Comparison of the effectiveness of freeze and spray-dried Bacillus thuringiensis preparations as a biopesticide for the control of cucumber moth, Diaphania indica (Pyralidae: Lepidoptera)

Abstract

A serious pest on cucumber crops in many tropical and subtropical regions is the cucumber moth (Diaphania indica), which causes significant yield losses. Bacillus thuringiensis (Bt), a well-known entomopathogenic bacterium effective against Lepidopteran pests, can be used as an eco-friendly alternative to chemical pesticides for biological pest control. The efficacy and shelf life of Bt biomass produced through two drying techniques, spray drying and freeze drying, were evaluated against D. indica larvae. Powdered formulations of the B. thuringiensis var. aizawai strain (Bta-VN116) were prepared using both methods by growing the strain in nutrient-rich media, harvesting, and processing it. Bt powders at different concentrations were tested in bioassays conducted in the Biological Control Laboratory at Can Tho University, Vietnam. Mortality rates of D. indica larvae were recorded over a 6-month storage period. The results showed that while both formulations remained active against D. indica larvae, spray-dried Bt retained efficacy more than 95% at recommended rates (28–32 g/ha) for up to three months post-drying, followed by a gradual decline. In contrast, freeze-dried Bt showed higher stability over time and remained effective for up to six months at 14–15 g/ha. Freeze-dried powders were found to retain a greater number of colony-forming units (CFU) over time compared to spray-dried powders, as revealed by viability testing. Although spray drying is more scalable and cost-effective, it has inferior long-term potency based on these findings. The resulting product from both spray and freeze drying (Bt) is demonstrated as a viable biopesticide formulation for controlling D. indica, while the trade-offs associated with drying techniques in terms of immediate efficacy, cost, and storage stability are highlighted.