Chemical Additive Effects on the Efficacy of Bacillus thuringiensis Berliner subsp. kurstaki Against Mamestra configurata (Lepidoptera: Noctuidae)

Abstract

Reduced lethal concentrations (that is, increased toxicity at LC 50 and LC 95 ) of Bacillus thuringiensis Berliner subsp. kurstaki (Dipel) were observed when fourth-instar bertha armyworm, Mamestra configurata Walker, were fed on artificial diet treated with the bacteria supplemented with various compounds. In 11 groups of compounds, 12 inorganic salts, 1 tannin, 2 detergents, 3 amino acids, 3 organic acids, 2 oxidized carbohydrates, 1 aromatic compounds, 0 inorganic sulphides, 0 aliphatic amines, 1 amide, and 1 serine protease inhibitor, at concentrations between 0.01 and 0.1%, reduced the LC 50 of Dipel when combined in artificial diet. However, when a 2-fold or higher increase in toxicity at both LC 50 and LC 95 was used as the acceptable criterion of effective enhancement, only boric acid, magnesium sulphate, magnesium chloride, and acetamide were effective. In diet, boric acid reduced LC 50 and LC 95 of B. thuringiensis subsp. kurstaki 3.5- and 2.3-fold; magnesium chloride, 2.1- and 2.1-fold; magnesium sulphate, 3.5- and 3.9-fold; and acetamide, 3.3- and 4.5-fold, respectively. These compounds also reduced weight gain in surviving larvae fed Dipel compared with larvae fed Dipel alone or untreated controls. In greenhouse trials on canola, Brassica napus L., sodium and calcium carbonates at 0.05% (which in diet enhanced Dipel toxicity 1.1- to 1.8-fold and reduced larval weight) enhanced the toxicity of Dipel 7.6- to 10.1-fold to third-instars but did not reduce larval weight compared with Dipel alone. Further laboratory and field trials of B. thuringiensis subsp. kurstaki supplemented with boric acid, magnesium sulphate, acetamide, sodium carbonate, calcium carbonate, or potassium carbonate against bertha armyworm on canola are warranted.