AN ASSESSMENT OF THE BIOLOGICAL CONTROL OF PURPLE LOOSESTRIFE (LYTHRUM SALICARIA) USING TWO LEAF-FEEDING BEETLES GALERUCELLA CALMARIENSIS AND GALERUCELLA PUSILLA IN BROWN COUNTY WISCONSIN
Jeremiah J. Schiefelbein
As part of the biological control program against purple loosestrife (Lythrum salicaria L.), two exotic leaf-feeding beetles Galerucella pusilla and Galerucella calmariensis have been distributed throughout North America. Limited monitoring of release sites has identified several important questions, including: 1) Is one species more prevalent than the other? 2) Are the numbers of individual adults released or the length of time they inhabit a site more important in terms of L. salicaria control? Site selection for this study was based on locations where the University of Wisconsin Extension Office of Brown County had released individuals in 1997, 1998, 2001, and 2002. Two methods of data collection were employed during the study. The first method examined the density (abundance/m2) of egg masses, larvae, adults, the number of shoots, the number of flowers, and an estimation of damage; the second method examined the same variables on a randomly selected individual shoot system. Flower heights also were measured during August sampling in quarter m2 quadrats. Adult beetles were also collected for identification to species level. A total of 298 Galerucella spp. (56% G. calmariensis and 43.9% G. pusilla) were collected during June sampling. The difference between the two species was not significant. Density (abundance/m2) of egg masses, adults, shoots and the estimated damage during June were all found to be significantly higher for 1997 release sites as compared to other release sites of 2001 and 2002. During August, egg masses and estimated damage were significantly higher for 1997 release sites, while number of shoots, flowers, and flower height were significantly lower for those release sites. The numbers of egg masses, adults, and the damage category per individual shoot system were all significantly higher for 1997 release sites during June sampling as compared to other release sites, while eggs, adults, damage, shoots, flowers, and flower height all showed significance during August sampling. Numbers of individuals released, density (abundance/m2) of egg masses, adults, and the damage category were all higher for releases of 500 individuals as compared to releases of 100 and 2000 individuals during June sampling while only shoot density, flower density, and flower height were significantly lower for releases of 2000 individuals during August sampling. Samples of individual shoot systems during June showed greatest numbers of egg masses and adults at sites where 500 individuals were released while damage was highest at sites where 2000 individuals were released. Due to the correlation between year of release and the number of individuals released, (greater numbers released during earlier years as compared later years) further analysis of 2001 and 2002 release sites suggests that year of release and therefore the length of time Galerucella spp. Inhabit a particular site to be more important than the number of individuals released in terms of control of L. salicaria. This is the first large scale study to examine post release monitoring of a number of variables.