A biogeographic perspective on watermilfoil declines: additional evidence for the role of herbivorous weevils in promoting declines?

Recent research suggests that a native, herbivorous weevil (Euhrychiopsis lecontei (Dietz)) may play a major role in promoting or causing Eurasian watermilfoil (Myriophyllum spicatum L.) declines in North America. When the locations of all unexplained (i.e., not related to a specific management practice) North American Eurasian watermilfoil declines were plotted there was a non-random distribution of declines. The majority of declines have occurred in northern states in the USA and some Canadian provinces which is the original range of E. lecontei and its native host, northern watermilfoil (Myriophyllum sibiricum Komarov (=M. exalbescens Fernald)). If other factors were important in producing declines (e.g., competition with native species, accumulation of toxins, changes in water clarity or sediment chemistry) one would not expect to see such a latitudinal bias in the location of declines. This pattern of declines supports the hypothesis that E. lecontei is an important agent in promoting or causing Eurasian watermilfoil declines. Alternative factors (e.g., climate, other herbivores) that could have produced this pattern were also considered.