Bacillus thuringiensis var. israelensis (B.t.i.) and B. sphaericus were evaluated for chironomid larvicidal activity in freshwater mesocosms. Of two B.t.i. formulations, the technical powder ABG-6164 provided excellent control of chironomines (94%) at the rate of 11.2 kg/ha whereas the liquid concentrate, Vectobac 6 AS, achieved only moderate control (57%) at the rate of 22.4 kg/ha. In contrast, similar rates of B. sphaericus products, ABG-6184 technical powder and BSP-2 flowable concentrate, produced no significant reduction. Lake studies were initiated to determine which B.t.i. formulations were practical and effective in large-scale situations. Two sinking granular corn grit formulations worked with varying degrees of success but were deemed too bulky for extensive applications. B.t.i. technical powder preparations with high potency ratings of 5,000 and 12,430 ITU/mg gave excellent control of Chironomus spp. (100% and 87%) when used at the rates of 6.7 and 2.8 kg/ha, respectively. None of the treatments reduced larval populations of the tanypodines, Procladius and Tanypus.
