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Insects play a fundamental role in ecosystem stability by providing important ecological services such as pollination, biological pest control, and nutrition for other organisms. Recent human-caused declines in insect populations mandate that we better understand how to improve and sustain insect diversity and abundance. The goal of our study was to identify the insects associated with 6 prairie plant species. We spent 248 hours over 3 months during 1 year monitoring insect visits to 10 replicates of ∼1-m2 patches each of Verbena brasiliensis (Brazilian Vervain, Verbenaceae), Pycnanthemum muticum (Blunt Mountainmint, Lamiaceae), Monarda fistulosa (Wild Bergamot, Lamiaceae), Eryngium yuccifolium (Rattlesnake Master, Apiaceae), Silphium gracile (Slender Rosinweed, Asteraceae), and Rudbeckia texana (Texas Coneflower, Asteraceae). We identified 11 bee species and 100 non-bee species representing 9 orders and 57 families. Insect richness varied widely among plant species: More taxa (n = 75) visited Blunt Mountainmint than any other plant species. Wild Bergamot had the fewest visiting taxa (n = 23). The majority of insects visited multiple plants, although we observed 2 bee species and 44 non-bee species on only 1 plant species. The number of unique insect visitors of each plant species varied from 1 to 19 taxa. Total insect richness was significantly greater in the morning than afternoon (χ2 = 3.7, P = 0.054). That plant diversity begets insect diversity is widely accepted, and our work suggests that inclusion of plants that are strong insect attractors could be integral to insect conservation and restoration initiatives.

Herein we provide direct evidence for the consumption of Pomacea maculata (Giant Applesnail) eggs by ants in the genus Crematogaster. The observations were made during removal of snail egg masses at the Hudson Woods Unit of the Texas Mid-Coast National Wildlife Refuge, TX. We observed acrobat ants (Crematogaster sp.) removing snail eggs from an egg mass and carrying eggs back to their nest. While predation on Pomacea applesnail eggs has been reported elsewhere, to our knowledge this is the first time that it has been observed in North America.

The charismatic bee-mimic robber fly Mallophora fautrix (Bee Killer) is distributed throughout the southwestern US and Mexico. Recent sightings of M. fautrix in Louisiana represent the easternmost reports of the species. We photographed and identified M. fautrix in north Lafayette Parish, LA, on 5 August 2015. This 2015 discovery and a second sighting in Bossier City, LA, on 28 July 2019, extend the eastern range of this charismatic bee-mimic robber fly.

Worldwide, trees are confronting increased temperature and aridity, exacerbating susceptibility to herbivory. Long-term studies comparing patterns of plant performance through drought can help identify variation among and within populations in vulnerability to climate change and herbivory. We use long-term monitoring data to examine our overarching hypothesis that the negative impacts of poor soil and herbivore susceptibility would be compounded by severe drought. We studied pinyon pine, Pinus edulis, a widespread southwestern tree species that has suffered extensive climate-change related mortality. We analyzed data on mortality, growth, male reproduction, and herbivory collected for 14–32 years in three areas with distinct soil-types. We used standardized precipitation-evapotranspiration index (SPEI) as a climate proxy that summarizes the impacts of drought due to precipitation and temperature variation on semi-arid forests. Several key findings emerged: (1) Plant performance measurements did not support our hypothesis that trees growing in stressful, coarse-textured soils would suffer more than trees growing in finer-textured soils. Stem growth at the area with coarse, young cinder soils (area one) responded only weakly to drought, while stem growth on more developed soils with sedimentary (area two) and volcanic (area three) substrates, was strongly negatively affected by drought. Male reproduction declined less with drought at area one and more at areas two and three. Overall mortality was 30% on coarse cinder soils (area one) and averaged 55% on finer soil types (areas two and three). (2) Although moth herbivore susceptible trees were hypothesized to suffer more with drought than moth resistant trees, the opposite occurred. Annual stem growth was negatively affected by drought for moth resistant trees, but much less strongly for moth susceptible trees. (3) In contrast to our hypothesis, moths declined with drought. Overall, chronically water-stressed and herbivore-susceptible trees had smaller declines in performance relative to less-stressed trees during drought years. These long-term findings support the idea that stressed trees might be more resistant to drought since they may have adapted or acclimated to resist drought-related mortality.

Arizona's Sunset Crater began erupting in 1064 AD and for the next 200 years buried over 2,000 km2 in ash, cinders, and lava. Soil analyses indicate that pinyon pines (Pinus edulis) currently colonizing the cinder fields are faced with a highly stressful environment. Many of these pinyons suffer chronic, intense insect herbivory that reduces plant growth and eliminates female cone production. In contrast, herbivory among pinyons growing in neighboring sandy-loam soils is minimal. Furthermore, numerous trees within the heavily infested cinder field population suffer relatively low herbivory and maintain normal growth and reproduction. We used four polymorphic enzymes to examine the relationship between herbivore attack, environmental stress and genotypes of the adjacent cinder field, and sandy-loam soil pinyon populations. Our results demonstrate that 1) resistant trees display significant genetic differences and are more heterozygous for two enzymes associated with herbivory than susceptible trees; and 2) the cinder-soil pinyons exhibit significant genetic differences and are more heterozygous for an enzyme associated with environmental stress than the neighboring sandy-loam soil pinyons. We conclude that heterozygosity of specific or closely linked loci may facilitate pinyon resistance to herbivory and environmental stress, and that strong selection across narrow geographic boundaries resulted in rapid genetic differentiation of pinyon populations.

