Competitive ability of parasitized Drosophila larvae

Knowledge of the cost of parasitism and the competitive ability of parasiti zed larvae is important for understanding the evolution of resistance. We u sed larvae of two Drosophila species as hosts for two parasitoid species wh ich differ in their counter-resistance mechanism. Parasitism by Leptopilina heterotoma leads to a reduction in survival, in contrast to parasitism by Asobara tabida. This can be explained by L. heterotoma having a counter-def ence mechanism that actively interferes with the host's immune system. Para sitized D. melanogaster larvae, which can encapsulate the parasitoid's egg to some degree, tend to suffer from a slight reduction in competitive abili ty, as opposed to parasitized D. subobscura larvae, which are unable to mou nt an immune response to parasitoids. Combined with earlier work, our resul ts suggest that, in this system, the costs of actual defence are lower than the costs of maintaining an efficient immune system.