Effects of intraguild predation on aphid parasitoid survival

To assess the potential selection pressure caused by intraguild predation b etween predators and parasitoids of aphids an estimate was made of the pred ation risk to Aphis fabae Scop. mummified by Lysiphlebus fabarum (Marshall, 1896) on sugar beet. Mummified aphids were exposed to a natural community of predators. Their survival time was estimated during a 10-day field surve y. Additionally, the role of alternative prey on parasitoid survival was in vestigated by adding unparasitised aphids to half of the mummy aggregations . The field data were evaluated by survival analysis. Two covariates were tes ted within a Cox proportional hazard model: (i) the presence of the alterna tive prey and (ii) the patch structure (number of proximal mummies attacked ). Within 4-5 days after exposure predators destroyed approx. 50% of the mu mmies. The model with both covariates revealed a significant difference con cerning survival of the mummies in the two treatments (Likelihood ratio tes t, chi(2) = 78.03, P = 0.0001). Alternative prey reduced the predation risk on mummies by 29%, while a high level of predation on proximal mummies inc reased the individual predation risk by 4%. The results are discussed in th e context of prey location by predators and the evolution of anti-predator mechanisms.