Competitive interactions between aphids Aphis nerii and Myzocallis asclepiadis as mediated by a host plant and a shared predator

Interactions among insect herbivores are important drivers for the population dynamics of all species involved. The interactions can be mediated by herbivores’ host plants and predators. Plants develop phytochemicals to protect themselves against herbivores. Some herbivores have evolved to sequester these plant toxins and use them as defense chemicals. However, it’s unclear how the phytochemical concentration influences interactions of two insect herbivores which share predators in the field. Milkweeds are known for using cardenolides as a phytochemical defense against herbivores. Aphis nerii and Myzocallis asclepiadis are two aphid species coexisting on common milkweed (Asclepias syriaca). Two aphid species differ in their ability to sequester cardenolides. A. nerii has lower sequestration of cardenolides than M. asclepiadis. Both aphids are consumed by the green lacewing, Chrysopa sp.. We hypothesized that green lacewing prefers the aphid species with the lower cardenolide concentration, and that this preference results in asymmetric indirect interactions between two aphid species. To test this hypothesis, we conducted a factorial field experiment on the milkweeds-aphids-lacewing system. Our result showed that competition and predation decrease the population size of both aphid species. When lacewing predators and A. nerii are present the population size of M. asclepiadis is larger compared to when A. nerii is absent. This demonstrates that the predation preference reduces the negative competitive effect on high sequestration aphid species. We are in the process of analyzing the cardenolide concentration of milkweeds and aphids which will shed light on the role of phytochemical on these interactions.