Silk Web Removal Enhances the Efficacy of Beauveria bassiana Treatment Against Hyphantria cunea: Insights from Immune and Physiological Parameters

Abstract

The construction of physical shelters represents a widespread survival strategy among insects. This study evaluated the effect of silk web removal on the efficacy of Beauveria bassiana treatment against Hyphantria cunea larvae. Results indicated that the removal of silk webs significantly increased larval mortality and enhanced conidia attachment to the larval body during B. bassiana infection. Removal of the silk web increased larval mortality from 23.06 ± 3.14% to 46.94 ± 3.20%, with the presence of the silk web thus corresponding to a 50.88% relative reduction in B. bassiana-induced mortality. Scanning electron microscopy showed that conidial deposition on the larval cuticle was 2.8-fold higher, and markedly more aggregated within cuticular folds, in web-removed larvae, indicating that the silk web substantially lowers the conidial exposure actually experienced by the larvae. This difference in conidial exposure between silk web-bearing and silk-web-removed larvae was accompanied by multi-level physiological responses in the host. Silk web removal decreased the cellular immune response of H. cunea larvae to B. bassiana, as evidenced by reduced hemocyte counts, lower encapsulation and melanization activity, and altered expression of cellular immune-related genes (e.g., Galectin-1 and PPO2). Silk web removal intensified the oxidative damage manifested by MDA and H2O2 and the cell apoptosis manifested by apoptosis regulatory genes (e.g., AIFM3 and Metacaspase-1-like), accompanied by a significant decrease in the antioxidant enzyme activity and total antioxidant capacity in B. bassiana-infected larvae. The depletion of nutrient reserves (e.g., glucose) by B. bassiana from the H. cunea larvae was exacerbated upon silk web removal. Moreover, larvae exhibited a heightened response to B. bassiana infection upon silk web removal, including upregulated expression of cuticular protein genes and humoral immunity regulators (e.g., Lebocin and Attacin), antioxidant genes, and key enzymes in the glycolytic and tricarboxylic acid cycles. Collectively, the silk web reduces the efficacy of B. bassiana treatment against H. cunea larvae; this reduction is accompanied by a markedly lower conidial load on the cuticle, together with changes in the host’s immune defense, oxidative stress, and energy metabolism.