A multi-lake field experiment to test the effectiveness of milfoil weevil for control of Eurasian watermilfoil

Background/Question/Methods

The invasive Eurasian water-milfoil (EWM) is a widespread aquatic plant that continues to spread among North American lakes. EWM grows to nuisance densities in many lakes and treatment with herbicides is a common control method. As an alternative to chemicals, biological control using several insect species has been explored. Most promising is the native milfoil weevil, Euhrychiopsis lecontei, which is known to favor EWM over native plants and be associated with population declines in the field. We designed a four-year field experiment to test the effectiveness of weevil augmentation on EWM biomass and recovery of native plants under natural conditions in 7 northern Wisconsin lakes.

Results/Conclusions

Preliminary surveys of 14 EWM beds during summer 2012 indicated that plant diversity varied from 7-16 native species per bed and EWM/native biomass varied over a 150-fold range among study beds. Background weevil densities varied over a 10-fold range and weevil damage to EWM stems was common in some lakes. The results from this 4-year study will guide our understanding of the conditions in which weevil augmentation can be effective in biological control of EWM.