PREMISE OF THE STUDY Gravitropism typically is generated by dense particles that respond to gravity. Experimental stimulation by high-gradient magnetic fields provides a new approach to selectively manipulate the gravisensing system. METHODS The movement of corn, wheat, and potato starch grains in suspension was examined with videomicroscopy during parabolic flights that generated 20 to 25 s of weightlessness. During weightlessness, a magnetic gradient was generated by inserting a wedge into a uniform, external magnetic field that caused repulsion of starch grains. The resultant velocity of movement was compared with the velocity of sedimentation under 1 g conditions. RESULTS The high-gradient magnetic fields repelled the starch grains and generated a force of at least 0.6 g. Different wedge shapes significantly affected starch velocity and directionality of movement. CONCLUSIONS Magnetic gradients are able to move diamagnetic compounds under weightless or microgravity conditions and serve as directional stimulus during seed germination in low-gravity environments. Further work can determine whether gravity sensing is based on force or contact between amyloplasts and statocyte membrane system.

PREMISE OF THE STUDY Iris pseudacorus spread rapidly into North America after introduction from Europe in the 1800s and now co-occurs with native I. hexagona in freshwater Louisiana wetlands. Native irises support and interact with multiple trophic levels, whereas I. pseudacorus is classified an invasive pest because it grows aggressively, reduces biodiversity, and displaces native vegetation. Salinity levels are increasing in coastal wetlands worldwide. We examined how salt-stress affects competitive interactions between these conspecifics. METHODS We established a three-way full-factorial common-garden experiment that included species (I. pseudacorus, I. hexagona), competition (no competition, intraspecific competition, and interspecific competition), and salinity (0, 4, 8 parts per thousand NaCl), with six replicates per treatment. KEY RESULTS After 18 mo, Iris pseudacorus produced much more biomass than the native species did (F1, 92 = 71.5, P < 0.0001). Interspecific competition did not affect the introduced iris, but biomass of the native was strongly reduced (competition × species interaction: F2, 95 = 76.7, P = 0.002). Salinity significantly reduced biomass of both species (F2, 92 = 21.8, P < 0.0001), with no species × salinity interaction (F2, 84 = 1.85, P = 0.16). CONCLUSIONS Our results demonstrate that salt stress strongly reduced clonal reproduction in native and introduced irises; however, the introduced iris had a competitive advantage over the native, regardless of environmental salinity levels. Based on patterns in clonal reproduction, the introduced iris could potentially threaten native iris populations. We are currently investigating seed production and mortality during competition and stress because both clonal and sexual reproduction must be considered when predicting long-term population dynamics.

Iris series Hexagonae is a small, monophyletic complex of five species and associated hybrid populations, popularly known as the “Louisiana irises.” The Hexagonae alliance of Iris has been recognized as a textbook case of introgressive hybridization on the basis of numerous studies in Louisiana. In light of field observations of phenotypic heterogeneity among Florida populations, we investigated genetic variation within and among 37 populations of Iris series Hexagonae from Florida, seven from Louisiana, and one from Texas with 19 microsatellite loci. We wished to test the hypothesis that the Lake Wales Ridge and Polk Uplands of Florida, which remained above sea level maxima during the early Pleistocene, might be the sites of origin for all extant iris populations in Florida and possibly the series Hexagonae in its entirety. We also hypothesized that phenotypic heterogeneity observed in Florida iris would be reflected in patterns of allelic diversity. Finally, we wished to seek evidence of Native American...

It is more important to improve the salt tolerance of crops in a salinized world with the situations of increasing populations, declining crop yields, and a decrease in agricultural lands. Attempts to produce salt-tolerant crops have involved the manipulation of existing crops through conventional breeding, genetic engineering and marker-assisted selection (MAS). However, these have, so far, not produced lines growing on highly saline water. Hence, the domestication of wild halophytes as crops appears to be a feasible way to develop agriculture in highly saline environments. In this review, at first, the assessment criteria of salt tolerance for halophytes are discussed. The traditional criteria for the classification of salinity in crops are less applicable to strong halophytes with cubic growth curves at higher salinities. Thus, realistic assessment criteria for halophytes should be evaluated at low and high salinity levels. Moreover, absolute growth rather than relative growth in fields during a crop's life cycle should be considered. Secondly, the use of metabolomics to understand the mechanisms by which halophytes respond to salt tolerance is highlighted as is the potential for metabolomics-assisted breeding of this group of plants. Metabolomics provides a better understanding of the changes in cellular metabolism induced by salt stress. Identification of metabolic quantitative trait loci (QTL) associated with salt tolerance might provide a new method to aid the selection of halophyte improvement. Thirdly, the identification of germplasm-regression-combined (GRC) marker-trait association and its potential to identifying markers associated with salt tolerance is outlined. Results of MAS/linkage map-QTL have been modest because of the absence of QTLs with tight linkage, the non-availability of mapping populations and the substantial time needed to develop such populations. To overcome these limitations, identification by GRC-based marker-trait association has been successfully applied to many plant traits, including salt tolerance. Finally, we provide a prospect on the challenges and opportunities for halophyte improvement, especially in the integration of metabolomics- and GRC-marker-assisted selection towards new or unstudied halophyte breeding, for which no other genetic information, such as linkage maps and QTL, are available.

 Premise of the study : Saltwater intrusion is one of the most widespread environmental threats to freshwater wetlands. Iris species worldwide are important members of these plant communities. Wetland irises reproduce clonally and sexually, which permits populations to spread and disperse in benign and stressful conditions. The ability of iris populations to tolerate and adapt to elevated salinity can play an important role in determining the long-term health of wetland ecosystems.  Methods : We used microsatellite markers to evaluate population structure and genetic diversity, and we performed a common garden experiment to examine the effect of salinity on the growth and reproduction of wild Iris hexagona collected from freshwater and brackish wetlands.  Key results : Colonies were genetically distinct, with average to high heterozygosity (0.55 – 0.66) for a clonal species. Salinity had negative linear effects on leaf mass (g), clonal growth (g), root mass (g), and fl ower numbers, and it had nonlinear effects on seed numbers and seed mass (mg). The greatest sexual reproduction occurred in the intermediate-salinity (4 parts per thousand) treatment. Flowering phenology was delayed for 5 days in the highest-salinity treatment.  Conclusions : We hypothesized that irises from brackish habitats would tolerate salinity better than freshwater irises would, but no difference in iris performance existed between the two habitats. The observed salinity tolerance and genetic diversity of I. hexagona indicate that populations will persist despite moderate increases in environmental salinity.

