Biological control of Eurasian watermilfoil by aquatic insects: Basic insights from an applied problem

Eurasian watermilfoil (Myriophyllum spicatum) is one of North America's most troublesome exotic weeds; it has infested 45 states and three Canadian provinces and millions are spent annually on control. Three insects have been considered for the biological control of Eurasian watermilfoil and all three taxa are present in the northern US and southern Canadian provinces. The naturalized pyralid caterpillar, Acentria ephemerella, has been associated with milfoil declines in some eastern states, but rarely reaches high densities in the midwest. The native midge, Cricotopus myriophylli, also does not generally reach high densities, but deserves further study. The native milfoil weevil, Euhrychiopsis lecontei, has been associated with milfoil declines across North America. The weevil, which is highly specific to watermilfoil species, has been shown to control Eurasian watermilfoil via stem mining in laboratory, tank and mesocosm studies, as well as in several field studies. During the summer, all life stages subsist on submersed watermilfoil, and after 3-5 generations are produced, adults move to shore to overwinter in shoreline leaf litter. The milfoil weevil has an induced preference for the exotic Eurasian watermilfoil and prefers and performs better on the exotic compared to native watermilfoils. Field declines of Eurasian watermilfoil due to weevil feeding will occur with sufficient weevil density and a positive native plant community response. At many sites, weevil populations are too low to effect control. Predation by sunfish (Lepomis) is likely an important factor limiting densities of adult weevils and other herbivores. Population modeling indicates that adult reproductive lifespan and fecundity are key factors for end-of summer population densities. The weevil and perhaps the other herbivores can be useful biocontrol agents if factors limiting their density can be ameliorated. This work demonstrates that herbivory by aquatic insects can be substantial, resulting in 50-95 % reductions in plant biomass and shifts in macrophyte community structure. Furthermore, important insights on the evolution of host range and host range expansion have been gained from study of this system and a better understanding of trophic cascades in littoral zones is being developed.