Host quality does not díctate the outcome of intrinsic competition between sympatric parasitoids: effect on their coexistence.

Abstract

Herbivores are frequently attacked by several parasitoid species. During their growth, herbivores can differ in their suitability and quality for the developing parasitoids and, therefore, interfere in their competition for the shared host. Here, we report the first results on whether acceptance of heterospecific parasitized hosts and the outcome of competition is affected by host instar using two parasitoid species of the genus Aphytis (Hymenoptera: Aphelinidae), which coexist in several areas. Aphytis melinus DeBach laid more eggs (clutch size) and accepted significantly more hosts than A. chrysomphali (Mercet) when they encountered heterospecific parasitized hosts of the third instar (higher quality), whereas there were not significant differences in the second instar. The proportion of A. melinus progeny in multiparasitized hosts (outcome of competition) was significantly higher than that of A. chrysomphali and it was independent of the host instar and order of attack. Overall, A. melinus was a superior competitor than A. chrysomphali independently of the host instar attacked and our results show that their direct competition strongly contributes to the displacement of A. chrysomphali by A. melinus. Therefore, their coexistence can be only explained by climatic conditions and the plasticity of A. chrysomphali to use different host instars depending on A. melinus density, but not because of the superiority of A. melinus diminished in the second instar.