Eurasian watermilfoil (Myriophyllum spicatum) and the watermilfoil weevil (Euhrychiopsis lecontei) in three Michigan lakes: Investigating demography at the whole-lake scale

Eurasian watermilfoil (Myriophyllum spicatum L., EWM), a nuisance exotic aquatic plant, can affect the physical, chemical and biological conditions of a lake, as well as impede recreational opportunities. The watermilfoil weevil (Euhrychiopsis lecontei), a native herbivore specialist of watermilfoil species, has shown promise as an EWM biocontrol agent. Although EWM suppression has been attributed to E. lecontei in small-scale studies and lake-wide EWM collapse has been scientifically documented in a few cases, E. lecontei densities remain low in many lakes. Little research has been done on E. lecontei demography at the whole-lake scale. Therefore, we investigated the variation of E. lecontei density across three EWM-infested Michigan lakes summer 2008. Each lake was sampled three times with 100+ samples per visit, providing us with EWM stem densities, tip densities, and biomass and E. lecontei densities. At the whole-lake scale, EWM and weevil densities varied depending upon the lake and time period without a discernable trend. EWM was similar over time in one lake, increased in one and decreased in another. Coincident with these results, E. lecontei decreased in one lake, did not vary and increased, respectively. These results help us to understand an important herbivore-plant relationship as well as EWM management.