Photothermal Resilience of Biopolymer-Based Btk Microcapsules: Implications for Sustainable Pest Control

The efficacy of <i>Bacillus thuringiensis var. kurstaki</i> (Btk) as a biocontrol agent is often compromised under field conditions due to its rapid degradation by ultraviolet (UV) radiation and thermal stress, leading to reduced spore viability and insecticidal potency. To address this, the present study developed and optimized two microencapsulated Btk formulations [CC-1+Btk (D) and CC-2+Btk (D)] using polyelectrolytic biopolymer-based complex coacervation. Both formulations exhibited excellent physicochemical properties, including >98% entrapment efficiency, high protein retention, and rapid wettability. Among them, CC-1+Btk (D) demonstrated superior stability under both UV-C and UV-B (254 and 385 nm) and thermal (30 and 50 °C) exposures, retaining significantly higher CFU counts and larval mortality against <i>Spodoptera litura</i> and <i>Achaea janata</i> compared to unencapsulated and commercial Btk products. Persistence studies on castor foliage further revealed extended survival of encapsulated Btk spores and consistent insecticidal activity up to 48 h after treatment, validating its resilience under simulated field conditions. These results highlight the potential of complex coacervation-based encapsulation in improving environmental robustness, residual efficacy, and shelf life of Btk formulations. This work contributes a scalable, biodegradable, and field-stable delivery system for sustainable pest management.