The "ecological law" of parasite aggregation among hosts, where a few hosts harbor the vast majority of parasites, is a major factor stabilizing host and parasite populations, preventing parasites from destroying the host population they depend on. Aggregation is thought to be the result of variation in host traits, such as resistance to parasites, or features of the parasite lifecycle, such as an immobile free-living stage that is transmitted en masse via ingestion. Aggregation may also significantly affect parasite-host coevolutionary dynamics -- selective pressure on the host population could be limited because only a small proportion experiences significant parasite loads, preventing or slowing adaptation of host defenses. Certain phytophagous insect herbivores have lifecycles similar to such parasites, with "parasitic" larvae and free-living adults that can colonize new host plants. However, the assumptions underlying parasite aggregation do not always translate to phytophagous insect herbivores. For example, lepidopteran adults select from many candidate host plants during oviposition, limiting the influence of plant heterogeneity and chance interactions. Phytophagous insect aggregation may have very different effects on population dynamics and coevolutionary relationships than parasite aggregation, but those implications remain unclear.
Results/Conclusions
We hypothesize that aggregation by phytophagous insects reduces selective pressure for plants to defend themselves, though sufficiently "intelligent" host choice by phytophagous insect herbivores may cancel out this effect. To address this question, we have adapted classic macroparasite models to reflect the lifecycles of phytophagous insect herbivores. Our model includes interaction traits that determine consumption rates and reproductive success of herbivores and damage sustained by host plants. To capture the breadth of phytophagous insect lifecycle diversity, we have also included model structures that allow insect adults to develop on host plants, later dispersing to other hosts. Periodic invasions of mutant plant and/or herbivore types allow comparisons of microevolutionary implications of aggregation on model populations. Preliminary results suggest that aggregation has less of a stabilizing effect on phytophagous insect herbivores than for macroparasites, producing persistent host and herbivore populations over a narrower range of parameter values than is observed for parasites. Furthermore, host choice of insect herbivores determines the degree to which aggregation influences coevolutionary outcomes -- random host choice with high aggregation creates evolutionary dynamics similar to those observed for parasites, while effective host choice with aggregation imposes strong selection on plants to improve defenses.
