Plant phytochemicals mediate interactions between Aphis nerii and Myzocallis asclepiadis that share a common predator

Plants have developed a variety of chemical mechanisms against herbivores. One of the chemical defense mechanisms is the synthesis of phytochemicals. Over evolutionary time, some herbivores have evolved mechanisms to sequester phytochemicals and use them as protection against their predators. As a result, phytochemicals may mediate interactions between insect herbivores. In this study, common milkweed (Asclepias syriaca) has evolved cardenolides as a phytochemical defense against herbivores. Specialist herbivores such as aphid species, Aphis nerii and Myzocallis asclepiadis, can sequester cardenolides but differ in the concentration of accumulated cardenolides, which provide different levels of protection against aphid predators. At the same time, A. nerii and M. asclepiadis coexisting on common milkweeds during the growing season at the study site. We verified how cardenolides mediate interactions between A. nerii and M. asclepiadis under different environmental conditions. We conducted a field experiment and examined the population growth rates of two aphid species under different competition, predation, and environmental conditions. The result showed when the predator was present, A nerii reduced predation pressure on M. asclepiadis. Because M. asclepiadis has higher sequestration of cardenolides than A nerii, this result suggests that the predation preference reduces the negative effect on high sequestration aphid species. The result also shows that the warming effect did not alter the interaction outcomes but slightly increased the population growth rate for both aphid species.