The Effects of Predation on Milfoil Weevils (Euhrychiopsis lecontei) and Impact on Eurasian Water Milfoil (Myriophyllum spicatum)

Abstract

Eurasian watermilfoil (EWM), Myriophyllum spicatum L, is an invasive aquatic macrophyte in North America. The aquatic milfoil weevil, Euhrychiopsis lecontei, is a native herbivore on milfoils that has been used as a biological control agent for EWM. The objective of this study was to determine if predation by sunfishes (Lepomis spp.) and damselfly nymphs (Lestidae spp.) can suppress weevil populations below the density necessary to control EWM. In Lake Joanis, Wisconsin, where supplemental weevil stocking had not led to an increase in weevil density, 250 gallon (944 L) mesh exclusion cages stocked with weevils were used to manipulate densities of small bluegill (Lepomis machrochirus) to 0, 2, and 4 per cage. Results indicated an inverse relationship between bluegill and weevil densities. Mean densities of weevils were significantly different among treatments (ANOVA, p=0.005). The average density of weevils in cages with no bluegill was 0.31 w/s. Cages with two bluegill averaged 0.02 w/s. Cages with four bluegill averaged 0.01 w/s. These results indicate that sunfish, even at relatively low densities, substantially reduce weevil densities. Protecting stocked weevils in cages may allow them to establish higher densities where they may serve as a control for milfoil in some lakes. Variable success of weevils in controlling EWM in different lakes reported in the literature may be attributable to variable densities of sunfish populations. A second study addressed the ability of damselflies (Lestidae spp.) to significantly suppress weevil densities. Ten gallon experimental tanks (37.5 L) were stocked with 8 adult weevils and 0, 3, 6, and 12 damselflies. Although damselflies have been observed feeding on weevils, differences between treatments were not significant (ANOVA, p=0.78). Further research into insect predation on weevils is warranted.