Bottom-up and top-down effects influence bruchid beetle individual performance but not population densities in the field

Can multitrophic interactions shape morphometry, allometry, and fluctuating asymmetry of seed-feeding insects?

Body size is commonly associated with biological features such as reproductive capacity, competition, and resource acquisition. Many studies have tried to understand how these isolated factors can affect the body pattern of individuals. However, little is known about how interactions among species in multitrophic communities determine the body shape of individuals exploiting the same resource. Here, we evaluate the effect of fruit infestation, parasitism rate, and seed biomass on size, allometric and asymmetric patterns of morphological structures of insects that exploit the same resource. To test it, we measured 750 individuals associated with the plant Senegalia tenuifolia (Fabaceae), previously collected over three consecutive years. Negative allometry was maintained for all species, suggesting that with increasing body size the body structure did not grow proportionally. Despite this, some variations in allometric slopes suggest that interactions in a multitrophic food web can shape the development of these species. Also, we observed a higher confidence interval at higher categories of infestation and parasitism rate, suggesting a great variability in the allometric scaling. We did not observe fluctuating asymmetry for any category or species, but we found some changes in morphological structures, depending on the variables tested. These findings show that both allometry and morphological trait measurements are the most indicated in studies focused on interactions and morphometry. Finally, we show that, except for the fluctuating asymmetry, each species and morphological structure respond differently to interactions, even if the individuals play the same functional role within the food web.

Host density and parasitoid presence interact and shape the outcome of a tritrophic interaction on seeds of wild lima bean

The interaction between the seed beetle Zabrotes subfasciatus and its parasitoid Stenocorse bruchivora, was investigated on seeds of two populations of wild lima bean, Phaseolus lunatus. By manipulating the number of beetle larvae per seed and the presence of parasitoids, we determined how factors related to beetle larvae density, the seed in which they feed and the parasitoid, may interact and affect host and parasitoid survival. Results showed that an increase in larval beetle density had a negative impact on beetle performance. This effect cascaded up to parasitoids, high larval density strongly reduced parasitoid emergence. Also, parasitoid presence resulted in faster beetle development and lower female weight. An interactive effect between larval host density and parasitoid presence affected the number of insects that emerged from the seeds. Beetle performance was better in the bean population with the largest seeds, while parasitoid emergence was the lowest in these seeds. This study shows that the impact of parasitoids on seed beetles is contingent on the interaction between density-mediated (direct mortality) and trait-mediated (e.g. non-consumptive) effects. Indirect trait-mediated effects of natural enemies are likely prevalent across insect communities, understanding their role in driving host-parasitoid interactions can have important implications for biological control.

Phytoplasma infection of a tropical root crop triggers bottom-up cascades by favoring generalist over specialist herbivores

Global interest on plant-microbe-insect interactions is rapidly growing, revealing the multiple ways in which microorganisms mediate plant-herbivore interactions. Phytopathogens regularly alter whole repertoires of plant phenotypic traits, and bring about shifts in key chemical or morphological characteristics of plant hosts. Pathogens can also cause cascading effects on higher trophic levels, and eventually shape entire plant-associated arthropod communities. We tested the hypothesis that a Candidatus Phytoplasma causing cassava witches' broom (CWB) on cassava (Manihot esculenta Grantz) is altering species composition of invasive herbivores and their associated parasitic hymenopterans. We conducted observational studies in cassava fields in eastern Cambodia to assess the effect of CWB infection on abundance of specialist and generalist mealybugs (Homoptera: Pseudococcidae), and associated primary and hyper-parasitoid species. CWB infection positively affects overall mealybug abundance and species richness at a plant- and field-level, and disproportionately favors a generalist mealybug over a specialist feeder. CWB phytoplasma infection led to increased parasitoid richness and diversity, with richness of 'comparative' specialist taxa being the most significantly affected. Parasitism rate did not differ among infected and uninfected plants, and mealybug host suppression was not impacted. CWB phytoplasma modifies host plant quality for sap-feeding homopterans, differentially affects success rates of two invasive species, and generates niche opportunities for higher trophic orders. By doing so, a Candidatus phytoplasma affects broader food web structure and functioning, and assumes the role of an ecosystem engineer. Our work unveils key facets of phytoplasma ecology, and sheds light upon complex multi-trophic interactions mediated by an emerging phytopathogen. These findings have further implications for invasion ecology and management.

