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In response to insect attack, many plants exhibit dynamic biochemical changes, resulting in the induced production of direct and indirect defenses. Elicitors present in herbivore oral secretions are believed to positively regulate many inducible plant defenses; however, little is known about the specificity of elicitor recognition in plants. To investigate the phylogenic distribution of elicitor activity, we tested representatives from three different elicitor classes on the time course of defense-related phytohormone production, including ethylene (E), jasmonic acid (JA), and salicylic acid, in a range of plant species spanning angiosperm diversity. All families examined responded to at least one elicitor class with significant increases in E and JA production within 1 to 2 h after treatment, yet elicitation activity among species was highly idiosyncratic. The fatty-acid amino acid conjugate volicitin exhibited the widest range of phytohormone and volatile inducing activity, which spanned maize (Zea mays), soybean (Glycine max), and eggplant (Solanum melongena). In contrast, the activity of inceptin-related peptides, originally described in cowpea (Vigna unguiculata), was limited even within the Fabaceae. Similarly, caeliferin A16:0, a disulfooxy fatty acid from grasshoppers, was the only elicitor with demonstrable activity in Arabidopsis thaliana. Although precise mechanisms remain unknown, the unpredictable nature of elicitor activity between plant species supports the existence of specific receptor-ligand interactions mediating recognition. Despite the lack of an ideal plant model for studying the action of numerous elicitors, E and JA exist as highly conserved and readily quantifiable markers for future discoveries in this field. ethylene | insect elicitor | jasmonic acid | plant defense | volatile organic compound

Patterns in food-web structure have frequently been examined in static food webs, but few studies have attempted to delineate patterns that materialize in food webs under nonequilibrium conditions. Here, using one of nature's classical nonequilibrium systems as the food-web database, we test the major assumptions of recent advances in food-web theory. We show that a complex web of interactions between insect herbivores and their natural enemies displays significant architectural flexibility over a large fluctuation in the natural abundance of the major herbivore, the spruce budworm (Choristoneura fumiferana). Importantly, this flexibility operates precisely in the manner predicted by recent foraging-based food-web theories: higher-order mobile generalists respond rapidly in time and space by converging on areas of increasing prey abundance. This 'birdfeeder effect' operates such that increasing budworm densities correspond to a cascade of increasing diversity and food-web complexity. Thus, by integrating foraging theory with food-web ecology and analyzing a long-term, natural data set coupled with manipulative field experiments, we are able to show that food-web structure varies in a predictable manner. Furthermore, both recent food-web theory and longstanding foraging theory suggest that this very same food-web flexibility ought to be a potent stabilizing mechanism. Interestingly, we find that this food-web flexibility tends to be greater in heterogeneous than in homogeneous forest plots. Because our results provide a plausible mechanism for boreal forest effects on populations of forest insect pests, they have implications for forest and pest management practices. budworm | food-web theory | foraging theory | herbivore-natural enemy interactions | insect outbreaks

Interspecific competition between plants and herbivory by specialized insects can have synergistic effects on the growth and performance of the attacked host plant. We tested the hypothesis that competition between plants may also negatively affect the performance of herbivores as well as their top-down effect on the host plant. In such a case, the combined effects of competition and herbivory may be less than expected from a simple multiplicative response. In other words, competition and herbivory may interact antagonistically. In a greenhouse experiment, Poa annua was grown in the presence or absence of a competitor (either Plantago lanceolata or Trifolium repens), as well as with or without a Poa-specialist aphid herbivore. Both competition and herbivory negatively affected Poa growth. Competition also reduced aphid density on Poa. This effect could in part be explained by changes in the biomass and the nitrogen content of Poa shoots. In treatments with competitors, reduced aphid densities alleviated the negative effect of herbivory on above- and belowground Poa biomass. Hence, we were able to demonstrate an antagonistic interaction between plant-plant interspecific competition and herbivory. However, response indices suggested that antagonistic interactions between competition and herbivory were contingent on the identity of the competitor. We found the antagonistic effect only in treatments with T. repens as the competitor. We conclude that both competitor identity and the herbivore's ability to respond with changes in its density or activity to plant competition affect the magnitude and direction (synergistic vs. antagonistic) of the interaction between competition and herbivory on plant growth. Key words: antagonistic interactions; aphids; insect herbivory; Plantago lanceolata; plant competition; Poa annua; Rhopalosiphum padi; Trifolium repens.

