Impact of Insecticide Exposure Duration on Heptageniid Mayflies.

Neonicotinoids like clothianidin are commonly applied in agricultural areas where they leach into aquatic systems and expose sensitive aquatic insects. While mayflies are known to be highly sensitive to clothianidin, the connection between laboratory toxicity assays and exposure dynamics in natural systems is poorly explored. We conducted low dose clothianidin exposures on the mayfly Stenacron interpunctatum to better understand how the temporal dynamics of insecticide pulses into stream systems might impact aquatic insects. We found that short duration (<24 hr) exposures to clothianidin (0.7-8.9 µg/L) failed to produce noticeable lethal or sublethal effects within 10 days after the exposure ended. With continuous exposure, we found that toxic effects began within 72 hr of exposure to our highest concentration (8.9 µg/L) while lower concentrations required longer exposures to have toxic effects. These results indicate that, given the low concentrations of neonicotinoids commonly observed in agricultural surface waters, a single typical pulse duration is unlikely to have acutely toxic effects on mayfly populations. However, repeated pulses could have additive effects resulting in greater impacts.