Host choice by Aphidius colemani:: effects of plants, plant-aphid combinations and the presence of intra-guild predators

1. This study first measured the effect of plant (radish, pepper, and wheat), aphid-plant combination (the green peach aphid Myzus persicae on pepper and radish, and the bird cherry-oat aphid Rhopalosiphum padi on wheat) and the host on which Aphidius colemani was reared (the cotton aphid Aphis gossypii on cucumber, M. persicae on radish and pepper, and R. padi on wheat) on host choice behaviour of the parasitoid and the performance of its offspring. Then, the effect of predator presence (Coccinella undecimpunctata larvae) on host preference of the wasps was tested. 2. When reared on M. persicae on either radish or pepper, wasps preferred the aphid-plant combination from which they had emerged. Wasps reared on A. gossypii (naive to all hosts offered) and R. padi preferred to parasitise M. persicae on radish and M. persicae on either radish or pepper, respectively. Rhopalosiphum padi on wheat was the least preferred and also the least suitable host, as determined by offspring body size. 3. Contrary to expectations, the presence of predators did not influence the host choice of A. colemani, even when predator-free plants were offered nearby. Results indicate that wasps are more likely to remain in some host systems but not in others, even when facing risk of predation. 4. Results are discussed in respect to field data and the relative importance of risk of predation and host preference to wasp fitness.