Compatibility between biological control and host plant resistance against the soybean aphid

Biological control and host plant resistance are essential components of most integrated pest management programs. While traditionally these approaches had been developed independently, current research has focused on tritrophic interactions involving insect-resistant cultivars, herbivores and their natural enemies, and how these influence pest control. Plant chemical defenses can mediate both bottom-up population regulation forces by reducing plant quality for the herbivores, and top-down forces by attracting natural enemies. Although alternate trophic levels (i.e. plants and natural enemies) frequently act in a mutualistic manner, this is not always the case and in many occasions plant defenses can be either directly or indirectly deterrent to herbivore natural enemies. This study investigated the effects of host plant resistance against the soybean aphid (SBA) on the native parasitoid Lysiphlebus testaceipes as a model for other SBA parasitoids. We compared aphid and parasitoid performance and parasitoid fitness traits in hosts feeding in four soybean lines with different degrees of resistance to SBA. SBA numbers were significantly greater in the susceptible soybean line than in any of the resistant lines at any point during the experiments. Parasitoid development time was on average one day longer in the resistant lines when compared to the susceptible ones. There were no significant differences in parasitoid performance or other offspring fitness traits among the lines associated with their degree of resistance. We conclude that host plant resistance against the SBA on soybean potentially decreases parasitoid fitness, and we discuss the implications of this finding.