Studies of how herbivory affects plant fitness often determine whether damage to one parent alters reproductive output (i.e., seed set or paternity) but ignore the possibility that the outcome may be different if both parents were damaged (i.e., the presence of maternal x paternal damage interactions). Using inbred lines of the common morning glory, Ipomoea purpurea, I conducted a series of greenhouse experiments to test whether foliar damage from a generalist insect herbivore, Trichoplusia ni, alters male and female fitness components when neither parent, one parent, or both I. purpurea parents had been damaged. In a single-donor experiment, flowers on both damaged and undamaged maternal plants received pollen from either damaged or undamaged paternal plants. I. purpurea flowers were more likely to be aborted when they received pollen from damaged paternal plants, or when maternal plants were both damaged and grown under low-resource conditions. Foliar damaged plants also produced less seed and pollen than undamaged plants, although seed mass and pollen viability were not affected by damage. In a multiple-donor experiment, flowers on damaged and undamaged maternal plants simultaneously received pollen from damaged and undamaged paternal plants, and F1 seeds were analyzed for paternity. Damaged paternal plants had reduced siring success compared to undamaged paternal plants, and this discrepancy was most pronounced when competition occurred on damaged maternal plants. Thus, damaged maternal plants were more 'selective' than undamaged maternal plants. Although previous studies have demonstrated that herbivory can alter fruit and seed production and paternity patterns, this is the first study to show that the magnitude of herbivore damage experienced by both parents can interact to influence maternal and paternal mating success. Key words: fitness components; herbivory; Ipomoea purpurea; morning glory; pollen competition; seed mass; seed production; selective abortion; siring success; Trichoplusia hi.

To elucidate the factors that affect the performance of plants in their natural environment, it is essential to study interactions with other neighboring plants, as well as with above- and belowground higher trophic organisms. We used a long-term field experiment to study how local plant community diversity influenced colonization by the biennial composite Senecio jacobaea in its native range in The Netherlands in Europe. We tested the effect of sowing later-succession plant species (0, 4, or 15 species) on plant succession and S. jacobaea performance. Over a period of eight years, the percent cover of S. jacobaea was relatively low in communities sown with 15 or 4 later-succession plant species compared to plots that were not sown, but that were colonized naturally. However, after four years of high abundance, the density of S. jacobaea in unsown plots started to decline, and the size of the individual plants was smaller than in the plots sown with 15 or 4 plant species. In the unsown plots, densities of aboveground leaf-mining, flower-feeding, and stem-boring insects on S. jacobaea plants were lower than on plants in sown plots, and there was a strong positive relationship between plant size and levels of herbivory. In a greenhouse experiment, we grew S. jacobaea in sterilized soil inoculated with soil from the different sowing treatments of the field experiment. Biomass production was lower when S. jacobaea test plants were grown in soil from the unsown plots than in soil from the sown plots (4 or 15 species). Molecular analysis of the fungal and bacterial communities revealed that the composition of fungal communities in unsown plots differed significantly from those in sown plots, suggesting that soil fungi could have been involved in the relative growth reduction of S. jacobaea in the greenhouse bioassay. Our results show that, in its native habitat, the abundance of S. jacobaea depends on the initial composition of the plant community and that, on a scale of almost a decade, its interactions with plant and soil communities and aboveground invertebrates may influence the dynamics of this colonizing species. Key words: biodiversity; biotic resistance; DGGE, grasslands; insect herbivory; plant--soil feedback; Senecio jacobaea.

