WATER-BALANCE, RESPIRATION AND IMMOBILIZATION IN RELATION TO DELTAMETHRIN POISONING AND PHYSICAL CONDITIONS IN THE EPIGEAL SPIDER OEDOTHORAX-APICATUS

The effect of a pyrethroid insecticide, deltamethrin, on water loss, m etabolic rate and immobilisation was studied, using the epigeal spider Oedothorax apicatus (Blackwall) (Linyphiidae) as a test species. Wate r loss was measured gravimetrically at different temperatures, air hum idities, and doses of deltamethrin. Metabolic rate and immobilisation were measured at different temperatures and doses. Immobilisation was measured at different relative humidities and temperatures. Water loss due to evaporation increased linearly with vapour pressure deficit. T he slope of the latter relationship increased with temperature, indica ting an increase in cuticle permeability with temperature. In addition to water loss by evaporation, deltamethrin treatment induced water ex cretion. This was positively related to dose and temperature, but inde pendent of air humidity. Metabolic rate increased with temperature and dose. Immobilisation was highest at low temperatures in combination w ith low air humidity. The consequences of pesticide-induced water excr etion are discussed with respect to field- and laboratory-based dose-e ffect relationships for pyrethroids presented in the literature. The p resent results seem to offer an explanation for hitherto unexplained m inima in temperature-response curves in pyrethroid toxicity in some ar thropod species. It is concluded that simultaneous measurements of met abolic rate, water-loss rate and pesticide effects are a prerequisite for an understanding of the effects of pesticides on arthropods.