Enemy-free space for parasitoids

Parasitism rate differs between herbivore generations in the univoltine, but not bivoltine, range

With climate change, plant-feeding insects increase their number of annual generations (voltinism). However, to what degree the emergence of a new herbivore generation affects the parasitism rate has not been explored. We performed a field experiment to test whether the parasitism rate differs between the first and the second generations of a specialist leaf miner (Tischeria ekebladella), both in the naturally univoltine and bivoltine parts of the leaf miner's distribution. We found an interactive effect between herbivore generation and geographical range on the parasitism rate. The parasitism rate was higher in the first compared to the second host generation in the part of the range that is naturally univoltine, whereas it did not differ between generations in the bivoltine range. Our experiment highlights that shifts in herbivore voltinism might release top-down control, with potential consequences for natural and applied systems.

Coevolution of Patch Selection in Stochastic Environments

AbstractSpecies interact in landscapes where environmental conditions vary in time and space. This variability impacts how species select habitat patches. Under equilibrium conditions, evolution of this patch selection can result in ideal free distributions where per capita growth rates are zero in occupied patches and negative in unoccupied patches. These ideal free distributions, however, do not explain why species occupy sink patches, why competitors have overlapping spatial ranges, or why predators avoid highly productive patches. To understand these patterns, we solve for coevolutionarily stable strategies (coESSs) of patch selection for multispecies stochastic Lotka-Volterra models accounting for spatial and temporal heterogeneity. In occupied patches at the coESS, we show that the differences between the local contributions to the mean and the variance of the long-term population growth rate are equalized. Applying this characterization to models of antagonistic interactions reveals that environmental stochasticity can partially exorcize the ghost of competition past, select for new forms of enemy-free and victimless space, and generate hydra effects over evolutionary timescales. Viewing our results through the economic lens of modern portfolio theory highlights why the coESS for patch selection is often a bet-hedging strategy coupling stochastic sink populations. Our results highlight how environmental stochasticity can reverse or amplify evolutionary outcomes as a result of species interactions or spatial heterogeneity.

Genetic variation in disease resistance in Drosophila spp. is mitigated in Drosophila sechellia by specialization to a toxic host

We found that Drosophila species vary in their susceptibility to the broad-spectrum entomopathogen, Metarhizium anisopliae (strain Ma549). Generalist species were generally more resistant than dietary specialists, with the cactophilic Drosophila buzzatii and Drosophila sechellia, a specialist of the Morinda citrifolia (Morinda) fruit, being most susceptible. Morinda fruit is reported to be toxic to most herbivores because it contains Octanoic Acid (OA). We confirmed that OA is toxic to Drosophila spp., other than D. sechellia, and we also found that OA is highly toxic to entomopathogenic fungi including Ma549 and Beauveria bassiana. Drosophila sechellia fed a diet containing OA, even at levels much less than found in Morinda fruit, had greatly reduced susceptibility to Ma549. This suggests that specializing to Morinda may have provided an enemy-free space, reducing adaptive prioritization on a strong immune response. Our results demonstrate that M. anisopliae and Drosophila species with divergent lifestyles provide a versatile model system for understanding the mechanisms of host-pathogen interactions at different scales and in environmental context.

Season and herbivore defence trait mediate tri-trophic interactions in tropical rainforest

Bottom-up effects from host plants and top-down effects from predators on herbivore abundance and distribution vary with physical environment, plant chemistry, predator and herbivore trait and diversity. Tri-trophic interactions in tropical ecosystems may follow different patterns from temperate ecosystems due to differences in above abiotic and biotic conditions. We sampled leaf-chewing larvae of Lepidoptera (caterpillars) from a dominant host tree species in a seasonal rainforest in Southwest China. We reared out parasitoids and grouped herbivores based on their diet preferences, feeding habits and defence mechanisms. We compared caterpillar abundance with leaf numbers ('bottom-up' effects) and parasitoid abundance ('top-down' effects) between different seasons and herbivore traits. We found bottom-up effects were stronger than top-down effects. Both bottom-up and top-down effects were stronger in the dry season than in the wet season, which were driven by polyphagous rare species and host plant phenology. Contrary to our predictions, herbivore traits did not influence differences in the bottom-up or top-down effects except for stronger top-down effects for shelter-builders. Our study shows season is the main predictor of the bottom-up and top-down effects in the tropics and highlights the complexity of these interactions.

