Comparative Toxicity of Delta-Endotoxin and Thuringiensin of Bacillus thuringiensis and Mixtures of the Two for the Bertha Armyworm (Lepidoptera: Noctuidae)

Abstract

Five formulations of delta-endotoxin, three of thuringiensin (β-exotoxin) derived from Bacillus thuringiensis , and mixtures of delta-endotoxin and thuringiensin, were tested against fourth-instar bertha armyworm, Mamestra configurata Walker, at 20 and 25°C. All compounds were more toxic at 25 than at 20°C. A mixture of half the concentration of the commercial formulation, Dipel 4L, and half the concentration of pure thuringiensin, ABC 6162A, fed to the larvae on canola, Brassica napus L., foliage was as toxic as the full dose of Dipel 4L. Median lethal times (LT50’s) of all treatments decreased, and slopes declined as dosage increased. LT50’s were always lower at 25 than at 20°C. LC50’s of the delta-endotoxins, the thuringiensins, and mixtures of the two were equally effective in reducing weight gain, frass deposition, and survival to adult among test larvae. Studies of interactions of commercial B. thuringiensis var. kurstaki and pure thuringiensin on canola foliage showed these compounds to be synergistic or additive at a concentration of thuringiensin that caused 32% mortality of bertha armyworm larvae. When incorporated with Dipel 4L into artificial diet, thuringiensin at concentrations of 0.01, 0.005, 0.001, and 0.0005%(AI), which by themselves caused 23, 18, 0, and 0% mortality, respectively, reduced the LC50 of Dipel 18-, 14-, 4-, and 5-fold, respectively, and resulted in weight loss among larvae, implying complete feeding inhibition. Fourth instars were more sensitive to the new formulation of delta-endotoxin, San 415, than third or fifth instars, and they produced less frass in the same time period, but the overall effectiveness of San 415 was similar to that of commercial Thuricide 32B.