Sea buckthorn (Hippophae L.) is an economically important, globally distributed shrub with unique biochemical properties. This long-juvenile and long-lived woody plant can be more difficult to cultivate than other crop plants. Dried-shrink disease is a dangerous pathogen that destroys sea buckthorn and halts commercial production. We estimated variability of ISSR molecular markers in 52 accessions of 16 sea buckthorn varieties and lines to seek markers associated with dried-shrink resistance and help identify potential breeding varieties and lines. Fifteen ISSR primers generated 346 bands, with an average of 23 bands per primer. Genetic similarities (Jaccard coefficient) between pairs of accessions ranged from 0.216 to 0.781. At a Jaccard coefficient of 0.39, the dendrogram generated with 342 polymorphic bands clustered 51 accessions of H. rhamnoides into five groups and one H. salicifolia isolate. Four ISSR markers (887190, 835700, 809290, and 811280) were significantly correlated with resistance to dried-shrink disease (P < 0.001). These markers provide a potential method for breeding programs that select lineages resistant to dried-shrink disease, especially when no other genetic information such as linkage maps and Quantitative Trait Loci are available.

Delayed selfing is a reproductive strategy that can increase mating opportunities and provide reproductive assurance in the absence of pollinators. When external pollination fails to occur in the herbaceous perennial, Kosteletzkya virginica, the styles curve downward to bring anthers into contact with the stigmal lobes, resulting in delayed selfing. Our goal was to evaluate experimentally the effects of weather conditions on pollinator activity, style curvature, and plant fitness. We discovered that adverse weather conditions significantly reduced pollinator visitation rates, and increased the incidence of style curvature, which resulted in delayed selfing. Furthermore, elimination of delayed selfing by experimental manipulation reduced fruit and seed set compared to non-emasculated flowers, even under adverse weather conditions. Offspring produced through selfing were similar to outcrossed progeny in the majority of fitness traits measured, and inbreeding depression was limited. Our study demonstrates that context-dependent style curvature promotes delayed selfing and provides reproductive assurance in the absence of pollinators, with minimal loss of progeny performance through inbreeding depression.

Mixed mating systems often evolve despite strong inbreeding depression, but there have been few experimental tests of this observation. Kosteletzkya virginica, a perennial dicot halophytic species of the Family Malvaceae, is native to brackish portions of coastal tidal marsh in the United States of America and was introduced into China in 1993. Pollination of K. virginica occurs through insect vectors and by delayed self-pollination via stylar movement. In the same two naturalized populations of K. virginica during 2005–2007, we measured (i) inbreeding depression across a range of life-cycle stages, (ii) pollination failure rate, and (iii) selfing rate estimated by AFLP markers. Results indicate that inbreeding depression values for the two naturalized populations during 2005–2007 ranged from 0.620 ± 0.036 to 0.656 ± 0.032, and mean inbreeding depression was 0.640 ± 0.006. Pollinator failure rates at the two naturalized K. virginica populations during 2005–2007 ranged from 0.091 ± 0.039 to 0.174 ± 0.061, and selfing rate ranged from 0.247 ± 0.018 to 0.468 ± 0.031. Population selfing rates are significantly and positively correlated with field estimates of pollinator failure (P = 0.008

We characterize the salinity stress response of Iris hexagona, a freshwater species, by measuring three putative osmolytes, betaine, proline, and dimethylsulphonio-propionate (DMSP) in plants after short (3 days) and long-term (4–5 months) exposure to NaCl. HPLC analyses show that untreated control leaves contained 2.1, 0.2 and 3.2 mg g−1 DW of betaine, proline, and DMSP, respectively, and establish the presence of these compounds in the Iridaceae. Within 4 days of salinity stress (200 mM NaCl) betaine, proline and DMSP increased significantly. Among vegetative tissues, the highest level of proline occurred in roots; betaine and DMSP were highest in leaves. Analyses of generative tissue (flowers and flower stalks) after long-term exposure showed the highest levels (>10 mg g−1 DW) of all examined compounds in petals but only proline and betaine increased with salinity. All three substances showed a basipetal gradient in flower stalks. Although the examined compounds responded to salinity, the osmometry of the sap indicated that they comprise less than 10% of the osmotically active solutes. The temporal and spatial changes in the distribution of the analyzed compounds indicate complex responses to salinity.

Salinity is an important cause of abiotic stress in wetland communities yet little is known about its consequences for freshwater plants and their insect herbivores. The goal of this study was to test the effect of salinity on a leafmining insect, Cerodontha iridiphora , and its herbaceous host plant, Iris hexagona . 2. Leafminer performance was evaluated on irises grown in control and saline treatments, and the effects of salinity and herbivory on leaf quality and mortality was measured. 3. Leafminer density and size were significantly lower on irises grown in saline water compared with freshwater. 4. Both salinity and herbivory accelerated leaf senescence and mortality, and their combined effects increased tissue loss by an order of magnitude compared with controls. 5. Leafminers acted as nutrient sinks. The undamaged regions of mined leaves contained 40% less nitrogen than unmined leaves, providing a mechanism for the premature leaf mortality. 6. Salinity was detrimental to the performance and survival of both the iris leafminer and its host plant. We propose that glycophytic host plants and their insect herbivores will suffer more than halophytic communities from environmental salinity because they lack the adaptive mechanisms to tolerate this potent physiological stress.

