| Abstract: | The effect of a pyrethroid insecticide, deltamethrin, on water loss, metabolic rate and immobilisation was studied, using the epigeal spider Oedothorax apicatus (Blackwall) (Linyphiidae) as a test species. Water loss was measured gravimetrically at different temperatures, air humidities, 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. The slope of the latter relationship increased with temperature, indicating an increase in cuticle permeability with temperature. In addition to water loss by evaporation, deltamethrin treatment induced water excretion. This was positively related to dose and temperature, but independent of air humidity. Metabolic rate increased with temperature and dose. Immobilisation was highest at low temperatures in combination with low air humidity. The consequences of pesticide-induced water excretion are discussed with respect to field- and laboratory-based dose-effect relationships for pyrethroids presented in the literature. The present results seem to offer an explanation for hitherto unexplained minima in temperature-response curves in pyrethroid toxicity in some arthropod species. It is concluded that simultaneous measurements of metabolic rate, water-loss rate and pesticide effects are a prerequisite for an understanding of the effects of pesticides on arthropods. |
