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Three   commercial  black   fly  and  mosquito   larvicidal   formulations   of  Bacillus

thuringiensis Berliner var. israelensis de Barjac were bioassayed to determine their potencies relative to the international standard powder, IPS-78. The potencies of the wettable powder (WP) formulations of Bactimos™ and Vectobac™ and the water dispersible concentrate (WDC) TeknarN were determined to be 4530, 5723, and  336 International Toxic Units (ITU)/mg, respectively, against Simulium spp.; their respective ITU/mg values against Aedes aegypti (Linnaeus) were 3556, 2317, and 1373. Little correlation (r2 = 0.47) was evident between the potency rating (ITU/mg) of the formulations against mosquitoes and black flies; this indicated that the potency rating of a formulation against black flies cannot be simply predicted from the A. aegypti ITU/mg rating on its label. A moderate correlation between formulation potency and mean particle size was observed in the black fly assays (r2 = 0.77), but not against mosquitoes (r2 = 0.20). Aqueou s suspensions of Teknar-WDC had a significantly smaller mean particle size and stayed significantly longer in suspension than other formulations. The relatively long duration of suspension of Teknar-WDC was viewed as important since the toxic crystals wou ld be less likely to settle out in mosquito and black fly habitats. A ratio based on the LC50 values determined from standardized bioassay methods is suggested as a convenient way to express the relative  potency of a formulation against mosquitoes and black flies.