Water depletion, an important cause of mortality in females of the spider Oedothorax apicatus after treatment with deltamethrin: A simulation study

Passive water loss and deltamethrin-induced water excretion were studied as a cause of mortality in the epigeal linyphiid spider Oedothorax apicatus. A quantitative deterministic model was constructed to predict mortality caused by water excretion in a laboratory population after topical application of the insecticide deltamethrin. Four behavioral states were distinguished, mobile, immobile, recovered, and dead. In immobile spiders, water excretion cannot be counterbalanced by active water uptake. Mortality will ensue unless recovery occurs before a lethal dehydration level is reached. Transition rates between behavioral states, passive and active water loss, lethal dehydration level and pesticide metabolization rates were estimated using data from three laboratory experiments and literature. The model was compared with independent data on spider mortality at a dose of 2.5 ng a.i. spider-1. Both onset of mortality and number of dead spiders were satisfactorily predicted at a range of combinations of temperature and relative humidity. The results strongly indicate that water loss is an important cause of death for spiders poisoned by deltamethrin. The results of the model support the existence of two independent, but simultaneous toxic effects of pyrethroid insecticides; an effect on behavior causing rapid immobilization or knockdown which is correlated positively with air humidity and negatively with temperature, and an effect on active water excretion causing dehydration which is correlated positively with temperature.