Pesticides play an important role in modern agriculture, but there are risks to non-target wildlife and habitats. Consequently, new pesticides must undergo rigorous tests for safety before authorisation for use in the EU. Pesticides can enter fresh water streams directly via spray drift or indirectly via surface runoff or drainflow. However, while pesticide contamination is often blamed for reducing biodiversity in freshwater, conclusive 'cause and effect' evidence is actually rare.
UK researchers combined the EU regulatory modelling approach for predicting pesticide exposure concentrations, with a field study carried out along a stream flowing through woodland, rough pasture and farmland planted with wheat. Three sections of the stream were surveyed: a control section where no pesticides were applied, and two sections where pesticides were applied, one with a 6 metre (about 20 foot) buffer zone to the stream edge and one without a buffer zone. This allowed researchers to separate contamination from the direct effects of spray drift from the indirect effects of rainfall and other land management practices, such as ploughing or irrigation.
Over two years, four chemicals, the pesticides cypermethrin (a pyrethroid pesticide) and chlorpyrifos (an organophosphorous pesticide) and the herbicides isoproturon and simazine, were applied on the wheat fields, three times each. The researchers surveyed more than 50 types of insects, or benthic macroinvertebrates, including shrimps, fly larvae, mayfly nymphs, snails and worms from each section of the stream to monitor responses to the pesticide applications. Researchers also populated the experimental sections with the crustacean Gammarus pulex as it is especially sensitive to organophosphorous insecticides.
The results revealed that pesticides entered the waterways mainly through spray-drift rather than drainage and surface runoff. In addition, the estimated pesticide concentrations in the 'no-buffer' zone were four times higher than in the buffer zone. Herbicide concentrations were too low to be toxic to invertebrates in both the no-buffer and buffer zones.
Although the concentration of the pesticides in the stream exceeded toxicity limits at times, the study suggests this only occurred for short periods. In addition, although pesticides had negative effects on individual organisms, such as reduced feeding rates, changes were not seen in the population as a whole. As the pesticides were only applied occasionally, it may be that organisms could cope in the short-term and that other organisms drifting downstream could help populations recover.