INSECTICIDAL POTENTIAL OF INDIGENOUS BACILLUS THURINGIENSIS STRAINS AGAINST PUMPKIN FRUIT FLY

ABSTRACT: The sustainable development of Bacillus thuringiensis (Bt)-based biopesticides against Zeugodacus tau, a destructive vegetable pest susceptible to Cry proteins, necessitates extensive laboratory and field-scale bioassays to replace hazardous chemical pesticides. In this study, 44 indigenous Bt strains harbouring various cry genes were evaluated against early third-instar larvae of Zeugodacus tau (Walker) (Diptera: Tephritidae: Dacinae) following enhanced Cry protein expression in T3 broth. Among the tested strains, nine demonstrated >70% larval mortality, with Bt JSd1 (92%), SaS6 (85%), JDc1 (83%), TaSa4 (75%), RaSa2 (74%), NaSc3 (72%), Ksa2 (72%), FhSb3 (70%), and MuSc2 (70%) showing the highest efficacy. Probit analysis was used to determine the lethal concentration (LC₅₀, LC₉₉) and lethal time (LT₅₀, LT₉₉) of each strain. The lowest LC₅₀ was observed for Bt JSd1 (0.472 mg/ml), while the highest was for FhSb3 (0.753 mg/ml). LC₉₉ values ranged from 3.992 mg/ml (Bt JSd1) to 6.926 mg/ml (MuSc2). The shortest LT₅₀ was recorded for Bt JSd1 (55.17 hrs), while the shortest LT₉₉ was noted for Bt RaSa2 (241.73 hrs). Biological quality parameters, including minimum number of pupae, pupal weight, adult emergence, sex ratio (M:F), percent adult emergence, wing length, f light ability (% fliers), percentage of half-emerged and un-emerged individuals, and prolonged larval and pupal durations were all significantly affected in Z. tau infested bitter gourd and cucumber treated with the highly potent Bt JSd1 strain. In contrast, the control group exhibited the highest values for all quality parameters. Bt kurstaki HD-73 served as the reference strain in this study. The combined dose and time-dependent bioassay results, along with quality parameter assessments, identified Bt JSd1 as the most promising candidate for environmentally safe and effective control of Z. tau. Upon successful field validation, Bt JSd1 holds strong potential for large-scale production and sustainable application in integrated pest management strategies. 

Keywords: Mortality, Quality parameters, Cry protein, Zeugodacus tau, Tephritidae