Zooplankton avoidance behavior following spray drift exposure to fenpyroximate

A 28-d zooplankton community level study with Fenpyroximate was performed in indoor microcosms of 1 m(3) volume including water and sediment from natural sources. The microcosms were treated by simulating spray-drift of the formulated product. The nominal concentrations (0.1, 0.32, 0.56, 1.0, 3.2, and 10 mu g/L) were confirmed by chemical analysis of the application solutions. At a water depth of 30 cm, the peak concentrations were measured after 24 h. The most sensitive taxa were cyclopoid copepods with a clear reduction of abundance at 1.0 mu g/L and a water depth of 15-45 cm. The very fast recovery at that concentration within one day indicates that the decrease in abundance was caused by flight (behavioral) reactions followed by re-migration into the sampled part of the water column. At 3.2 mu g/L recovery took 28 days. For cladocerans, a similar but weaker response was observed at the next dose level. Active avoidance of chemical stress should be taken into account when predicting effects in the field or interpreting post application monitoring results. In micro- or mesocosm studies, zooplankton sampling strategies should be adjusted to include avoidance behavior.