Can preexisting trait variation predict the outcomes of parasitoid competition in novel habitats?

Background/Question/Methods

Parasitic wasps (parasitoids) often specialize on one or a small number of hosts and host associations often define reproductively isolated populations. Shifting and adapting to novel hosts may explain the tremendous amount of diversity among insect parasitoids if parasitoids using novel hosts become reproductively isolated from their source population. In this study we ask if different parasitoids compete for a novel resource and whether some species possess preexisting traits that enable them to more easily colonize particular novel hosts (i.e., “win” the competition). Further, we ask if we can predict successful host shifts based on analyses of characteristics of the ancestral system. Using existing host-parasite models as a basis, we have designed a mathematical model to describe the effects of morphological differences between competing parasitoid populations on successful colonization of a novel host. As a case study to parameterize this general model, we use races of the wasp Diachasma alloeum that attack Rhagoletis larvae in hawthorn and blueberry fruit  to determine their ability to attack Rhagoletis larvae in a new plant host, apple. The morphological character of interest in this system is ovipositor length, which is directly correlated with successful oviposition.

Results/Conclusions

The evolutionary simulations of the model allow us to explore the population dynamics of host-mulitparasite systems. Specifically, this model evaluates preexisting morphological characters that may allow one D. alloeum race to successfully utilize its host in a novel fruit over the other. Preliminary analyses focus on the effect of ovipositor length on the ability of a parasitoid population to colonize a novel host when in competition with another population. We find threshold values of ovipositor lengths that result in competitive exclusion of one population by the other. Preliminary results show no instances of coexistence. While the empirical evidence used to parameterize the model come from a specific system, the model is broad in scope and is able to be expanded to use in other models of insect host shifts.