Eurasian watermilfoil biomass associated with insect herbivores in New York

Abstract

A study of aquatic plant biomass within Cayuga Lake, NewYork spans twelve years from 1987-1998. The exotic Eurasianwatermilfoil(Myriophyllum spicatumL.) decreased in thenorthwest end of the lake from 55% of the total biomass in1987 to 0.4% in 1998 and within the southwest end from50% in 1987 to 11% in 1998. Concurrent with the watermilfoildecline was the resurgence of native species of submersedmacrophytes. During this time we recorded for thefirst time in Cayuga Lake two herbivorous insect species: theaquatic mothAcentria ephemerella, first observed in 1991, andthe aquatic weevilEuhrychiopsis lecontei, first found in 1996.Densities ofAcentriain southwest Cayuga Lake averaged 1.04individuals per apical meristem of Eurasian watermilfoil forthe three-year period 1996-1998. These same meristems hadEuhrychiopsisdensities on average of only 0.02 individuals perapical meristem over the same three-year period. A comparisonof herbivore densities and lake sizes from five lakes in1997 shows thatAcentriadensities correlate positively withlake surface area and mean depth, whileEuhrychiopsisdensitiescorrelate negatively with lake surface area and meandepth. In these five lakes,Acentriadensities correlate negativelywith percent composition and dry mass of watermilfoil.However,Euhrychiopsisdensities correlate positively with percentcomposition and dry mass of watermilfoil. Finally,Acentriadensities correlate negatively withEuhrychiopsisdensitiessuggesting interspecific competition.