Impact of a systemic insecticide on the community of arthropods associated with elm trees in the urban forest

Imidacloprid is one of the most widely used insecticides in the world. It offers many benefits, such as ease and safety of application and decreased exposure to non-target organisms. Recently, imidacloprid has been used to treat thousands of elm trees as a preventative measure in Asian long-horned beetle eradication efforts in Central Park, New York. In response to reports of pest outbreaks on these trees, we investigated effects of these applications, on the arthropod community in Central Park and at a replicate site at the University of Maryland, College Park, Maryland from 2005 to 2007. Elm trees at both locations received applications of imidacloprid, and arthropod fauna was compared between treated and untreated trees by destructive sampling. Principle Response Curves and analyses of variance were used to evaluate differences in arthropod abundance and community structure between treatments. Our results indicated that elms treated with imidacloprid housed significantly more insects and arachnids. Additionally, spider mites (Tetranychus schoenei, McGregor) were the taxon driving this pattern. Applications of the insecticide result in secondary outbreaks of the mites at both sampling locations across all sampling years. Our findings could not point to natural enemy disruption as a mechanism of the increased abundance of mites on treated elm trees. Thus, we evaluated elm spider mite fecundity, and found that mites consuming leaves from imidacloprid treated elms laid significantly more eggs than mites feeding on untreated leaves. Greater abundance of mites on elms receiving imidacloprid did not seem to arise from elimination of natural enemies, but rather through a plant-mediated or direct positive effect of the insecticide on mite fecundity. Further experiments are planned to elucidate the exact mechanism that leads to increased fecundity exhibited by the spider mites.