Iris hexagona is a perennial freshwater species indigenous to Louisiana wetlands. Coastal iris populations that are exposed to saline conditions often suffer heavy floral browsing. This paper is the first to photographically document florivory by white-tailed deer (Odocoileus virginianus Zimmerman), which consume iris flowers and immature seed capsules. Severe florivory can prevent sexual reproduction in plants and eliminate habitat for floral arthropods. In this three-year study, we monitored environmental salinity, assessed natural levels of florivory, and established deer exclosures to test the effects of florivory on arthropod abundance and diversity. In two out of three years, florivory rates were positively correlated with the level of salinity at individual I. hexagona populations. In all three years, florivory in the brackish marsh was an order of magnitude greater than florivory in intermediate and freshwater sites. In 2003, the number and diversity of arthropods was significantly greate...

We used field, common garden, and laboratory studies to investigate the influence of environmental stress on a native wetland species, Iris hexagona (Iridaceae), and an iris leafminer, Cerodontha iridiphora (Diptera: Agromyzidae). There were strong effects of salinity, herbivory, and phenology on jasmonic acid (JA) and salicylic acid (SA), two important signal phytohormones. Within 48 h, foliar JA increased an order of magnitude, and SA declined 50% in plants grown under salinity stress compared to freshwater controls. Leafminer performance declined on plants exposed to the salinity treatment. Both herbivory and salinity stress induced JA and suppressed SA, indicating that plants can respond to abiotic and biotic stress through shared hormonal pathways. Surprisingly, leafminer pupae contained JA and SA, and we discuss the implications of this novel discovery. Unrelated to salinity, there was a sharp decline in foliar JA between April and September. Despite its strong impact on hormonal signaling in plants, the abiotic environment is often ignored in ecological studies of induced plant defenses. Importantly, natural seasonal cycles and/ or environmental stress can induce stronger changes in phytohormones than herbivory, and should be incorporated into the design of ecological experiments whenever possible, for a balanced understanding of phytohormonal ecology.

Summary 1 Salinization is a growing environmental stress in wetland ecosystems world-wide. Several models have been proposed that predict clonal plant responses to stress, including that environmental stress stimulates sexual reproduction. 2 We conducted a common-garden experiment to investigate the effects of salinity on 10 natural populations of Iris hexagona, a clonal perennial endemic to freshwater and brackish wetlands of the North American Gulf Coast. 3 Salinity reduced vegetative growth but either increased or had neutral effects on sexual reproduction, consistent with the clonal stress hypothesis. Salinity of 4 µg g−1 more than doubled the number of seeds produced compared with freshwater controls, but flower number and seed mass were unaffected. 4 Salinity reduced total below-ground mass by nearly 50% compared with controls, with no significant change in rhizome numbers. 5 Plants from 10 randomly selected I. hexagona populations differed dramatically in growth and reproduction, independent of salinity. Total biomass that accumulated over the 20-month experiment ranged across all treatments from 52 to 892 g, and flower numbers varied from 2.3 to 11.3 per replicate. 6 Populations did not respond differently to salinity, except with respect to above- : below-ground ratios, thus providing no conclusive evidence for local adaptation to salinity stress. 7 Our results concur with published models of plant reproductive strategies in variable environments, in that environmental stress stimulated sexual reproduction at the expense of growth. However, these models do not predict the observed sharp decline in seed production at near lethal salinity levels.

Adults and larvae of the loberine erotylid beetle Loberus impressus (LeConte) were found associated with fungi growing on corolla and seed capsule tissue of the blue flag iris, Iris hexagona. We examined adult beetle specimens using light and scanning electron microscopy to determine if specialized structures (mycangia) may function in transporting fungi. Two pairs of deep pits on the ventral aspect of the gena between the eyes and the maxillae of both sexes contained fungal spores and hyphae, suggesting a possible role as mycangia in addition to their role as glandular outlets. Inoculation from the surface of cleaned beetle specimens produced colonies of Cladosporium and Fusarium. These genera are widespread, usually air-dispersed conidial fungi that sometimes are associated with insects.

Canopy disturbance is a major factor affecting forest structure and composition and, as a result of habitat alterations, can influence insect communities. We initiated a field study to quantify the effects of canopy disturbance on aerial insect abundance and distribution within a bottomland hardwood forest along the Cache River, Arkansas, USA. We used passive flight-intercept traps to sample insects in canopy gap and forest interior habitats from May to July in 1996, 1997, and 1998. The hydrologic conditions of our study site varied among years: 1996 was relatively dry, 1997 incurred a long-duration flood, and 1998 was moderately wet. Of the 34,000+ Homopterans collected, many groups were distributed in a non-uniform manner among years and between habitats. Total Homopterans, two families of Homopterans, and six morphospecies were more abundant in canopy gaps than interior forest. Many Homopteran taxa were least abundant in 1997 following almost six months of flooding. Alternatively, relatively large Homopteran abundances were associated with the dry conditions of 1996 and the moderately wet conditions of 1998. Differences in Homopteran abundance among years and habitats may be related to differences in vegetation density. Canopy gaps supported more vegetation cover than the interior forest in all but the first sampling interval. In addition, similar to Homopteran abundance, vegetation density was lower in 1997 than in 1998. These results demonstrate that natural disturbance and flooding contribute to Homopteran abundance and distribution patterns in bottomland hardwood forests of the south central United States.

Phytohormones play critical roles in regulating plant responses to stress. We investigated the effects of salinity on abscisic acid (ABA), indole-3-acetic acid (IAA), salicylic acid (SA), and jasmonic acid (JA) in leaves, stalks, fruits, and seeds of Iris hexagona, a native wetland species. Using gas chromatography-mass spectroscopy with selected ion monitoring, our experiments demonstrated significant and different short- and long-term changes in iris phytohormones. ABA and JA generally increased and IAA and SA declined in response to salinity. We conclude that these phytohormones may have separate and interactive effects on how plants respond and adapt to stress in natural environments.

