Coleopteran-active cry gene profiling and insecticidal activity of indigenous Bacillus thuringiensis strains against Holotrichia serrata

Abstract White grub (Holotrichia serrata F.) is a destructive soil-dwelling pest causing severe yield losses in several agricultural and horticultural crops. Owing to its subterranean habit, chemical control is often ineffective, necessitating sustainable biological alternatives. This study investigated the distribution of coleopteran-active cry genes among indigenous Bacillus thuringiensis strains and evaluated their insecticidal activity against H. serrata. A total of 100 indigenous B. thuringiensis strains isolated from the Western Ghats of India were screened using gene-specific PCR for 12 coleopteran-active cry genes. The most prevalent genes were cry7 (66%), cry26 (65%), cry1b (64%), and cry34 (63%), whereas cry8 and cry9 occurred in fewer than 10% of the isolates. Preliminary bioassays at 380 µg mL⁻¹ identified 20 strains causing > 50% larval mortality. Concentration-dependent bioassays showed that strain NBAIR Bt68 was the most virulent, causing 73% mortality with an LC₅₀ of 149.95 µg mL⁻¹. Highly toxic strains consistently possessed the cry7–cry26–cry34–cry35 gene combination, whereas the total number of cry genes did not correlate with high insecticidal activity. Molecular profiling combined with bioassay evaluation identified indigenous B. thuringiensis strains with promising biocontrol potential and provides insights into the association between cry gene combinations and insecticidal activity against H. serrata.