EFFECTS OF THE INSECTICIDE DURSBAN(R) 4E (ACTIVE INGREDIENT CHLORPYRIFOS) IN OUTDOOR EXPERIMENTAL DITCHES .1. COMPARISON OF SHORT-TERM TOXICITY BETWEEN THE LABORATORY AND THE FIELD

Using the insecticide Dursban(R) 4E (active ingredient chlorpyrifos) a s the test compound, results of laboratory acute single-species toxici ty tests with indigenous and standard test species were compared with short-term direct effects in outdoor experimental ditches (mesocosms). In the mesocosms a regression experiment was performed with nominal i nitial chlorpyrifos concentrations of 0.1, 0.9, 6, and 44 mu g/L. The mesocosms were sprayed once. Effects were investigated by sampling mac roinvertebrates and zooplankton and by doing in situ cage experiments with several species. Chlorpyrifos concentrations showed highest spati al and temporal variation within 2 d of treatment. Acute effects were observed on arthropods only and essentially were manifest on day 0. Sh ort-term direct effects in the mesocosms could be quantified by a regr ession method for seven of 120 species. For these species, 48- and 96- h median effective concentrations (EC50s) ranged from 0.1 to 3.4 mu g/ L and were in the same order of magnitude as their laboratory EC50s. S usceptibility of the most sensitive standard test species (Daphnia mag na; 48-h median lethal concentration [LC50], 1 mu g/L) was more or les s representative of susceptible indigenous species. In the mesocosms e ffects were negligible at the 0.1-mu g/L treatment level. A safety fac tor of 0.1 (48-h LC50 of Daphnia magna) may have protected almost all of the species in the community in the mesocosms against short-term di rect effects. A safety factor of 0.01 probably protected the most susc eptible taxa we found (laboratory 96-h EC10 for Gammarus pulex, 0.02 m u g/L; no-observed-effect concentration for Copepoda, <0.1 mu g/L). Th e question remains, however, of whether long-term (in)direct effects o n the populations or the community may occur at the 0.1-mu g/L treatme nt level.