Wednesday, August 4, 2010 - 3:20 PM

COS 70-6: Specialist weevil mediates negative indirect interactions between a threatened plant, Mead’s Milkweed (Asclepias meadii), and weedy congeners

Steven M. Roels, University of Kansas


Asclepias meadii (Mead’s Milkweed) is a federally threatened plant found on virgin tallgrass prairies. This species has a very slow population growth rate, which is a challenge for both recovery and restoration efforts. Although many factors contribute to observed low population recruitment, I found that herbivory by a milkweed specialist weevil, Rhyssomatus lineaticollis, is responsible for a substantial reduction in reproductive success. Literature descriptions of R. lineaticollis life history infrequently describe host species other than Asclepias syriaca (Common Milkweed), but large populations of this species, or others in the genus Asclepias, may act as reservoirs for weevil populations, which then migrate to the rare A. meadii. The negative interaction between A. meadii and its more abundant congeners, mediated via a shared enemy, may be an example of an ecological phenomenon often called “apparent competition.”


Observations of a large population of marked plants revealed that at least 25% of flowering A. meadii ramets failed to produce viable seeds due to weevil activity on stems and developing fruit. Damage to stems occurs early in the season, and invariably leads to ramet death in A. meadii. Damage to follicles occurs later, and results in seed destruction and premature follicle dehiscion. Each damage type corresponds with a generation of the weevil, which is bivoltine in Kansas. In field surveys of northeast Kansas, I found weevil use of seven Asclepias species, including A. syriaca and A. viridis (Green Antelopehorn Milkweed), which are the most abundant milkweeds in the region. In order to evaluate the relationships between R. lineaticollis and each milkweed host, as well as the indirect interactions between hosts, I evaluated the timing and degree of weevil infestation in several populations of A. meadii, A. syriaca, and A. viridis. I also compared weevil activity patterns to the phenologies of each host species. My results have practical implications for efforts to manage and reestablish populations of Mead’s Milkweed.