Predation on immature parasitoids and its impact on aphid suppression.

Many predatory arthropods eat both unparasitized herbivores and herbivores that are parasitized and contain the immature stages of endoparasitoids, a form of intraguild predation. Thus, the biological control of herbivorous arthropods can be either enhanced or disrupted by introducing a predator species to an existing host-parasitoid system. We evaluate the impact of introducing a predator, the convergent ladybird beetle, Hippodamia convergens, on the biological control of the cotton aphid, Aphis gossypii, by the parasitoid Lysiphlebus testaceipes, under field conditions. Predation on immature parasitoids by H. convergens was intense: 98-100% of aphid mummies were consumed by the end of the experiment, and H. convergens substantially reduced immature parasitoid populations. Despite the negative impact of H. convergens on aphid parasitoids, aphid population suppression was greatest in treatments containing both H. convergens and parasitoids. The parasitoid alone or in combination with H. convergens suppressed cotton aphids in a density-dependent manner and increased total plant leaf area and biomass, H. convergens did not substantially alter the percentage of aphids mummified by parasitoids and showed a partial feeding preference for unparasitized aphids over aphid mummies. We conclude that under conditions where a predator shows both a partial preference for unparasitized hosts and high levels of predation on unparasitized hosts, we may expect the predator to improve suppression of herbivores even if it produces high levels of intraguild predation. While intraguild predation is an important ecological interaction in the early-season cotton agroecosystem, it does not disrupt cotton aphid biological control.