Potential impact of native natural enemies on Galerucella spp. (Coleoptera: Chrysomelidae) imported for biological control of purple loosestrife: a field evaluation

Can resident natural enemies impede the action of herbivores introduced for biological control of weeds, and if so, can their level of activity be predicted from tests that use resident herbivores as hosts? To examine these questions, exclusion experiments were done at three sites in central New York State which focused on the leaf beetle Galerucella nymphaeae in stands of the introduced weed, purple loosestrife (Lythrum salicaria). This beetle is congeneric with two European species (G. calmariensis and G. pusilla) that are being imported and distributed in North America for biological control of purple loosestrife. General predators, including the ubiquitous lady beetle Coleomegilla maculata, preyed on G. nymphaeae eggs from late spring to the end of summer. During this period, approximately one-third of G. nymphaeae's egg masses were attacked, whereas the proportion of eggs within each egg mass that were damaged or consumed increased from about 50 to 90%. At all sites, the survival of G. nymphaeae larvae and pupae was lower in open than in closed cages during mid- and late summer, but not earlier. The presence of arthropod predators and the absence of parasitized or diseased beetles indicate that predators were largely responsible for the reduced survival in open cages. From these results, we predict that resident species of general predators, at times, may hinder the colonization or effectiveness of the European G. calmariensis and G. pusilla. Therefore, the continued use of protective cages when making introductions and during subsequent distribution of these natural enemies is desirable. Moreover, the action of general predators should be considered in subsequent evaluations of biological control efforts involving Galerucella spp.