Stilbosis quadricustatella leafminers are microlepidopteran specialists of sand-live oak (Quercus geminata). These tiny moths produce one generation per year and have a parasitic life-cycle and long larval stage that develops entirely within a single oak leaf. Differences in host-plant age, phenotype, and phenology generate a coarse-grained, spatially heterogeneous environment for the leafminer population. Previous reciprocal trans- fers of leafminer eggs among mature oaks revealed that S. quadricustatella are locally adapted to individual oak trees. In this paper we use genetic markers and an extinction- recolonization experiment to explore further variation in leafminer population structure. Allozyme loci indicate significant interdemic genetic structure among recent colonists of new host trees, which weakens in the 10th generation and disappears by the 40th generation. In contrast, adaptive demic structure is evident by the 10th generation and is strong in the 40th generation, despite the potential for substantial intertree dispersal. We propose that host heterogeneity combined with leafminer fidelity to natal trees promotes divergent se- lection and rapid demic evolution on individual oaks, despite potentially high gene flow between the leafminers inhabiting them.

The larva of Loberus impressus LeConte is described based on adult-associated and reared specimens collected from corollas and seed pods of Iris hexagona on the gulf coast of Louisiana. The larva of L. impressus is similar to larvae of Zavaljus brunneus (Gyllenhal) Hapalips prolixus Sharp, Pharaxonotha spp., and Bolerus angulosus (Arrow), the other described larvae within the Xenoscelinae. Larval characters typical for the Loberini that are found in L. impressus include arrangement of integumental granules into rows, frayed and aciculate setae borne on spiny tubercles, and urogomphi spiny or tuberculate and recurved. The tarsungular setae of L. impressus and B. angulosus are single, in contrast to the dual setae present in other described larvae of Languriidae. Larva and adults of L. impressus were common in dried corollas and seed pods of I. hexagona that had begun to split open or had been invaded by Crematogaster ants. Fusarium and Cladosporium fungi were potential sources of food for L. impressus and other fungivorous beetles in the iris seed pods and on corollas.

The adaptive deme formation (ADF) hypothesis pre- likely to become locally adapted than are dispersive in- dicts that herbivorous insects become locally adapted to their host sects; and haplodiploid insects are more likely to become plants over time. Since its inception, approximately 17 indepen- dent studies have tested ADF, and they are divided in support and locally adapted than are diplodiploid species. rejection of the hypothesis. This field of insect evolutionary ecol- Since its inception, the predictions of the ADF hypoth- ogy has a contentious history, and the contradictory studies ob- esis have been examined in approximately 17 indepen- scure our understanding of the general evolutionary importance of dent studies. Recent reviews of these studies (Mopper adaptive deme formation in phytophagous insects. We conducted 1996; Boecklen and Mopper 1998; Strauss and Karban a meta-analysis in an attempt to clarify this issue. Meta-analysis is 1998) conclude that they are about evenly divided in a statistical method for quantitatively comparing and synthesizing support and rejection of the ADF hypothesis. Addition- the results of different studies in a way that is more objective than a traditional literature review. Our analysis indicates that local ad- ally, Alstad (1998) recently retracted the experimental re- aptation is an important phenomenon in diverse insect systems. sults and deme formation hypothesis proposed in the Contrary to predictions of the original hypothesis, there was no original study by Edmunds and Alstad (1978). This field evidence that insect dispersal ability, and ostensibly gene flow, was of insect evolutionary ecology has been somewhat con- associated with local adaptive differentiation. There was some in- tentious, and combined with the inconsistent results of dication that breeding (parthenogenetic, haplodiploid, diplodi- the experimental field studies, a clear picture of the bio- ploid) and feeding (exophagous, endophagous) modes may influ- logical role of adaptive deme formation in the evolution ence the evolution of locally adapted demes. Our analysis supports the theory of adaptive deme formation and provides guidance for of phytophagous insect populations has been impossible future research directions. to obtain using traditional literature reviews. Meta-analysis offers a solution to this dilemma be

Using 6 years of observational and experimental data, we examined the hypothesis that water and nutrient stress increase the susceptibility of pinyon pine (Pinus edulis) to the stem- and cone-boring moth (Dioryctria albovittella). At two geographic levels, a local scale of 550 km2 and a regional scale of 10,000 km2, moth herbivory was strongly correlated with an edaphic stress gradient. At a local scale, from the cinder soils of Sunset Crater to nearby sandy-loam soils, nine of ten soil macro- and micronutrients, and soil water content were lowest in cinder-dominated soils. Herbivore damage was six times greater on trees growing in the most water and nutrient deficient site at Sunset Crater compared to sites with well-developed soils. Percentage silt-clay content of soil, which was highly positively correlated with soil nutrient and soil moisture at a local scale, accounted for 56% of the variation in herbivory at a regional scale among 22 sites. Within and across sites, increased stem resin flow was positively associated with reduced moth attack. On the basis of moth distribution across a stress gradient, we predicted that pinyons growing in highly stressful environments would show increased resistance to herbivores if supplemented with water and/or nutrients. We conducted a 6-year experiment at a high-stress site where individual trees received water only, fertilizer only, and water + fertilizer. Relative to control trees, stem growth and resin flow increased in all three treatments, but only significantly in the water + fertilizer treatment. Although there was no significant difference in herbivore damage among these three treatments, there was an overall reduction in herbivore damage on all treatment trees combined, compared to control trees. This experiment suggests that release from stress leads to increased resistance to insect attack and is consistent with our observational data. While other studies have predicted that short-term stress will result in herbivore outbreaks, our studies extend this prediction to chronically stressed host populations. Finally, while flush-feeders are not predicted to respond positively to stressed host plants, we found a positive association between herbivore attack and stressed pinyon populations.

