Biological Control of Mile-a-Minute Weed: Effects of Abiotic Environmental Conditions.

Classical biological control of the invasive vine mile-a-minute weed, Persicaria perfoliata, by the imported Asian weevil Rhinoncomimus latipes has been effective in reducing P. perfoliata percent cover and biomass, and delaying and reducing seed production. However, the impact in the field has varied from year to year. We conducted greenhouse studies examining the impact of the weevil on the plant under controlled moisture conditions, and environmental chamber studies at different temperatures to determine the lower developmental threshold and growing degree-day requirements for weevil development. Results were compared with long-term field monitoring studies. In greenhouse trials, both water limitation and weevil herbivory reduced mile-a-minute reproduction and growth compared to well-watered plants. In environmental chambers, the lower developmental threshold for the weevil was estimated at 10.2 °C, and the number of degree days required for development from egg to adult estimated at 358, with an additional 139 degree days needed for the pre-oviposition period. Monitored field sites had relatively high P. perfoliata cover and low R. latipes densities in 2009 compared to either 2008 or 2010. Based on degree-day accumulations for these three years applied to a simple population growth model, fewer than half the number of weevils would have been produced by the end of the season in 2009 as in 2008, and only about a quarter as many as in 2010. Substantially higher spring rainfall probably also helped P. perfoliata develop more abundant cover in 2009 than in 2008 or 2010. Both temperature and moisture effects can help explain varying impacts of the weevil in different years with different weather conditions, and in different parts of the plant’s range.