REPRODUCTIVE POTENTIAL - ITS INFLUENCE ON THE SUSCEPTIBILITY OF A SPECIES TO PESTICIDES

Acute lethal concentration estimates (72-hr LC50) and population growt h rates (7-day instantaneous rate of increase) of two mite species, an herbivore, the two-spotted spider mite Tetranychus urticae Koch, and the generalist predator mite Iphiseius degenerans ans Berlese, were de veloped after exposure to two pesticides, dicofol and Neemix. For each pesticide, LC50 estimates for both species mere similar, yet the two species exhibited completely different susceptibility when population growth rate was the endpoint evaluated; I. degenerans was much more su sceptible than T. urticae to either pesticide. For example, population s of I. degenerans became extinct after exposure to 250-ppm azadiracht in, the active ingredient in Neemix, while T. urticae populations beca me extinct after exposure to 1000 ppm. A similar relationship was foun d for dicofol. The no observable effect concentration (NOEC) for popul ation growth rates after Neemix exposure mas 4 ppm for I. degenerans a nd 125 ppm for T. urticae. These NOEC values were equivalent to the ac ute LC2 for the immature stage of I. degenerans and the acute LC65 for the immature stage of T. urticae. Consequently, populations of T. urt icae were able to compensate for high losses of individuals while I. d egenerans populations could not compensate for losses. An analysis of reproduction data indicated that unexposed T. urticae produced four to five times more offspring than I. degenerans. This in itself was impo rtant because it indicated that I. degenerans was intrinsically more s usceptible than T. urticae because similar effects on reproduction wou ld be more devastating to the species with a lower reproductive rate. Results indicate that a species' reproductive potential can greatly in fluence its susceptibility to toxicants. (C) 1997 Academic Press.