Tritrophic chemical transfer of piperidine alkaloids: characterizing the epigeal and foliar food webs on invasive poison hemlock (Conium maculatum L.)

Poison hemlock, Conium maculatum L. (Apiales: Apiaceae), is an invasive plant that has spread throughout the United States and other regions outside its native range in Eurasia. It is categorized as a harmful and undesirable weed in agricultural, ecological, and urban settings, due to its toxic piperidine alkaloids. Previous studies revealed a paucity of specialists for poison hemlock, but there remains a lack of information regarding generalists, especially those at higher trophic levels that may be exposed to alkaloids via predation. This study characterizes arthropod diversity and the mode of alkaloid movement in poison hemlock communities, hypothesizing that the generalists Harmonia axyridis (Coleoptera: Coccinellidae) and various species of Carabidae (Coleoptera) are receiving detectable concentrations of alkaloid by consuming herbivores of poison hemlock. Predator gut contents were analyzed for either the presence of piperidine alkaloids or the presence of prey DNA. The former illustrates the potential significance of the tri-trophic transfer of toxic compounds into higher parts of the food chain while the later characterizes the route of alkaloid transfer. Both foliar and ground dwelling predator gut contents were screened for Hyadaphis foeniculi (Hemiptera: Aphididae) DNA, a known herbivore of poison hemlock. Carabidae were also screened for Mesodon sp.(Gastropoda: Polygyridae), which is particularly abundant in habitats containing poison hemlock. Laboratory feeding assays further categorized the chemical links between trophic levels. Chemical and field data support the tri-trophic movement of poison hemlock alkaloids within this community, and have major implications for generalist predator foraging behavior driven by plant chemistry.