Individual trees in a North Florida population of Quercus geminata, (sand live oak) vary widely in rates of herbivory by a leaf-mining moth (Stilbosis quadricusta- tella). Some oaks are always heavily infested while neighboring trees remain lightly at- tacked. We conducted field experiments in 1991 and 1992 to determine if leafminer per- formance on heavily and lightly attacked trees contributed to this pattern. We also compared the leaf production phenology, leaf area, and stem length of heavily and lightly attacked trees to determine if tree phenotypic traits influenced herbivory. Our results indicate that: (1) Lightly attacked trees produced leaves earlier in the spring than heavily attacked trees, and the difference among trees in leaf production phenology explained 61% of the variation in leafminer densities (P = 0.008). (2) Lightly attacked trees produced significantly smaller leaves than heavily attacked trees, but leaf size explained only 13% of the variation in herbivory (P = 0.048). (3) There were no differences in larval performance on heavily and lightly attacked trees in either 1991 or 1992, with the exception of egg mortality, which was significantly higher on lightly attacked trees in 1991 (P = 0.04). (4) Host plants caused greater leafminer mortality than natural enemies, but neither differed significantly between heavily and lightly attacked trees. Our transfer experiments and observations of wild leafminers revealed little difference in leafminer performance on heavily and lightly attacked Quercus geminata trees. However, tree phenotypic traits differed, especially the timing of spring foliation, which occurred significantly earlier among trees with low rates of herbivory. These results suggest that leafminer ovipositional preference, rather than performance, could be an important deter- minant of differential herbivory. There may be an unknown selective advantage to pref- erential oviposition on trees with delayed spring leaf production.

Saltwater intrusion into wetland ecosystems has destroyed or damaged many native plant populations. Iris hexagona is a salt-sensitive species that exhibits intraspecific variation in salinity tolerance. To investigate the effect of salinity on flowering, we exposed I. hexagona collected from natural populations to salt treatments in a common garden. Experimental salinity additions strongly delayed flowering phenology, but the effect was not apparent until the second year, when less than 4 g/L NaCl delayed flowering up to 3 d. In the field, soil salinity and flowering phenology varied substantially within I. hexagona populations. Iris flowers are receptive to pollinators for 2 d or less, therefore a 3-d delay could affect outcrossing dynamics, and ultimately, the evolutionary ecology of iris populations. Salinity also had a carryover effect; prior salinity exposure delayed flowering in irises that had been replanted in freshwater conditions for 6 mo. This is an important result because it suggests that episodic stress (such as tropical storms) can influence performance well after the stress has disappeared. Our research further underscores the importance of long-term studies because a 1-yr experiment would have failed to reveal the strong effects of salinity that emerged in the second year.

Genetic variation in insect populations is frequently structured into discrete groups, or demes, that form in response to stochastic forces or natural selection. Because host-plant populations are often highly heterogeneous, phytophagous insects may form demes that are adapted to the unique traits of individual plants. Recent field experiments indicate that selection pressures imposed by host-plants can promote rapid adaptive evolution in natural insect populations at very fine spatial scales. Adaptive deme formation may be more common among endophagous insects, which feed and reside within plant tissue, than for externally feeding insects, because internal feeders experience stronger plant-mediated selection pressures.

Plant stress theory predicts and many observational studies support the notion that abiotic stress can increase plant quality and benefit the performance of phytophagous insects. Paradoxically, numerous experiments contradict this assumption. To test the theory, we conducted experiments to determine the influence of pinyon pine (Pinus edulis) stress on pinyon sawfly (Neodiprion edulicolis) performance. In a natural pinyon population, we transferred first—instar sawfly larvae to 41 trees divided into four groups of 11; each group received either water, fertilizer, water and fertilizer, or no supplements. Upon completion of their penultimate larval instar, we collected sawflies and determined their fecundity and sex ratios. Although sex ratios are rarely used to estimate insect performance in natural systems, we believe that their sensitivity to changes in plant quality may play an overlooked role in insect—plant interactions. Most of our results refuted theoretical predictions, because sawflies performed better on trees receiving both water and fertilizer. However, in some cases, sawflies did benefit from pinyon stress. We contend that "simultaneous plant stress," such as low precipitation while the insect is feeding or ovipositing, can reduce performance. In contrast, "sustained plant stress," such as chronically poor soil conditions or prolonged drought, can benefit insect performance. These different effects on herbivores may parsimoniously explain some discrepancies between theoretical and empirical plant stress studies.

The association between host-plant mineral nutrients and insect herbivory is complex, idiosyncratic, and dynamic. Because of this we employed an unusual statistical method to evaluate the relationship between tenthredinid sawfly herbivory and concentrations of foliar nutrients in arroyo willow,Salix lasiolepis. We discovered that among 20 willow clones and between two years arroyo willow varied in the size (absolute concentrations) and shape (relative concentrations) of five mineral nutrients (Ca, Mg, N, P, and K). Similarly, the densities of five sawfly species varied in size and shape among clones and between years. In 1984, relative densities, or shape, of the sawfly community was associated with relative concentrations, or shape, of foliar nutrients in 20 willow clones. In contrast, in 1985 absolute densities, or size, was associated with absolute concentrations, or size, of foliar nutrients. This shift from a shape to a size association between herbivory and foliar mineral nutrients may reflect the large difference in winter precipitation and sawfly densities between years: precipitation and sawfly densities were much greater in 1985 than in 1984.

