Environmental Toxicology VARIABLES DETERMINING THE IMPACT OF DIAZINON ON AQUATIC INSECTS: TAXON, DEVELOPMENTAL STAGE, AND EXPOSURE TIME

Section Aquatic Ecology and Ecotoxicology, ARISE, University of Amsterdam, Kruislaan 320, 1098 SM Amsterdam, The Netherlands(Received 9 October 1998; Accepted 12 May 1999)Abstract—Several variables determine the impact of a pesticide on aquatic invertebrates. In this study, aquatic insectsweresubjectedto the common insecticide diazinon; we analyzed the variables taxon, developmental stage, and exposure time. Effects of diazinonon the caddis fly Hydropsyche angustipennis and the midge Chironomus riparius were determined in the laboratory during differentexposure times (48 and 96 h) using mortality, activity, and growth as end points. Last instars of both species displayed a clearbehavioral response at concentrations much lower than those affecting survival. Doubling the exposure time from 2 to 4 d decreasedsurvival of midges and caddis flies by a factor 1.4 to 8.4. The 96-h 50% lethal concentrations were: 1.3mg/L (first instar of thecaddis fly), 29 mg/L (fifth instar of the caddis fly), 23 mg/L (first instar of the midge) and 167mg/L (fourth instar of the midge).Within the spectrum of tested insects (nine species for which 48-h 50% lethal concentrations have been reported in the literature),H. angustipennis is the second most sensitive, and C. riparius the most tolerant species. However, the ranking of species stronglydepends on the developmental stage; differences between species are often smaller than differences between instars of one species.The large difference in sensitivities between young and old larvae imply that the impact of a pesticide strongly depends on theseason of occurrence. Runoff from pesticide applications on crops is more likely to occur in spring and summer and may have arelatively greater impact on insect communities since young larvae prevail in these seasons. In addition, recovery of typical riverineinsects such as H. angustipennis from incidental exposure will be slow, considering their relatively long life cycle.Keywords—Diazinon Hydropsyche angustipennis Chironomus riparius Variables ToxicityINTRODUCTION