Interspecific interactions are key mechanisms that shaping the structure of a community. While a community is shared by multiple species, the interaction between two can be influenced by other co-occurring species. In most of the terrestrial communities, the plant is the food source that supports and has cascade effects through a community. phytochemicals, one of the plant defense mechanisms that against herbivores. Over evolutionary time, some herbivores have evolved mechanisms to sequester phytochemicals and use them as an insect’s defense mechanism. As a result, phytochemicals may have cascade effects in a community by mediating interactions between insect herbivores. In this study, we used a two-prey-one-predator system to explore how do phytochemicals influence interspecific interactions. In this system, 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. Our result suggests that the predation preference reduces the negative 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 phytochemicals in these interactions.
