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 a rthropods can be either enhanced or disrupted by introducing a predator spe cies to an existing host-parasitoid system. We evaluate the impact of intro ducing a predator, the convergent ladybird beetle, Hippodamia convergens, o n the biological control of the cotton aphid, Aphis gossypii, by the parasi toid Lysiphlebus testaceipes, under field conditions. Predation on immature parasitoids by H. convergens was intense: 98-100% of aphid mummies were co nsumed by the end of the experiment, and H. convergens substantially reduce d immature parasitoid populations. Despite the negative impact of H. conver gens on aphid parasitoids, aphid population suppression was greatest in tre atments 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. conve rgens did not substantially alter the percentage of aphids mummified by par asitoids and showed a partial feeding preference for unparasitized aphids o ver aphid mummies. We conclude that under conditions where a predator shows both a partial preference for unparasitized hosts and high levels of preda tion on unparasitized hosts, we may expect the predator to improve suppress ion of herbivores even if it produces high levels of intraguild predation. While intraguild predation is an important ecological interaction in the ea rly-season cotton agroecosystem, it does not disrupt cotton aphid biologica l control.