Wetland habitats are besieged by biotic and abiotic disturbances such as invasive species, hurricanes, habitat fragmentation, and salinization. Predicting how these factors will alter local population dynamics and community structure is a monumental challenge. By examining ecologically similar congeners, such as Iris hexagona and I. pseudacorus (which reproduce clonally and sexually and tolerate a wide range of environmental conditions), one can identify life-history traits that are most influential to population growth and viability. We combined empirical data and stage-structured matrix models to investigate the demographic responses of native (I. hexagona) and invasive (I. pseudacorus) plant populations to hurricanes and salinity stress in freshwater and brackish wetlands. In our models I. hexagona and I. pseudacorus responded differently to salinity stress, and species coexistence was rare. In 82% of computer simulations of freshwater marsh, invasive iris populations excluded the native species within 50 years, whereas native populations excluded the invasive species in 99% of the simulations in brackish marsh. The occurrence of hurricanes allowed the species to coexist, and species persistence was determined by the length of time it took the ecosystem to recover. Rapid recovery (2 years) favored the invasive species, whereas gradual recovery (30 years) favored the native species. Little is known about the effects of hurricanes on competitive interactions between native and invasive plant species in marsh ecosystems. Our models contribute new insight into the relationship between environmental disturbance and invasion and demonstrate how influential abiotic factors such as climate change will be in determining interspecific interactions.

Host plants not only influence herbivore performance, they can also mediate interactions between herbivores. We conducted a 5—yr field study to test the effects of pinyon pine (Pinus edulis) phenotype and competitive interactions on the colonization success, mortality, fecundity, and sex ratios of a foliage—feeding sawfly, Neodiprion edulicolis. Our experiments revealed four major patterns. (1) Sawfly survival was significantly higher on susceptible trees than on resistant trees. (2) In contrast, sawfly fecundity was significantly lower on susceptible trees than on resistant trees. (3) Interactions with a stem moth, Dioryctria albovitella, caused significantly reduced sawfly fecundity on susceptible trees. (4) Sawfly mortality, rather than sawfly fecundity, was the dominant factor influencing population growth rates: after four generations, sawflies transferred to resistant trees. A model developed with performance information from our field experiments accurately described patterns of population increase in the field: sawflies rapidly attain high densities on susceptible tress, and accumulate slowly on resistant trees. Trees displaying a susceptible phenotype are therefore better hosts than resistant phenotypes, despite the negative competitive interactions that occur between sawflies and moths on heavily infested susceptible trees.

We document through correlative and experimental analyses sex—biased herbivory in the arroyo willow, Salix lasiolepis. Male willows typically support higher densities of five species of tenthredinid sawflies than do female willows. Significant intersexual variation sawfly densities exists in naturally occurring willow clones and in clones grown in a common garden. Arroyo willow is sexually dimorphic in leafing phenology and in aspects of leaf chemistry. Male willows have significantly longer vegetative shoots than do females at the time of sawfly oviposition (P = .01). Males also have significantly higher foliar concentrations of nitrogen, phosphorus, and potassium than do females. Female willows have a marginally significant (P = .093) higher concentration of total phenols. Of the dimorphic traits, only variation in leaf phenology is a consistent predictor of variation in sawfly densities among willow clones. See full-text article at JSTOR

Evolutionary ecologists are interested in microgeographic genetic structure in herbivore populations and its importance to herbivore population dynamics (Slatkin 1987, Mopper 1996a, Gandon et al., Chapter 13, this volume, McCauley and Goff, Chapter 9, this volume, Peterson and Denno, Chapter 12, this volume). Two major patterns have emerged from the pioneering work by Bush (1969) and Edmunds and Alstad (1978): Polyphagous herbivores exhibit distinct “races” adapted to sympatrically distributed host species (Bush 1969; Pashley 1988; Aki-moto 1990; Feder et al. 1990, Chapter 16, this volume; Via 1991; Carroll and Boyd 1992), and specialist herbivores display genetic differentiation—demes—at the spatial scale of individual conspecific host plants (McCauley et al. 1988; McPheron et al. 1988; Alstad and Corbin 1990; Komatsu and Akimoto 1995). The fine-scale partitioning of herbivore populations into demes has been implicated as an important factor promoting discontinuous distributions of herbivores within natural populations of host plants (Edmunds and Alstad 1978; Wainhouse and Howell 1983; Hanks and Denno 1994; Mopper 1996a), and has been cited as an important variable in understanding herbivore outbreaks (Wainhouse and Howell 1983).

There is strong evidence for local genetic variation and demic structure in phytophagous insect populations (McCauley and Eanes 1987; McPheron et al. 1988; Gittman et al. 1989; Rank 1992; and see Chapter 9 in this volume by McCauley and Goff; Chapter 12 by Peterson and Denno; and Chapter 14 by Thomas and Singer). Research in agricultural systems indicates that a major cause of this structure is genetic variation in resistance among individual host plants in a population (Denno and McClure 1983). Genetic variation within natural plant populations is also well established (Hiebert and Hamrick 1983; Plessas and Strauss 1986; Mop-per et al. 1991; Berg and Hamrick 1995; Strauss and Karban 1994a, 1994b; Berenbaum and Zangerl, Chapter 5 this volume), but perhaps because of inherently greater spatial and temporal complexity, its relation to insect population structure is not as well understood as in managed systems (Michalakis et al. 1993).

