Did a North American Herbivore Cause the Decline of an Exotic Plant?
The Eurasian watermilfoil (Myriophyllum spicatum) population in Brownington Pond, Vermont, declined between 1986 and 1989. Watermilfoil covered 0-11 ha of the littoral zone in 1986. Less than 0.5 ha remained in 1989. An herbivorous weevil (Euhrychiopsis lecontei), which is native to North America, was found associated with this watermilfoil population and we hypothesized that this weevil played a role in the decline. We monitored watermilfoil and E. lecontei populations in Brownington Pond from 1990 through 1992 by (1) mapping the location and extent of beds in the pond, and (2) determining watermilfoil biomass along permanent transects. The abundance of weevil eggs, larvae, pupae, and adults were monitored by sampling individual watermilfoil stems. Watermilfoil cover increased to ha by 1991 and then declined again to ha by 1992. The reduction in watermilfoil biomass from 1991 to 1992 ranged from 4- to 30-fold depending on location. Mean weevil abundance increased from 1990 (< l/stem)
