth and temperature- and size-dependent predation. Attack rates on caterpillars increased with warming and ant recruitment, but decreased with caterpillar size. Caterpillar growth rates increased with temperature, narrowing the window of vulnerability. The model predicted that net caterpillar survival would decrease with temperature, suggesting that A. virginalis populations could be depressed with future climate warming. Theoretical work suggests that the net outcome of predator–prey interactions with increasing temperature depends on the respective responses of interacting species in terms of velocity across space, whereas the present study suggests the importance of effects of temperature on prey window of vulnerability, or “velocity” across time.

We examined genotypic variation in constitutive and induced resistance in grapes against Willamette spider mites, Eotetranychus willametti Ewing, and Pacific spider mites, Tetranychus pacificus McGregor, 2 common species of tetranychid mites found in California vineyards. We found evidence that early-season injury by Pacific mites induced resistance against subsequent Willamette mite populations but early-season injury by Willamette mites did not induce resistance against subsequent Willamette mite populations. Significant levels of induction were detected for several cultivars of the Old World species Vitis vinifera L. as well as the North American species V. calif arnica Bentham. Phylogenetic relationships among grape genotypes explained little of the variation we observed in induced resistance. Phylogenetic relatedness among grapes did help explain patterns of constitutive resistance for Pacific mites; cultivars of V. vinifera L. tended to be susceptible, whereas North American species were resistant. Wi11amette mites, however, performed well on some Old World cultivars and 2 North American species of Vitis that are native to California. We did not find any strong evidence of a negative correlation between constitutive resistance and strength of induction for these grape genotypes. Our results show that several factors contribute to variation in constitutive and induced resistance in grapes against these 2 species of spider mites, including grape genotype, previous history of mite injury (induction), the species of mite causing previous injury, and to some extent, phylogenetic relatedness among grapes. We also suspect that mite genotype has important influences.

Anti-herbivore defense shows high levels of both inter- and intraspecific variability. Defending against herbivores may be costly to the plant when it requires a tradeoff in allocation between defense and other missed opportunities, such as reproduction. Indeed, the plastic expression of defensive traits allows the plant to invest resources in defense only when the risk of being damaged actually increases, avoiding wasted resources. Plants may assess risk by responding to volatile cues emitted by neighbors that are under attack. Most plastic responses likely depend on environmental conditions. In this experiment, we investigated the effect of water availability on resistance induced by volatile cues in sagebrush. We found that plants receiving additional water over summer and/or volatile cues from neighbor donor plants showed reduced herbivore damage compared to control plants. Interestingly, we found no evidence of interactions between additional water and volatile cues. We performed an inferential analysis comparing historical records of the levels of herbivore damage during different years that had different temperature and precipitation accumulations. Results confirmed findings from the experiment, as the regression model indicated that sagebrush was better defended during wetter and hotter seasons. Reports from the literature indicated that sagebrush is extremely sensitive to water availability in the soil. We suggest that water availability may directly affect resistance of herbivory as well as sensitivity to cues of damage. Costs and benefits of allocating resources to defensive traits may vary with environmental conditions.

© 2017, Springer Science+Business Media Dordrecht. Models of climate change predict more variable precipitation for much of western North America, including more severe multi-year droughts. Droughts are known to increase mortality to trees although less is known about effects on shrubs from arid environments and about effects on reproduction. In this study, we followed a cohort of young sagebrush plants from 2010 to 2016, a period that included a severe drought from 2012 to 2015. Plants experienced little mortality preceding and during the drought. However, in the year following the drought, 14% of individuals died and 33% of branches on living plants died. There was little flowering in the years preceding the drought and flowering increased in each successive year from 2014 to 2016. Plants that produced more flowers in 2015 had more dead branches in 2016. Larger plants had fewer branches that died. Contrary to expectations, afternoon shade was not associated with greater survival or flowering, perhaps because shaded plants were in proximity to large trees which likely competed for water. Plants of the two common chemotypes had similar rates of survival and flowering. Experimental watering during the summer of 2015 did not affect survival and may have increased flowering in 2016. If multi-year droughts become more common in the future, even drought-adapted shrubs may be expected to suffer high rates of mortality.

The possibility of communication between plants was proposed nearly 20 years ago, although previous demonstrations have suffered from methodological problems and have not been widely accepted. Here we report the first rigorous, experimental evidence demonstrating that undamaged plants respond to cues released by neighbors to induce higher levels of resistance against herbivores in nature. Sagebrush plants that were clipped in the field released a pulse of an epimer of methyl jasmonate that has been shown to be a volatile signal capable of inducing resistance in wild tobacco. Wild tobacco plants with clipped sagebrush neighbors had increased levels of the putative defensive oxidative enzyme, polyphenol oxidase, relative to control tobacco plants with unclipped sagebrush neighbors. Tobacco plants near clipped sagebrush experienced greatly reduced levels of leaf damage by grasshoppers and cutworms during three field seasons compared to unclipped controls. This result was not caused by an altered light regime experienced by tobacco near clipped neighbors. Barriers to soil contact between tobacco and sagebrush did not reduce the difference in leaf damage although barriers that blocked air contact negated the effect.

© 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.

It is becoming increasingly accepted that plant–plant signals can affect plant defense against herbivores. In a few taxa, both the mechanisms of plant–plant signaling and the ecological consequences are well described. However, the current adaptive explanations for plant–plant signaling rely heavily on many ecological contingencies, such as the behavior of herbivores and genetic structure of plant neighborhoods, making it unclear how frequently plant–plant signaling is important across the plant kingdom. Moreover, these same ecological contingencies mean that it may be beneficial for a plant to release a publicly available cue to coordinate its (or its neighbor's) defenses following herbivory in some cases, but it may be disadvantageous in others. In this paper, we focus on recent studies that expand the taxonomic breadth of plant–plant signaling about herbivores that bring classic plant–plant signaling systems into a broader ecological context, and those that find either no consequence or counte...