Elucidating the relative strength of top-down and bottom-up forces in communities of phytophagus insects has been a major historical focus. Current consensus is that both forces play a role, but it is poorly known if these forces act differently on herbivores in the same assemblage and what factors underlie this variation. Using manipulative experiments with an assemblage of sap-feeding phytophagous insects (six species of planthoppers, leafhoppers, and heteropteran bugs) inhabiting intertidal Spartina marshes, we examined the association between herbivore behavior, risk of predation, and ultimately the relative impact of top-down (wolf spider predation) and bottom-up (host-plant nutrition) factors on the population density of each sap-feeding herbivore. A factorial experiment on open Spartina islets in the field (two levels of plant nutrition crossed with two levels of spider predation) showed that bottom-up and top-down manipulations differentially affected the various sap-feeders. Overall, bottom-up effects dominated in this sap-feeder community, whereby the density of all six sap-feeders increased when the nitrogen content of Spartina was elevated. By contrast, wolf-spider addition significantly suppressed populations of only the Prokelisia species and had little impact on the other four sap-feeder species in the community. Functional-response experiments and behavioral studies revealed that certain species (Prokelisia planthoppers) were at much higher risk of attack by wolf spiders than other sap-feeders in the assemblage and that risk of predation was associated with a species' particular 'escape/defensive behavior.' Moreover, risk of spider predation was roughly linked to the strength of top-down impacts in the field, because species with ineffective escape behaviors and a high risk of spider attack (Prokelisia planthoppers) were the only sap-feeders whose populations were suppressed by spider predation in the field. Thus, specific behavioral characteristics of the sap-feeders on Spartina influenced risk of predation and the relative strength of top-down and bottom-up impacts on their population dynamics. Notably, all herbivores in this system were positively influenced by elevated plant nutrition, only the common sap-feeder species (Prokelisia planthoppers) were adversely affected by spider predation, and it was the rarer sap-feeders in the assemblage that were least impacted by predation. These results call into question the overall pervasiveness of top-down forces and underscore the primacy of basal resources in structuring this community of phytophagous insects. Key words: bottom-up vs. top-down impact; phytophagous insect community; planthopper; plant nutrition; population suppression; predation; Prokelisia; risk of predation; sap-feeding insects; Spartina alterniflora; spider; trophic cascades.

Research is presented on the population dynamics of the planthopper, a phytophagous insect, in a Florida salt marsh habitat. Environmental conditions studied included changes in soil salinity and parasitism levels. Results show that environmental alterations have a significant impact on planthopper density, which in turn impacts trophic dynamics in a given habitat.

Wood-borers are compared to leaf-feeding herbivores regarding their impact on canopy in mangrove forests. Wood-boring insects killed more than 50% of the canopy via girdling, hollowing and pruning, compared to leaf-feeders which killed less than 6%. The effects of wood-borers on external and internal nutrient cycling processes within mangrove forest ecosystems is also discussed.

We conducted a 3-yr field experiment that manipulated the presence of ungulates (domestic sheep and Spanish ibex Capra pyrenaica, Bovidae) and the monophagous beetle Timarcha lugens (Chrysomelidae) to evaluate (1) the effects on beetle abundance and interaction with the woody crucifer Hormathophylla spinosa; (2) the reciprocal effect of this insect on the interaction between ungulates and the plant; (3) their impact on H. spinosa reproductive output; and (4) the abundance of an associated taxon, Ceutorhynchus sp. nov. (Curculionidae). Our experiment showed that ungulate exclusion increased Timarcha abundance, whereas Timarcha had no effect on ungulates. H. spinosa reproductive output increased threefold promptly when ungulates were excluded, although in the third year Timarcha began to have effects on shrub reproduction. Ungulates affected weevils both by exploitative competition and by incidental predation, with the total number of weevils emerging per shrub being more than fourfold higher in shrubs excluded from these mammals than in shrubs open to them. In addition, during the last year, the abundance of weevils was also affected by Timarcha removal. Our study demonstrated that phytophagous insects can compete with other herbivores highly dissimilar in size, ecology, and taxonomy, such as ungulates. These interactions usually produce asymmetrical results, sometimes closer to amensalism or predation than to pure competition. Key words: asymmetrical interactions; competition, long-term effects; herbivory; incidental omnivory; interphyletic competition; interspecific competition; Mediterranean high-mountains; phytophagous insect; Timarcha lugens; ungulates.

