Characterizing the epigeal and foliar food webs on poison hemlock

Invasive species have the potential to impact native communities by affecting biodiversity and ecosystem functioning. Poison hemlock (Conium maculatum L. (Apiales: Apiaceae)) is an invasive weed in the United States and produces the toxic piperidine alkaloids coniine, γ-coniceine, conhydrinone, and conmaculatin. After more than 100 years since introduction, relatively little is known about the interactions of arthropods with poison hemlock. The movement of plant chemicals and predator-prey relationships are necessary components for an accurate description of the potential impacts of poison hemlock on native communities. To test the hypothesis that generalist predators foraging in poison hemlock are receiving detectable levels of plant alkaloids, pitfall traps were established in areas of poison hemlock for the capture of Carabidae, and hand collection of foliar predators (Coccinellidae) was undertaken. Following capture, DNA-based molecular gut-content analysis calculated predation frequency on the dominant prey species, Hyadaphis foeniculi (Hemiptera: Aphididae). Predators were also analyzed for the presence of signature alkaloids. In combination, results revealed that Harmonia axyridis (Coleoptera: Coccinellidae) was frequently consuming H. foeniculi under field conditions and also contained poison hemlock alkaloids in their digestive system. Carabidae showed no evidence for consumption of H. foeniculi; however, Harpalus pensylvanicus (Coleoptera: Carabidae) is obtaining alkaloids, likely through seed consumption. Furthermore, feeding trials revealed that H. axyridis may benefit from consuming H. foeniculi and that many Carabidae will readily consume poison hemlock seeds. These results illustrate the potential impacts imparted on predator communities by invasive weeds and thus, the importance of characterizing new interactions arising within invaded areas.