Reproductive Potential: Its Influence on the Susceptibility of a Species to Pesticides

Acute lethal concentration estimates (72-hr LC50) and population growth rates (7-day instantaneous rate of increase) of two mite species, an herbivore, the two-spotted spider miteTetranychus urticaeKoch, and the generalist predator miteIphiseius degeneransBerlese, were developed after exposure to two pesticides, dicofol and Neemix. For each pesticide, LC50estimates for both species were similar, yet the two species exhibited completely different susceptibility when population growth rate was the endpoint evaluated;I. degeneranswas much more susceptible thanT. urticaeto either pesticide. For example, populations ofI. degeneransbecame extinct after exposure to 250-ppm azadirachtin, the active ingredient in Neemix, whileT. urticaepopulations became extinct after exposure to 1000 ppm. A similar relationship was found for dicofol. The no observable effect concentration (NOEC) for population growth rates after Neemix exposure was 4 ppm forI. degeneransand 125 ppm forT. urticae.These NOEC values were equivalent to the acute LC2for the immature stage ofI. degeneransand the acute LC65for the immature stage ofT. urticae.Consequently, populations ofT. urticaewere able to compensate for high losses of individuals whileI. degeneranspopulations could not compensate for losses. An analysis of reproduction data indicated that unexposedT. urticaeproduced four to five times more offspring thanI. degenerans.This in itself was important because it indicated thatI. degeneranswas intrinsically more susceptible thanT. urticaebecause similar effects on reproduction would be more devastating to the species with a lower reproductive rate. Results indicate that a speciesʹ reproductive potential can greatly influence its susceptibility to toxicants.