This chapter discusses population dynamics of leaf-mining insects. Leaf mining is a means by which some insects consume foliage while simultaneously dwelling inside it. Worldwide approximately 10,000 species of insects are leaf miners, but density estimates and demographic data are available for only 1% of these species. Most leaf-mining species apparently suffer little egg mortality from natural enemies. Virtually every study examining mortality of leaf miners, whether at latent or eruptive densities, has identified parasitism of larvae, and sometimes pupae, as an important source of mortality. Interactions with conspecifics and other phytophagous insect species have generally been viewed as having little effect on the population dynamics, largely because the characteristically low densities of most phytophages were presumed to result in little competition for limiting resources. Tests of the regulatory role of dominant mortality sources, usually with regression-based tests for spatial and or temporal density dependence, have been equivocal. One additional caveat must be added to any discussion of population dynamics of leaf-mining insects. An extensive 3-year study of egg, larval, and pupal mortality revealed that most mortality occurred during the larval stage. In 1984, when population densities were 3.59 mines/leaf, intraspecific competition accounted for substantial mortality, and was spatially density dependent. Stilbosis quadricustatella is a univoltine leaf miner commonly found on Quercus geminata in northern Florida. No studies have simultaneously examined latent and eruptive populations of the same species over several years, so it is presently impossible to determine if these species have multiple stable states with low- and high-density populations regulated around different equilibrium levels.

Because of the modular structure and enormous phenotypic plasticity of plants, herbivores influence plant architecture in diverse ways that range from superficial modifications to transformations of the entire plant form. Potential consequences for the plant of these structural changes are numerous, and may result in repercussions at the organismal, population, or community levels. The importance of plant structure in determining the species richness of insect herbivore communities is well documented (Lawton and Schroder, 1977, 1978; Strong and Levin, 1979; Moran, 1980; Southwood et al., 1982; Lawton, 1983). In general, these studies found that larger, more structurally complex plants, support a greater number of herbivore species because they provide a greater diversity of habitats.

The deme-formation hypothesis states that selection can produce adaptive genetic variation within and among phytophagous insect populations. We conducted three field experiments and tested this prediction by transferring eggs and measuring performance of a mobile leafmining insect, Stilbosis quadricustatella. In Experiment 1, we com- pared the rate of mine initiation of leafminers transferred to natal and novel sites. In Experiment 2, we compared mine-initiation rate of leafminers transferred to natal and novel host-plant species. In Experiment 3, we compared the mine-initiation rate, mine-completion rate, and sources of mortality of miners transferred to neighboring natal and novel Quercus geminata trees. In the first, second, and third experiments, leafminer larvae initiated significantly more mines at the natal site, on the natal plant species, and on the natal Q. geminata tree, evidence for adaptive differentiation. Furthermore, plant-mediated mortality was significantly lower among miners transferred to natal Q. geminata trees. This result supports a key assumption of the deme-formation hypothesis: insects adapt to the defensive phenotypes of individual trees. However, natural-enemy mortality was significantly higher among miners transferred to natal trees, essentially reversing the plant effect. Therefore, rates of successful mine completion were similar on natal (19%) and novel (17%) trees. This experiment suggests that host plants and natural enemies may represent opposing forces of selection. Leafminers adapted to individual trees may realize a selective advantage only when natural-enemy densities are low.

1. We compared high and low density populations of a leaf miner (Stilbosis quadricustatella (Cham.)) on two host oaks to ascertain mechanisms influencing abundance. High density miner populations occurred on sand live oak, Quercus geminata (Fagaceae), found in homogeneous stands at inland and coastal sites. Quercus nigra, water oak, a patchily distributed inland species, supported a low density leaf miner population. 2. Average foliar nitrogen of Q.geminata was significantly lower than that of Q.nigra, and lad mining period on Q.geminata was correspondingly longer than on Q.nigra. 3. The average leaf area of Q.nigra was significantly greater than that of Q.geminata. 4. Parasitism was significantly greater in Q.geminata miner populations and predation was significantly higher in Q.nigra populations. However, parasitism and predation rates were roughly reciprocal so that overall larval mortality did not differ significantly between hosts. 5. In a laboratory experiment, pupal overwintering survivorship did not differ significantly between moist and dry treatments of the sand and loam soil types that typify Qgeminata and Q.nipra habitats, respectively. 6. Within-leaf miner density on Q.geminata significantly affected larval survivorship, parasitism and predation. Leaf area and within-leaf miner density were positively correlated. 7. We detected no host-patch area or isolation effect on miner densities on coastal Qgeminata and there was no significant gradient of local coastal conditions affecting larval survivorship or natural enemies. 8. Coastal leaf miner densities were significantly higher on oak patch edges than in interiors.

Pinyon pine, Pinus edulis (Engelm.), in Northern Arizona is exposed to recurring high levels of herbivory by the moth Dioryctria albovitella (Hust.). During a 3-year period, infested trees experienced on average a 30 percent reduction in annual shoot production. This herbivory affects tree architecture, growth rate, reproductive output, and sexual expression. Less infested trees produce 47 percent more trunk wood, 43 percent more branch wood, and are monoecious. Architectural changes in infested trees can result infunctionally male plants due to a complete loss of normal female cone-bearing ability. When herbivores are experimentally removed, normal growth and reproduction patterns resume. These strong herbivore impacts should represent a potent selection pressure in the evolution of host traits.

We monitored abundances and occurrences of leaf-mining insects on three oak species, Quercus falcata, Q. nigra, and Q. hemisphaerica, that vary in leaf persistence times. When densities and species richness are compared among oak species, we found no significant differences as predicted by Rhoades and Cates' and Opler's hypotheses regarding leaf miners that feed on oaks with different leaf durations. Furthermore, densities of leaf miners are significantly and negatively correlated with total nitrogen content of leaves, a result contradictory to Feeny's study on lepidopteran species that feed on another oak. We conclude that if leaf persistence or, more directly, nitrogen content, at all affects abundances and numbers of leafmining species on oaks, the effect is inconsequential compared to that of parasitism, predation, abiotic factors, and overwintering success. We did find significant variation in leaf miner densities within and between trees and season. Much of the variation within and between trees may be attributable to ovipositional preference of adult females or secondarily to differential mortality of eggs or larvae. The within and between season variability appears to be caused by the specificity of some leaf miners to certain leaf stages, the build-up of densities through successive cohorts of multivoltine species, and large fluctuations in year-to-year densities of a few species.