Immigration and subtle trait variation can mitigate or reverse competitive outcomes for parasites attacking a novel host

Background/Question/Methods

Parasitic wasps (parasitoids) often specialize on one or a small number of hosts and specialization helps avoid or reduce competition. When parasitoids shift to novel hosts, traits resulting from coevolution with their ancestral host may determine their success in their new environment – particularly when many parasitoids may compete for the new host. In this study we model the population dynamics of two parasitoids competing for a novel host. We posit that we can predict successful host shifts based on subtle differences in ancestral morphological characteristics relevant to successful parasitism of the novel host. 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. We use ovipositor length (a trait that affects the proportion of hosts available to the parasitoid) and total body size (which influences searching and attack efficiency) as our morphological characters of interest. As a case study to parameterize this general model, we use two morphologically different host races of the wasp Diachasma alloeum that attack Rhagoletis larvae in hawthorn and blueberry fruit and model their competition for host larvae in a new plant environment, apple fruit.

Results/Conclusions

Our model allows us to explore the population dynamics of host-multiparasite systems. Specifically, this model evaluates preexisting morphological characters that may allow one D. alloeum race to better compete for a host in a novel fruit. Our null model addresses a system with no immigration into the novel environment from the source (ancestral) populations. We also determine the parameter space necessary for coexistence under a range of morphological differences and rates of immigration. Finally, using data from the R. pomonella – D. alloeum host – parasitoid system as a case study, we test the model with stochastic immigration from the source population to determine what level of immigration can rescue an inferior competitor. While the empirical evidence used to parameterize the model come from a specific system, the model is broad in scope and may be expanded for use with other characters (e.g., behavior, life history differences) and for a range of applications (e.g., biocontrol, predicting insect host shifts).