Toxicity of Bacillus thuringiensis var. Israelensis formulations, spinosad, and selected synthetic insecticides to Chironomus tepperi larvae

Three Bacillus thuringiensis var. israelensis (BTi) formulations, the bacterial metabolite spinosad, and 7 synthetic insecticides were bioassayed against 4th instars of Chironomus tepperi, a serious pest of rice in southern Australia. The BTi formulations returned 48-h product median lethal concentration (LC50) values (25 +/- 1 degrees C) of between 0.59 mg/liter (VectoBac (R) water-dispersible granule [WDG], 3,000 international toxic units [ITU]/mg) and 2.15 mg/liter (Teknar (R) suspension concentrate [SC], 1,200 ITU/mg). When LC50 values were adjusted to reflect nominal ITU values of the 3 products, there was still substantial variation, with LC50 values ranging from 1,770 ITU/liter (VectoBac WDG) to 2,580 ITU/Iiter (Teknar SC). Aquabac (R) SC (1,200 ITU/mg) showed intermediate activity. Differential activity between formulations may reflect faster settling rates in the more active formulations, which may be a beneficial characteristic when controlling benthic species such as C. tepperi. Spinosad (24-h LC50 = 28.9 mu g active ingredient [Al]/liter) and the synthetic insecticides we evaluated were all substantially more active than BTi. The highest activity was shown by the neonicotinoid compounds thiacloprid, acetamiprid, and clothianidin, which all returned 24-h LC50 values between 1 and 3 mu g Al/liter. Indoxacarb and thiomethoxam showed the lowest activity of the synthetic compounds evaluated.