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.

© 2016 by the Ecological Society of America. Plants respond to volatile cues emitted by damaged neighbors to increase their defenses against herbivores. We examined whether plants communicated more effectively with local neighbors than distant neighbors in a reciprocal experiment at two sites. Three branches on focal plants were incubated with air from (1) a control, (2) an experimentally clipped foreign plant from 230 km away, or (3) an experimentally clipped local plant from the same population as the focal plant. Branches incubated with air from the controls experienced 50-80%25 more leaf damage than those receiving air from experimentally clipped plants. Of more interest, branches receiving volatiles from experimentally clipped local plants received 50-65%25 of the leaf damage as those receiving volatiles from experimentally clipped foreign plants. Sabinyl compounds and related terpinenes were found to differ consistently for plants from southern and northern sites. These results indicate that cues vary geographically in their effectiveness and suggest that sagebrush responds more strongly to local than foreign dialects.

Plants that are damaged by herbivores emit complex blends of volatile compounds that often cause neighboring branches to induce resistance. Experimentally clipped sagebrush foliage emits volatiles that neighboring individuals recognize and respond to. These volatiles vary among individuals within a population. Two distinct types are most common with either thujone or camphor as the predominate compound, along with other less common types. Individuals respond more effectively to cues from the same type, suggesting that some of the informative message is contained in the compounds that differentiate the types. In this study, we characterized the chemical profiles of the two common types, and we examined differences in their microhabitats, morphologies, and incidence of attack by herbivores and pathogens. Analysis by gas chromatography coupled with mass spectrometry revealed that the camphor type had higher emissions of camphor, camphene, and tricyclene, while the thujone type emitted more α-thujone, β-thujone, (Z)-salvene, (E)-salvene, carvacrol, and various derivatives of sabinene. We were unable to detect any consistent morphological or microhabitat differences associated with the common types. However, plants of the thujone type had consistently higher rates of damage by chewing herbivores. One galling midge species was more common on thujone plants, while a second midge species was more likely to gall plants of the camphor type. The diversity of preferences of attackers may help to maintain the variation in volatile profiles. These chemical compounds that differentiate the types are likely to be informative cues and deserve further attention.

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.

Volatile communication allows plants to coordinate systemic induced resistance against herbivores. The mechanisms responsible and nature of the cues remain poorly understood. It is unknown how plants distinguish between reliable cues and misinformation. Previous experiments in which clipped sagebrush branches were bagged suggested that cues are emitted or remain active for up to 3 days. We conducted experiments using plastic bags to block emission of cues at various times following experimental clipping. We also collected headspace volatiles from clipped and unclipped branches for 1 h, transferred those volatiles to assay branches, and incubated the assays for either 1 or 6 h. We found that assay branches that received volatile cues for less than 1 h following clipping of neighbors failed to induce resistance. Assay branches that received volatile cues for more than 1 h experienced reduced herbivory throughout the season. Branches incubated for 6 h with volatiles that had been collected during the first hour following clipping showed induced resistance. These results indicate that sagebrush must receive cues for an extended time (>1 h) before responding; they suggest that the duration of cue reception is an important and overlooked process in communication allowing plants to avoid unreliable, ephemeral cues.

Plants can release chemicals into the environment that suppress the growth and establishment of other plants in their vicinity: a process known as 'allelopathy'. However, chemicals with allelopathic functions have other ecological roles, such as plant defense, nutrient chelation, and regulation of soil biota in ways that affect decomposition and soil fertility. These ecosystem-scale roles of allelopathic chemicals can augment, attenuate or modify their community-scale functions. In this review we explore allelopathy in the context of ecosystem properties, and through its role in exotic invasions consider how evolution might affect the intensity and importance of allelopathic interactions.

Legacy effects occur when particular species or their interactions with others have long-lasting impacts, and they are increasingly recognized as important determinants of ecological processes. However, when such legacy effects have been explicitly explored, they most often involve the long-term direct effects of species on systems, as opposed to the indirect effects. Here, we explore how a legacy of small mammal exclusion on the abundance of a shrub, bush lupine (Lupinus arboreus), influences the abundance of a native land snail (Helminthoglypta arrosa) in coastal prairie and dune habitats in central California. The factors that limit populations of land snails are very poorly known despite the threats to the persistence of this group of species. In grasslands, prior vole (Microtus californicus) exclusion created long-lasting gains in bush lupine abundance, mediated through the seedbank, and was associated with increased snail numbers (10×) compared to control plots where mammals were never excluded. Similar plots in dune habitat showed no difference in snail numbers due to previous mammal exclusion. We tested whether increased competition for food, increased predation, and/or lower desiccation explained the decline in snail numbers in plots with reduced lupine cover. Tethering experiments supported the hypothesis that voles can have long-lasting impacts as ecosystem engineers, reducing woody lupine habitat required for successful aestivation by snails. These results add to a growing list of studies that have found that non-trophic interactions can be limiting to invertebrate consumers.

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.

There are now approximately 10 documented examples of volatile plant communication that affect resistance to herbivores. For several of these cases, plants have been found to experience fitness ben...

Plants progress through a series of distinct stages during development and plant developmental phases link with changing seasons through a year. Recent work indicated that many plants activate syst...

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.