Research on insect-plant relationships indicates that learning in egg laying females is an advantageous strategy when larval development is dependent on multiple host plants. A formal mathematical model is included.

The impact of solar UVB radiation on plant-consumer interactions is investigated. This is because research on solar UVB radiation and the behavioral responses of animals toward it has received little attention. A simple agroecosystem was used for the study, consisting of soybeans as the primary producers and a few species of insects as consumers. The results show solar UVB radiation triggers direct behavioral responses to the insects, aside from affecting the attractiveness of plant hosts.

I conducted a field test of the hypothesis that the dynamics of a predatorprey interaction should be affected by the number of interacting subpopulations within an assemblage. I used a 'successful' biological control system, the phytophagous European red mite (Panonychus ulmi) and its predator, and phytoseiid mite Typhlodromus pyri. An orchard was established where apple trees were arranged in groups of 1, 4, of 16 and the dynamics of the mite populations on the trees followed over a season. The pests reached highest densities and were more persistent on trees in the largest groups. These results are explained in terms of an interaction between prey immigration/emigration and predation.

A simultaneous analysis of the species richness of vascular plants and their insect herbivores in semi-natural grasslands with varying grazing intensities was conducted. The results showed that grazing management leads to a decrease in the species richness of the grassland insects, with a notable increase in the species density and richness of vascular plants.

The relationship between pairwise similarity of assemblages of exploiters (phytophagous insects and parasitic fungi) and pairwise genetic distance, range overlap, niche overlap and habitat overall of host trees is investigated. It was observed that the host switches of exploiters between related hosts are more significant than host switches between host co-occurring in the same habitat and a strong connection was found between pairwise similarity of assemblages of insects and the pairwise similarity of assemblages of fungi.

Previous studies of insect-plant interactions have produced the contradictory Plant Stress Hypothesis (that stressed plants are more suitable hosts for phytophages) and Plant Vigor Hypothesis (that vigorous plants are more suitable hosts for phytophages). However, experimental studies of phytophage responses to host stress have involved only one, or a few, related phytophagous species, not whole communities of organisms associated with a particular plant species. We evaluated responses of various arthropods associated with creosotebush Larrea tridentata to manipulated water availability and plant stress in southern New Mexico during 1990-1991. Of 44 arthropod groups (taxa or functional groups) evaluated in our study only two taxa (including a lepidopteran folivore) showed significant negative response to water availability, thereby supporting the Plant Stress Hypothesis. Ten taxa (including eight phytophages) responded positively to water availability, supporting the Plant Vigor Hypothesis. One phytophage showed a nonlinear response, supporting neither hypothesis. Detrended Correspondence Analysis significantly distinguished the arthropod community on water-deprived shrubs from the communities on watered shrubs. The variation in responses among phytophagous insects on creosotebush indicated that the effect of plant water stress likely reflects the choice of phytophage, and perhaps the host plant, being studied. Therefore, neither the Plant Stress Hypothesis nor the Plant Vigor Hypothesis can explain responses of all phytophages on a particular plant species., INTRODUCTION Field observations that outbreaks of phytophagous arthropods are frequently associated with stressed plants generated the Plant Stress Hypothesis, i.e., that stressed plants are more susceptible hosts for phytophages (White, [...]