Distribution of biotypes and hybrids of Myriophyllum spicatum and associated Euhrychiopsis lecontei in lakes of Central Ontario, Canada

Eurasian watermilfoil (Myriophyllum spicatum) has been present in Ontario's waterways since the 1960s, leading to various forms of mechanical, chemical, and biological control to manage nuisance milfoil growth. New concerns have recently arisen surrounding increased invasiveness and resiliency to the biological control agent, the milfoil weevil (Euhrychiopsis lecontei), through hybridization of M. spicatum with the native northern watermilfoil (Myriophyllum sibiricum) and biotypic variation within M. spicatum. In 2010 we performed a survey of 21 lakes in Central Ontario, Canada, to determine the distribution of M. spicatum biotypes and hybrids and associated E. lecontei populations. We identified 2 distinct biotypes of M. spicatum and M. spicatum x M. sibiricum hybrids. One biotype of M. spicatum was widespread throughout the sampling region and co-occurred within the same lakes as hybrid milfoil populations and the lesser-observed M. spicatum biotype. Hybrid populations were only identified in lakes where both parental species have been historically present. Although there were noticeable differences in the geographic distribution of Eurasian watermilfoil biotypes and hybrids, differences in E. lecontei density did not seem to be impacted by milfoil type. E. lecontei occurred naturally in all lakes, with densities ranging up to 0.7 weevils per stem, and was not significantly different by milfoil type. Instead, variation of E. lecontei density was closely related to environmental variables including patch depth and conductivity, and weevil herbivory likely affected shoot tissue nitrogen content.