Rate of ingestion by mosquito larvae (Diptera: Culicidae) as a factor in the effectiveness of a bacterial stomach toxin.

Abstract

To study the impact of ingestion rate on the effectiveness of the stomach toxin produced by Bacillus thuringiensis var. israelensis, fourth-instar larvae of Aedes aegypti, Culex quinquefasciatus, Anopheles albimanus, and Anopheles quadrimaculatus were exposed to a series of toxin suspensions at larval densities ranging from 0.15 to 2.4 individuals per ml water. Toxin was dosed as weight per larva; consequently, toxin concentration increased with increasing larval density. Because filtration rates of individual larvae are not influenced by larval density, it was hypothesized that the toxin particles are ingested by larvae at a speed relative to particle concentration. With increasing larval density, increasing toxin concentrations were necessary to elicit equal levels of mortality. Based on regression analysis of the dosage-mortality correlation, toxin amounts inducing 50% mortality in groups at various densities were calculated. The average amount of toxin per larva inducing this level of mortality decreased by 62–63% (An. quadrimaculatus), 43–64% (An. albimanus), and 34–44% (Ae. aegypti) when larval density increased from 0.15 to 1.2 individuals per ml water. This effect was independent of toxin formulation and also was present when density was varied without surface crowding of larvae. Previously documented species-specific differences in susceptibility of test species were significant at low larval densities (<0.6 individuals per ml water), but were not observed at the optimized ingestion rate (larval density > 1.2 individuals per ml water). We conclude that the relative tolerance of Anopheles larvae to suspensions of the bacterial toxin is a result of their low filtration rates. In Cx. quinquefasciatus, toxin effectiveness did not change with larval density, possibly due to feeding inhibition caused by the bacterial agent.