Effects of the Herbicide Atrazine on Aquatic Insect Community Structure and Emergence

Abstract

The herbicide atrazine was applied to 0.045—ha experimental ponds in two replicate concentrations of 0, 20, 100, and 500 mg/L each. Physical, chemical, and biological variables were measured and aquatic insect community structure was monitored in the eight ponds using partially submerged funnel emergence traps. Atrazine had no effect on water temperature or oxygen concentration. Mean turbidity did increase significantly with atrazine concentration, but turbidity could not be correlated with abundance or species richness of emerging insects. Macrophyte production decreased with increasing herbicide concentration; however, the macroalga Chara sp. showed resistance up to 100 mg/L. The benthic insect community was also negatively affected by atrazine. Abundance of emerging individuals of the chironomid Labrundinia pilosella was significantly reduced at atrazine concentrations as low as 20 mg/L, while other, less abundant species showed similar declines. Benthic insect species richness (S), species equitability (J'), and total emergence all declined significantly with atrazine addition. In general, abundances of nonpredatory insects were greatly reduced with the addition of atrazine (20 mg/L), while predatory insects showed no response to the herbicide. In addition, emergence periods of several herbivorous insect species shifted to an earlier time in atrazine—treated ponds; the shift was statistically significant for Oxyethira pallida. The lowest concentration at which atrazine affected aquatic insects in the pond system (20 mg/L) is one order of magnitude lower than the lowest concentration previously shown to have a direct toxic effect on the midge Chironomus tentans in single—species laboratory experiments. Furthermore, because of the differential response of predatory and nonpredatory insect species to atrazine in this study, it is suggested that the effect of the herbicide on the insect community in the experimental ponds may be primarily indirect, presumably through reduction of the food of nonpredators (periphyton, macrophytes) and, to some extent, their habitat (macrophytes).