THE EFFECT OF MALATHION-60 ON GAMMARUS-PULEX (L) BELOW WATERCRESS BEDS

The organophosphorus insecticide Malathion 60 is used to control terrestrial arthropod pests on watercress, Rorippa nasturtium-aquaticum L. (Hayek). The use of this pesticide could damage aquatic invertebrate communities in nearby chalk streams, so an in situ experiment was undertaken with the amphipod Gammarus pulex L. to determine the extent of this risk. Caged G. pulex were placed below two selected watercress beds and their associated settling pools, and their mortality, feeding rate, and acetylcholinesterase activity measured before and after the application of Malathion 60 to the watercress crop. The differences found between prespray and postspray samples of G. pulex taken from the stations immediately below both watercress beds follow: (a) the acetylcholinesterase activity was significantly lower at both stations after spray application; (b) the mortality rate was significantly higher at one station after spray application, but not at the other; (c) there were no clear effects on feeding rates at either station. Also, there were no significant detrimental effects at either of the stations located below the settling pools. Results from this study suggest that the use of Malathion 60 on watercress beds at recommended rates presents little risk to G. pulex living in streams below watercress farms if bed effluent passes through a settling pool before release.