Timing matters: impact of different frequencies of low pesticide pulses on aquatic invertebrates

Background

Aquatic communities are constantly exposed to multiple pulses of pesticides caused by spray drift or runoff that can have significant short- and long-term effects. In particular, short pulses can lead to chronic effects that may only occur considerable time after the end of exposure. Furthermore, in the environment, pesticide pulses vary in frequency, duration and intensity. For this reason, we conducted a higher tier study with a long observation period (98 days) using artificial indoor streams to compare the effects on an aquatic community, consisting of the oligochaete Lumbriculus variegatus, the snail Potamopyrgus antipodarum, the amphipod Gammarus pulex and the mayfly Ephemera danica. Four pesticide pulses with different recovery times (from one to seven days) between pulses were applied using the model insecticide deltamethrin. Two different deltamethrin concentrations (1st, 3rd and 4th pulses with 0.64 ng/L, 2nd pulse with 2 ng/L) were tested.

Results

For E. danica, we observed a significant increase in mortality and a significantly reduced emergence success with increasing days of recovery between the pulses. The snails appeared to benefit from the toxicity-induced mortality of the mayfly larvae; they showed an opposite pattern compared to the mayfly larvae that was reflected in significantly higher energy levels. In addition, the juvenile gammarids were very sensitive to pyrethroid exposure, reflected by the significantly lower number of offspring. No adverse effects were observed for L. variegatus.

Conclusion

Our results indicate that the temporal spacing of the insecticide pulses is important. Exposure over longer periods at longer intervals is more harmful than the same number of pulses in quick succession, particularly for insects. In addition, it is important to ensure that toxicity tests are conducted over a sufficiently long period of time to reliably detect chronic effects after short-term exposures. Furthermore, our results show that the current regulatory thresholds for deltamethrin do not adequately protect aquatic insects and should therefore be lowered.