The Potential Role of Sunfish in Limiting Biological Control of Eurasian Watermilfoil with Herbivorous Insects

Eurasian watermilfoil (Myriophyllum spicatum) is one of the most troublesome invasive submersed aquatic macrophyte world-wide. It can be controlled with herbivorous insects, such as the milfoil weevil (Euhrychiopsis lecontei). In many lakes, however, the insects fail to sustain sufficient densities to control the plant.  Previous work in Minnesota and Washington suggested that predation by sunfish in lakes with high sunfish densities may prevent development of adequate herbivore populations.  Additional evidence from several lakes in Carver County Minnesota supports this hypothesis. Milfoil weevil populations in Lakes Ann and Riley are low with total weevil densities of 0.13 ± 0.1 (2SE) per stem and 0.2 ± 0.18 per stem respectively.  Adult weevil densities are less than 0.03 per stem and bluegill catches in both lakes exceed 50 per trapnet set.  Eurasian watermilfoil biomass exceeds 150 g/m² dry in Lake Ann and there is no evidence of milfoil control by herbivores in either lake. Nearby Lake Susan has a low population of sunfish (< 6 per trapnet) and supports up to 1 weevil per milfoil stem with 0.15 adults per stem.  Eurasian watermilfoil biomass has remained less than 6 g/m². Strategies to reduce sunfish density are needed for biological control to be successful.