Eurasian watermilfoil biomass associated with insect herbivores in New York

A study of aquatic plant biomass within Cayuga Lake, New York spans twelve years from 1987-1998. The exotic Eurasian watermilfoil (Myriophyllum spicat um L.) decreased in the northwest end of the lake from 55% of the total bio mass in 1987 to 0.4% in 1998 and within the southwest end from 50% in 1987 to 11% in 1998. Concurrent with the water milfoil decline was the resurgenc e of native species of submersed macrophytes. During this time we recorded for the first time in Cayuga Lake two herbivorous insect species: the aquat ic moth Acentria ephemerella, first observed in 1991, and the aquatic weevi l Euhrychiopsis lecontei, first found in 1996. Densities of Acentria in sou thwest Cayuga Lake averaged 1.04 individuals per apical meristem of Eurasia n watermilfoil for the three-year period 1996-1998. These same meristems ha d Euhrychiopsis densities on average of only 0.02 individuals per apical me ristem over the same three-year period. A comparison of herbivore densities and lake sizes from five lakes in 1997 shows that Acentria densities corre late positively with lake surface area and mean depth, while Euhrychiopsis densities correlate negatively with lake surface area and mean depth. In th ese five lakes, Acentria densities correlate negatively with percent compos ition and dry mass of watermilfoil. However, Euhrychiopsis densities correl ate positively with per cent composition and dry mass of watermilfoil. Fina lly, Acentria densities correlate negatively with Euhrychiopsis densities s uggesting interspecific competition.