PERTURBATION OF THE AQUATIC INVERTEBRATE COMMUNITY OF ACIDIC BOG PONDS BY THE INSECTICIDE FENITROTHION

In forest areas receiving aerial application of pesticides, small bodies of water, which are often difficult to see from low-flying spray planes, are at risk of receiving high concentrations of contaminants. To determine the sensitivity of bog pond invertebrate communities, formulated fenitrothion was applied at ground level with a mist blower to four acidic bog ponds near Lake George, New Brunswick, Canada. Fenitrothion fate and persistence, insect emergence, gyrinid populations, and the invertebrate community inhabiting the floating sphagnum fringe of the ponds were sampled before and after treatment. The invertebrate community was sensitive to formulated fenitrothion. Concentrations of fenitrothion in bog pond water were in ranges that would be expected after a direct aerial application, with no buffer zone, of 2 x 210 g active-ingredient/ha, the operational emission rate used for spruce budworm. Insect emergence was reduced compared to control ponds for 6-12 weeks following fenitrothion treatment. The population densities of Chironomidae and Ceratopogonidae were reduced by more than 50% for 1 month after treatment. The densities of most other benthic insect taxa were also reduced. Some reductions in density lasted over winter into the next year. This contrasted with the usual small and transitory effects of fenitrothion on aquatic invertebrate communities in streams or large lakes. Insecticide-induced suppression of aquatic insects and adult insect emergence in the small bog ponds allowed Hydrachnellae, Oligochaeta, and Nematoda to dominate the aquatic invertebrate community. Export of energy and nutrients to the terrestrial system through insect emergence was thus reduced and cycling or retention within the aquatic system increased. Sustained suppression of insect emergence could result in nutrient accumulation in the ponds. Substantially reduced insect emergence would be expected to have effects on dependent predators in the bog ecosystem.