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 studi ed as a cause of mortality in the epigeal linyphiid spider Oedothorax apicatus. A quantitative deterministic model was constructed to predic t 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 i mmobile spiders, water excretion cannot be counterbalanced by active w ater uptake. Mortality will ensue unless recovery occurs before a leth al dehydration level is reached. Transition rates between behavioral s tates, passive and active water loss, lethal dehydration level and pes ticide metabolization rates were estimated using data from three labor atory experiments and literature. The model was compared with independ ent 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 pre dicted 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 mode l support the existence of two independent, but simultaneous toxic eff ects of pyrethroid insecticides; an effect on behavior causing rapid i mmobilization or knockdown which is correlated positively with air hum idity and negatively with temperature, and an effect on active water e xcretion causing dehydration which is correlated positively with tempe rature. (C) 1997 Academic Press.