INTERPRETATION OF DENSITY DEPENDENT DATA IN COMPARISONS OF BACILLUS THURINGIENSIS VAR. ISRAELENSIS FORMULATIONS1

Abstract

The effect of larval density on estimates of effectiveness of larvicides with Bacillus thuringiensis var. israelensis (BTI) as the active ingredient (a.i.) was evaluated. Bioassays with Aedes aegypti (L.) fourth instars showed that the toxic effect of a standard powder (IPS-78) and a wettable powder formulation (Bactimos®) was independent of volume because the effect of varying the water volume in test vessels was usually negligible and had no obvious relationship to larval mortality. Data were best interpreted as a function of dose (i.e., the absolute amount of a. i.) and the number of larvae in each test vessel rather than as a function of concentration (dose rate). This indicated that accurate comparisons of acute toxicity estimates derived from different tests with differing numbers of larvae (i.e., differing densities) can be made if results are reduced to and expressed as the EC50/larva.