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

Published source details Newman R.M. (2004) Biological control of Eurasian watermilfoil by aquatic insects: basic insights from an applied problem. Archiv fur Hydrobiologie, 159, 145-184.
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Summary

Eurasian watermilfoil Myriophyllum spicatum is a widespread invasive species in North America and has infested most (45) US states and three Canadian provinces, and millions of dollars are spent annually on control. Herbivory by aquatic insects can be substantial, and has been shown to result in 50 to 95% reductions in Eurasian watermilfoil biomass.

A review was undertaken of the potential of three insect species (milfoil weevil Euhrychiopsis lecontei (native), a pyralid moth Acentria ephemerella (native to Europe) and a midge Cricotopus myriophylli (native)) already present in northern USA and southern Canada, as biological control agents of Eurasian watermilfoil Myriophyllum spicatum.

Euhrychiopsis lecontei: Of the three insects investigated, the native milfoil weevil appears to offer the greatest potential and has been associated with milfoil declines across North America. The weevil is highly specific to Myriophyllum species as its food plant. During the summer, all life stages subsist on submersed watermilfoil, and after 3-5 generations are produced, adults move to shore to overwinter in leaf litter. The weevil has been shown to control Eurasian watermilfoil (by stem mining) in several field studies, and in laboratory and mesocosm experiments (using artificial pools designed to approximate natural conditions within which environmental factors can be manipulated). The milfoil weevil has a preference for Eurasian watermilfoil and prefers and performs better on it compared to native Myriophyllum species. Declines of Eurasian watermilfoil in the field due to weevil infestation occur at sufficient weevil density, and resultant declines have been shown to induce positive responses of native aquatic plants. However, at many sites, weevil populations are too low to be effective in control. Predation by sunfish Lepomis is likely to be an important factor limiting densities of adult weevils although this has not been demonstrated.

Acentria ephemerella: The naturalized pyralid moth A.ephemerella (native to Europe) whose caterpillar feeds on Myriophyllum spp. and some other aquatic plants, has been associated with milfoil declines in some eastern states, but rarely reaches high enough densities in the Midwest to be effective as a control.

Cricotopus myriophylli: Similarly, the native midge C.myriophylli, does not generally reach high densities and is therefore ineffective.

Conclusions: The weevil, and perhaps the pyralid moth and midge, may be a useful biocontrol agent if factors limiting their population density can be ameliorated.

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