Variables determining the impact of diazinon on aquatic insects: Taxon, developmental stage, and exposure time

Several variables determine the impact of a pesticide on aquatic invertebra tes. In this study, aquatic insects were subjected to the common insecticid e diazinon: we analyzed the variables taxon, developmental stage, and expos ure time. Effects of diazinon on the caddis fly Hydropsyche angustipennis a nd the midge Chironomus riparius were determined in the laboratory during d ifferent exposure times (48 and 96 h) using mortality, activity, and growth as end points. Last instars of both species displayed a clear behavioral r esponse at concentrations much lower than those affecting survival. Doublin g the exposure time from 2 to 4 d decreased survival of midges and caddis f lies by a factor 1.4 to 8.4. The 96-h 50% lethal concentrations were: 1.3 m u g/L (first instar of the caddis Ay), 29 mu g/L (fifth instar of the caddi e fly), 23 mu g/L (first instar of the midge) and 167 mu g/L (fourth instar of the midge). Within the spectrum of rested insects (nine species for whi ch 48-h 50% lethal concentrations have been reported in the literature), H. angustipennis is the second most sensitive, and C. riparius the most toler ant species. However, the ranking of species strongly depends on the develo pmental stage; differences between species are often smaller than differenc es between instars of one species. The large difference in sensitivities be tween young and old larvae imply that the impact of a pesticide strongly de pends on the season of occurrence. Runoff from pesticide applications on cr ops is more likely to occur in spring and summer and may have a relatively greater impact on insect communities since young larvae prevail in these se asons. In addition, recovery of typical riverine insects such as H. angusti pennis from incidental exposure will be slow, considering their relatively long life cycle.