Latitudinal gradient in the intensity of biotic interactions in terrestrial ecosystems: Sources of variation and differences from the diversity gradient revealed by meta-analysis

The Latitudinal Biotic Interaction Hypothesis (LBIH) states that the intensity of biotic interactions increases from high to low latitudes. This hypothesis, which may partly explain latitudinal gradients in biodiversity, remains hotly debated, largely due to variable outcomes of published studies. We used meta-analysis to identify the scope of the LBIH in terrestrial ecosystems. For this purpose, we explored the sources of variation in the strength of latitudinal changes in herbivory, carnivory and parasitism (119 publications) and compared these gradients with gradients in the diversity of the respective groups of animals (102 publications). Overall, both herbivory and carnivory decreased towards the poles, while parasitism increased. The latitudinal gradient in herbivory and carnivory was threefold stronger above 50-60° than at lower latitudes and was significant due to interactions involving ectothermic consumers, studies using standardised prey (i.e. prey lacking local anti-predator adaptations) and studies aimed at testing LBIH. The poleward decrease in biodiversity did not differ between ectothermic and endothermic animals or among climate zones and was fourfold stronger than decrease in herbivory and carnivory. The discovered differences between the gradients in biotic interactions and biodiversity suggest that these two global macroecological patterns are likely shaped by different factors.

Experimental shelter-switching shows shelter type alters predation on caterpillars (Hesperiidae)

Caterpillars build various shelters that protect them from natural enemies, but whether specific shelters provide different protection is unknown. To disentangle a caterpillar species’ shelter from the rest of its phenotype, we performed a field experiment in which two caterpillar species (Urbanus dorantes and U. proteus) were removed from their original shelters, placed into shelters made by conspecifics or heterospecifics, and monitored for predation and parasitism. Predation was intense, with 0–48% of caterpillars surviving depending on treatment. Shelter builder identity significantly affected predation independent of occupant identity, with caterpillars placed in U. proteus shelters experiencing higher predation than those in U. dorantes shelters. The effect of shelter builder identity was related to shelter type: shriveled leaf shelters built by U. dorantes had a lower risk of predation than cut-and-fold shelters built by either species. Cut-and-fold shelters built by the two species did not have significantly different shapes. Caterpillar stage also significantly affected predation (mid-instars were more successful than early instars), but caterpillar species identity did not. Surprisingly, parasitism was rare, but both shriveled leaf shelters and cut-and-fold shelters resulted in similar overall caterpillar mortality. The differences in predation and overall mortality between shelter types suggest a trade-off between protection from predators and parasitoids. This experiment demonstrates that shelter type determines the fate of the caterpillar inside, independent of the identity of the caterpillar that built the shelter. This is the first experimental evidence that predation may select for shelter type and associated shelter-building behavior in Lepidoptera.

Detoxification of plant defensive glucosinolates by an herbivorous caterpillar is beneficial to its endoparasitic wasp

Plant chemical defences impact not only herbivores, but also organisms in higher trophic levels that prey on or parasitize herbivores. While herbivorous insects can often detoxify plant chemicals ingested from suitable host plants, how such detoxification affects endoparasitoids that use these herbivores as hosts is largely unknown. Here, we used transformed plants to experimentally manipulate the major detoxification reaction used by Plutella xylostella (diamondback moth) to deactivate the glucosinolate defences of its Brassicaceae host plants. We then assessed the developmental, metabolic, immune, and reproductive consequences of this genetic manipulation on the herbivore as well as its hymenopteran endoparasitoid Diadegma semiclausum. Inhibition of P. xylostella glucosinolate metabolism by plant-mediated RNA interference increased the accumulation of the principal glucosinolate activation products, the toxic isothiocyanates, in the herbivore, with negative effects on its growth. Although the endoparasitoid manipulated the excretion of toxins by its insect host to its own advantage, the inhibition of herbivore glucosinolate detoxification slowed endoparasitoid development, impaired its reproduction, and suppressed the expression of genes of a parasitoid-symbiotic polydnavirus that aids parasitism. Therefore, the detoxification of plant glucosinolates by an herbivore lowers its toxicity as a host and benefits the parasitoid D. semiclausum at multiple levels.

