A decline of Eurasian watermilfoil in Minnesota associated with the milfoil weevil, Euhrychiopsis lecontei

The native milfoil weevil, Euhrychiopsis lecontei Dietz, is a candidate biological control agent for the exotic Eurasian watermilfoil (Myriophyllum spicatum L.) in northern North America. Declines of Eurasian watermilfoil populations have been associated with the weevil but many of these examples are poorly documented. We report the first documented decline of Eurasian watermilfoil in Minnesota due to the milfoil weevil. Eurasian watermilfoil in Cenaiko Lake declined from 123 ± 45 g dm/m² (±2SE) in July 1996 to 23 ± 14 g/m² in September 1996 and remained at <5 g/m² in 1997. It increased to 44 g/m² in June and July of 1998, but declined to 12 ± 10 g/m² in September 1998; the decline persisted through 1999. Biomass of other aquatic macrophytes increased while milfoil biomass decreased and other macrophytes remained >95 g/m², or more than 90% of plant biomass in 1997, and >200 g/m² and >85% of plant biomass in 1998. In July 1996, milfoil weevil densities in Cenaiko Lake were the highest yet observed in Minnesota (103 ± 42/m²; 1.6 per stem), but declined with decreasing milfoil density, from 8.1/m² in September 1996 to below detection in September 1997; weevil density increased to over 2 per stem in September 1998. Densities of two herbivorous lepidopterans, Acentria ephemerella and Parapoynx sp. increased after the decline of Eurasian watermilfoil, however, these insects were associated primarily with Ceratophyllum, Potamogeton and Zosterella and did not appear to be the main cause of the decline. A decline in stem and leaf carbohydrates of milfoil and an increase in sediment ammonium accompanied the decline of Eurasian watermilfoil. These observations indicate that Euhrychiopsis lecontei can reach adequate densities to effect persistent declines in Eurasian watermilfoil in Minnesota, but the lack of declines in other lakes indicates that more research is needed.