Insecticidal activity of indigenous Bacillus thuringiensis strains against Tephritids fruit fly pests affecting fruits and vegetables

Abstract

The extensive and indiscriminate use of synthetic chemical insecticides in agriculture and forestry has led to several unintended and detrimental consequences, including environmental pollution, the development of pest resistance, the mortality of non-target organisms, and negative public health effects. These challenges are particularly pronounced in developing countries like Bangladesh, where pest infestations significantly reduce crop productivity and economic gains. As a sustainable and environmentally safer alternative, microbial biopesticides, particularly those derived from Bacillus thuringiensis (Bt), have emerged as a promising solution. Bt-based biopesticides are host-specific, biodegradable, and effective, offering significant potential to reduce chemical pesticide dependency. This study aimed to evaluate insecticidal activity of previously isolated, characterized and preserved indigenous Bacillus thuringiensis (Bt) strains from different eco-regions of Bangladesh against four economically important Tephritid fruit fly pests, Bactrocera dorsalis, B. zonata, Zeugodacus cucurbitae and Z. tau. These pests are known to cause extensive damage to fruits and vegetables, impacting local consumption, export opportunities, and national food security. A total of 44 Bt strains were isolated, identified, and screened through a comprehensive suite of phenotypic, genetic, proteomic, and toxicity analyses. The overarching goal was to identify highly potent Bt strains and validate their effectiveness under both laboratory and field conditions. Initial screening of the 44 native Bt isolates showed that 16 strains could induce greater than 50% and three potential strains JSd1, SaS6, and JDc1 exceeded 80% larval mortality in all four Tephritid species tested. Bt strain JSd1 consistently demonstrated the highest efficacy, inducing 97% mortality in B. dorsalis, 95% in B. zonata, 95% in Z. cucurbitae, and 92% in Z. tau, outperforming even the reference Btk HD-73 and Bts T84A1. Further toxicological analyses, including lethal concentrations (LC₅₀) and lethal time (LT₅₀) values, confirmed JSd1’s superior performance. LC₅₀ values ranged from 0.431 to 0.472 mg/ml and LT₅₀ values ranged between 54.09 and 55.17 hours, which are significantly lower than other strains, indicating high potency at reduced concentrations, while faster action and quicker pest knockdown. Beyond bioassays, the study also evaluated the biological quality parameters of insects exposed to Bt treatment infested with preferred hosts, which are critical in understanding sub-lethal effects and potential population suppression in pest communities. Bt JSd1, SaS6, and JDc1 significantly reduced pupal yield, pupal weight, adult emergence percentage, and flying ability iii across all four Tephritid species. For instance, Bt JSd1-treated groups consistently demonstrated the lowest pupal yield (as low as 99 ± 2.081 in Z. tau), lowest pupal weight (~8 mg), and lowest adult emergence (ranging from 39–53%). Emergence of malformed or half-emerged adults was also significantly higher in treated groups, while sex ratio distortion was minimal, suggesting that these strains primarily impacted general viability rather than sex-linked mortality. These results indicate a substantial decline in the reproductive and survival potential of these pest populations upon exposure to Bt biopesticides, making them highly effective components in Integrated Pest Management (IPM) strategies. To understand the genetic basis of its high efficacy, whole genome sequencing of Bt JSd1 was conducted. Genomic analysis revealed the presence of multiple Cry and Vip insecticidal genes, notably Cry22A and Vip3A, which are known for their effectiveness against Dipteran insects. Additionally, several virulence factors and biosynthetic gene clusters associated with secondary metabolite production were identified, contributing to the strain’s broad-spectrum insecticidal capability and environmental adaptability. This genetic richness further reinforces the potential of Bt JSd1 as a candidate for next-generation bioinsecticide development. Finally, field validation was carried out using the formulated Bt biopesticide (compared with chemical pesticides) on Solanum melongena (brinjal), a crop heavily impacted by the Eggplant Fruit and Shoot Borer (EFSB. Four-time foliar applications (Each of 100 ml volume containing 25.7 mg spore crystal mixture) of the Bt preparation reduced EFSB infestation to just 10%, compared to significantly higher levels in untreated controls, with no significant difference with the chemical pesticide. The average yield per plant increased from 1.45 kg (control) to 3.85 kg in the treated group, effectively matching or surpassing yields from chemically treated plots. Importantly, there were no observed negative impacts on beneficial insect populations or surrounding flora, highlighting the ecological safety of the product. Overall, this study provides compelling evidence for the viability and impact of indigenous Bt strains, particularly JSd1, as potent, safe, and affordable biocontrol agents, offering a sustainable solution to the challenges of chemical pesticide overuse, contributing to improved agricultural productivity, ecological health, and food security. Their local origin also enables domestic production and reduces reliance on imported formulations, supporting national bioeconomy goals.