Effects of plant intraspecific diversity across three trophic levels: Underlying mechanisms and plant traits

PREMISE OF STUDY

Although there is increasing recognition of the effects of plant intraspecific diversity on consumers, the mechanisms by which such effects cascade-up to higher trophic levels remain elusive.

METHODS

We evaluated the effects of plant (lima bean, Phaseolus lunatus) intraspecific diversity on a suite of insect herbivores (leaf-chewers, aphids, and seed-eating beetles) and their third trophic-level associates (parasitoids and aphid-tending ants). We established plots of three plants, classified as monocultures of one population source or polycultures with mixtures of three of the four population sources (N = 16 plots per level of diversity). Within each plot, plants were individually placed in pots and canopy contact was prevented, therefore eliminating diversity effects on consumers arising from changes in plant traits due to plant physical interactions.

KEY RESULTS

Plant diversity reduced damage by leaf-chewers as well as aphid abundance, and the latter effect in turn reduced ant abundance. In contrast, plant diversity increased the abundance of seed-eating beetles, but did not influence their associated parasitoids. There were no effects of diversity on seed traits potentially associated with seed predation, suggesting that differences in early season herbivory between monocultures and polycultures (a likely mechanism of diversity effects on plants since plant interactions were prevented) did not drive concomitant changes in plant traits.

CONCLUSIONS

This study emphasizes that effects of plant intraspecific diversity on consumers are contingent upon differences in associate responses within and among higher trophic levels and suggests possible mechanisms by which such effects propagate up this food web.

How does plant chemical diversity contribute to biodiversity at higher trophic levels?

Plants, perhaps Earth's most accomplished chemists, produce thousands of specialized metabolites having no direct role in cell division or growth. These phytochemicals vary by taxon, with many taxa producing characteristic substance classes; and within taxa, with individual variation in structural variety and production patterns. Observations of corresponding variation in herbivore metabolism, behavior, and diet breadth motivated the development of chemical ecology research. We discuss the importance of plant biodiversity in general and phytochemical diversity in particular for biodiversity and ecological interactions at higher trophic levels. We then provide an overview of the descriptive, molecular and analytical tools which allow modern biologists to investigate phytochemical diversity and its effects on higher trophic levels, from physiological mechanisms to ecological communities.

Uncovering Cryptic Parasitoid Diversity in Horismenus (Chalcidoidea, Eulophidae)

Horismenus parasitoids are an abundant and understudied group of eulophid wasps found mainly in the New World. Recent surveys based on morphological analyses in Costa Rica have quadrupled the number of named taxa, with more than 400 species described so far. This recent revision suggests that there is still a vast number of unknown species to be identified. As Horismenus wasps have been widely described as parasitoids of insect pests associated with crop plants, it is of high importance to properly establish the extant diversity of the genus, in order to provide biological control practitioners with an exhaustive catalog of putative control agents. In this study, we first collected Horismenus wasps from wild Phaseolus bean seeds in Central Mexico and Arizona to assess the genetic relatedness of three morphologically distinct species with overlapping host and geographical ranges. Sequence data from two nuclear and two mitochondrial gene regions uncovered three cryptic species within each of the three focal species (i.e., H. missouriensis, H. depressus and H. butcheri). The monophyly of each cryptic group is statistically supported (except in two of them represented by one single tip in which monophyly cannot be tested). The phylogenetic reconstruction is discussed with respect to differences between gene regions as well as likely reasons for the differences in variability between species.

Crop domestication and its impact on naturally selected trophic interactions

Crop domestication is the process of artificially selecting plants to increase their suitability to human requirements: taste, yield, storage, and cultivation practices. There is increasing evidence that crop domestication can profoundly alter interactions among plants, herbivores, and their natural enemies. Overall, little is known about how these interactions are affected by domestication in the geographical ranges where these crops originate, where they are sympatric with the ancestral plant and share the associated arthropod community. In general, domestication consistently has reduced chemical resistance against herbivorous insects, improving herbivore and natural enemy performance on crop plants. More studies are needed to understand how changes in morphology and resistance-related traits arising from domestication may interact with environmental variation to affect species interactions across multiple scales in agroecosystems and natural ecosystems.