Locomotory behaviour in the freshwater amphipod Gammarus pulex exposed to the pyrethroid cypermethrin

The use of pesticides in modern agriculture results in contamination of streams. Pyrethroid insecticides in particular may lead to long-term effects at the population and ecosystem levels. A key phenomenon observed upon pulse exposure to pyrethroids is drift, where freshwater invertebrates are carried along by the current and disappear from the contaminated stretch of the stream. The ecologically important freshwater amphipod Gammarus pulex is among the most sensitive species, and the aim of this study was to provide a detailed description of the locomotory behaviour displayed by this species during exposure to the pyrethroid cypermethrin, using an automated video tracking system. Marked changes in locomotory behaviour were observed within minutes of exposure to very low, environmentally realistic cypermethrin concentrations at which mortality was observed only after several days of continuous exposure. Exposure resulted in a biphasic behavioural response: at 0.01 mu g.L-1 G. pulex displayed hyperactivity, which was maintained, until at higher exposure concentrations (>= 1 mu g.L-1) immobilisation followed. The results indicate that the methodology constitutes a very powerful tool for detecting sublethal effects of pesticides on non-target stream invertebrates; effects that may be predictive of impacts at the population level.