Submitted to: Journal of Chemical Ecology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 4/7/2005
Publication Date: 6/15/2005
Citation: Castells, E., Berhow, M.A., Vaughn, S.F., Berenbaum, M.R. 2005. Geographic variation in alkaloid production in Conium maculatum populations experiencing differential herbivory by Agonopterix alstroemeriana. Journal of Chemical Ecology. 31(8):1693-1709. Interpretive Summary: Poison hemlock is a biennial Eurasian weed that is extensively naturalized in the United States. The weed is generally regarded as noxious, as all aerial parts are poisonous to livestock and to humans. Its tendency to invade fields of alfalfa and other forage crops has led to livestock death through contamination of green-chopped hay. Due to its toxicity, as well as to its rank odor and profuse growth, poison hemlock is frequently a target of eradication programs in populated areas. Relative to other introduced weed species, poison hemlock is attacked by few insect herbivores. Recently, the hemlock defoliating moth has been reported at various sites in the U.S. This moth is an excellent potential biological control agent as it is extremely host-specific on poison hemlock and does not feed on economically-important plants. In this paper we found that plants where the moth was present had significantly higher levels of toxins than plants from areas where the moth was absent. In addition, plants with lower levels of toxins in areas where the moth was present had higher levels of herbivory than plants with high levels of toxins. These results suggest that the hemlock defoliating moth could be an excellent biological control agent for poison hemlock.
Technical Abstract: Conium maculatum, a Eurasian weed naturalized in North America, contains high concentrations of piperidine alkaloids that may act as chemical defenses against insect herbivores. In the U.S., C. maculatum was free from herbivory until 30 years ago when it was re-associated with its European monophagous associate, the moth Agonopterix alstroemeriana. At present, A. alstroemeriana is found in a continuum of re-association time and intensities with C. maculatum across the U.S. In some areas, such as in the western U.S., A. alstroemeriana can cause severe injuries to its hostplant resulting in a complete defoliation. The Evolution of Increased Competitive Ability (EICA) hypothesis predicts that introduced plants free from herbivores will invest fewer resources in chemical defenses, but when re-associated with a significant herbivore the investment in defenses should increase. In the present study three locations in the U.S. (New York, Washington and Illinois) where C. maculatum is under different degrees of herbivory intensity by A. alstroemeriana were studied. We aimed to determine whether chemical defense production in C. maculatum could change in response to herbivory selective pressure. Total alkaloids in C. maculatum were positively correlated with A. alstroemeriana herbivory intensity, being higher in Washington and lower in Illinois. This pattern was similar for the most common alkaloid, lambda-coniceine. Plants from New York and Washington, with higher herbivory intensity, invested two and four times more N to alkaloid synthesis compared to plants from Illinois. However, individual plants with lower concentrations of alkaloids from a single location in Illinois presented higher herbivory, indicating a preference of A. alstroemeriana for plants with low chemical defenses. These results support the EICA evolutionary hypothesis and suggest that A. alstroemeriana could be a selective agent for C. maculatum.