Relative toxicity of subspecies of Bacillus thuringiensis kurstaki HD-1 and HD-73 against the larvae of legume pod borer, Maruca vitrata, F. (Lepidoptera: Crambidae)

The legume pod borer, Maruca vitrata F. (Lepidoptera: Crambidae), is a major insect pest of many edible legumes in various regions of America, Asia and Africa. The larvae cause serious damage to the reproductive parts of cowpea, pigeon pea and beans in India. Promotion and use of biopesticide containing Bacillus thuringiensis (Bt) is viewed as a viable alternative to synthetic pesticides. In this study, field populations of M. vitrata were collected from intensive legume-growing regions of India during years 2023 and 2024. A commercial formulation, Bt sub.sp. kurstaki HD-1 (Delfin®), along with reference strains of Bt sub.sp. kurstaki HD-1 and HD-73 were tested on the larvae of different M. vitrata populations collected across India to evaluate their relative effectiveness. The LC50 values ??estimated for Bt kurstaki HD-1 and Bt kurstaki HD-73 strains and a commercial formulation of Bt kurstaki HD-1 (Delfin®) against different field-collected populations of M. vitrata ranged from 1.097 to 1.829 ppm, 6.228 to 7.236 ppm and 2.894 to 4.930 ppm, respectively. The Bt kurstaki HD-1 strain harbouring multiple crystal proteins (Cry1Aa, Cry1Ab, Cry1Ac, Cry2A, Cry2B) were relatively more toxic to the larvae of M. vitrata than Bt kurstaki HD-73 which harbours a single Cry toxin i.e., Cry1Ac.