Ecological effects of mosquito control on zooplankton, insects, and birds

We completed an integrated, 6-year study on the potential ecological effects of two mosquito control agents, methoprene (applied as Altosid(TM) sand granules) and Bacillus thuringiensis var. israelensis (Bti, applied as Vectobac-G(TM) granules), on zooplankton, insects, and breeding birds in wetlands of central Minnesota, USA, from 1988 to 1993. The study was a before-and-after design with pretreatment (1988-1990) and posttreatment (1991-1993) of 27 wetlands. Study sites were randomly selected and placed within one of three groups of sites, nine control, nine Bti-treated, and nine methoprene-treated. Selected populations of zooplankton, insects, and breeding birds were sampled within each of these wetlands. Insect densities were reduced by 57 to 83% and biomass was reduced by 50 to 83% in the second (1992) and third (1993) years of treatment. No negative effects on zooplankton or breeding birds could be attributed to treatment or changes to insect communities. Many factors may explain the lack of effects on breeding birds including, reductions in insects occurred after the nesting season was over, nest loss rates due to predation were very high (70%) and may have been a greater limiting factor to birds than mosquito control, and the density of breeding birds may be below carrying capacities, especially because not all wetlands in the landscape were treated and sufficient food may have been available. It is unclear what the long-term consequences of insect reductions mean to wetland health. The lack of close coupling between zooplankton, insects, and breeding birds probably reflects the ecological complexity of these wetlands such as the presence of other limiting factors on population distribution and abundance. Although the study period was relatively long (3 years of treatment) compared with most ecological studies of pesticides, it may not have been long enough to fully predict the effects of decades of continued mosquito control.