Does host quality dictate the outcome of interference competition between sympatric parasitoids? Effects on their coexistence

The suitability and quality of herbivorous insect hosts for hymenopteran parasitoids is dynamic, varying with host development. Generally, within a host species, large hosts (i.e. older instars) are considered of higher quality for parasitoid development. Studies of interspecific competition between parasitoids have considered the effect of host instar on indirect competition but its effect on interference competition remains unknown. Here, we report the first results on whether the quality of host instars might dictate the outcome of interference competition between sympatric parasitoids of the genus Aphytis (Hymenoptera: Aphelinidae) when they attack low-quality (second) and high-quality (third) instars of their common host Aonidiella aurantii (Hemiptera: Diaspididae). Oviposition behaviour (host acceptance and clutch size) in low- and high-quality host instars was similar for both Aphytis species in the absence of competition. When they found heterospecific parasitized hosts of high quality, Aphytis melinus laid more eggs and accepted significantly more hosts than Aphytis chrysomphali, whereas there were no significant differences in the low-quality instar. This result suggests that interference competition is mediated by host quality. However, the progeny proportion of both parasitoids in multiparasitized hosts (outcome of competition) was independent of host quality and A. melinus always emerged at higher rates. Therefore, the result of interference competition between these sympatric parasitoids was not affected by host quality and this competition will contribute to the displacement of the native A. chrysomphali by the introduced A. melinus, which has been observed in some areas of the Mediterranean basin.