Host plant iridoid glycosides mediate herbivore interactions with natural enemies

Many insect herbivores are dietary specialists capable of sequestering the secondary metabolites produced by their host plants. These defensive compounds have important but complex implications for tritrophic interactions between plants, herbivores, and natural enemies. The sequestration of host plant secondary metabolites defends herbivores from attack by generalist predators, but may also compromise the immune response, making insect herbivores more vulnerable to parasitism. Here, we investigate the role of plant secondary metabolites in mediating interactions between a specialist herbivore and its natural enemies. The host plants are two Penstemon species, Penstemon glaber and Penstemon virgatus, which are chemically defended by iridoid glycosides (IGs). First, we examined how Penstemon iridoid glycoside content influences the sequestration of IGs by a specialist herbivore, Euphydryas anicia. Then, we performed ant bioassays to assess how host plant species influences larval susceptibility to predators and phenoloxidase assays to assess the immunocompetence and potential vulnerability to parasitoids and pathogens. We found that the concentration of IGs sequestered by E. anicia larvae varied with host plant diet. Larvae reared on P. glaber sequestered more IGs than larvae reared on P. virgatus. Yet, ant predators found larvae unpalatable regardless of host plant diet and were also repelled by sugar solutions containing isolated IGs. However, E. anicia larvae reared on P. glaber showed higher levels of phenoloxidase activity than larvae reared on P. virgatus. Our results suggest that the sequestration of some secondary metabolites can effectively protect herbivores from predation, yet may also increase vulnerability to parasitism via decreased immunocompetence.

Enemy escape: A general phenomenon in a fragmented literature?

Many populations are thought to be regulated, in part, by their natural enemies. If so, disruption of this regulation should allow rapid population growth. Such “enemy escape” may occur in a variety of circumstances, including invasion, natural range expansion, range edges, suppression of enemy populations, host shifting, phenological changes, and defensive innovation. Periods of relaxed enemy pressure also occur in, and may drive, population oscillations and outbreaks. We draw attention to similarities among circumstances of enemy escape and build a general conceptual framework for the phenomenon. Although these circumstances share common mechanisms and depend on common assumptions, enemy escape can involve dynamics operating on very different temporal and spatial scales. In particular, the duration of enemy escape is rarely considered but will likely vary among circumstances. Enemy escape can have important evolutionary consequences including increasing competitive ability, spurring diversification, or triggering enemy counteradaptation. These evolutionary consequences have been considered for plant–herbivore interactions and invasions but largely neglected for other circumstances of enemy escape. We aim to unite the fragmented literature, which we argue has impeded progress in building a broader understanding of the eco-evolutionary dynamics of enemy escape.

Community ecology of the 'other' parasitoids

The study of parasitoid communities is an active and dynamic field. Most studies, however, are focused primarily on parasitic wasps, despite the thousands of other insect parasitoids distributed across many lineages. Although questions in parasitoid community ecology are much the same for different groups, answers to these questions may not be due to differing biological traits. The ecology of non-hymenopteran ('NH') parasitoid communities is poorly known, but recent work indicates that habitat and host traits have strong impacts on the size and composition of these parasitoid assemblages. Recent food-web analyses indicate that host ranges vary widely within and among taxa and associations are shaped by host ecology and defenses. Evidence is also accumulating for strong 'bottom-up' and 'top-down' multi-trophic interactions between NH-parasitoids and nonadjacent trophic levels, as well as trait-mediated indirect effects on communities. Recent technical and conceptual advances in characterizing and comparing food webs, consideration of phylogenetic history, and increasing anthropogenic impacts provide many new and stimulating areas of research on parasitoid communities.

Interactions between parasitoids and higher order natural enemies: intraguild predation and hyperparasitoids

Parasitoids kill and live at the expense of their hosts, but they also serve as food for intraguild predators and hyperparasitoids. Natural enemy diversity can thus challenge herbivore suppression by parasitoids, but this depends on the ecological niches of the species involved and their functional diversity. The spatial context is another important layer of complexity, particularly in areas with reduced habitat complexity and increased fragmentation. Parasitoids have evolved strategies to locate their host, but this can be affected by risk of intraguild predation or hyperparasitism. To better understand these interactions we need more long-term experiments and trophic-web studies. This will provide fundamental knowledge, improve pest control, and allow ecologists to better predict the impact of human activities on species extinctions.

Indirect plant-parasitoid interactions mediated by changes in herbivore physiology

In occupying an intermediate trophic position, herbivorous insects serve a vital link between plants at the base of the food chain and parasitoids at the top. Although these herbivore-mediated indirect plant-parasitoid interactions are well-documented, new studies have uncovered previously undescribed mechanisms that are fundamentally changing how we view tri-trophic relationships. In this review we highlight recent advances in this field focusing on both plant-driven and parasitoid-driven outcomes that flow up and down the trophic web, respectively. From the bottom-up, plant metabolites can impact parasitoid success by altering host immune function; however, few have considered the potential effects of other plant defense strategies such as tolerance on parasitoid ecology and behavior. From the top-down, parasitoids have long been considered plant bodyguards, but in reality the consequences of parasitism for herbivory rates and induction of plant defensive chemistry are far more complicated with cascading effects on community-level interactions.