Mythimna separata, an important insect pest of grain crops, exhibits density-dependent prophylaxis (DDP). Most studies have focused on DDP from the perspective of immune system regulation. In this report, we show that two cuticular protein genes, MsCP1 and MsCP2, are involved in DDP of M. separata. MsCP1 and MsCP2 were highly expressed in 4th and 5th instar larvae of the M. separata gregarious high-density phase as compared to solitary, low-density individuals. The expression levels of MsCP1 and MsCP2 were significantly higher in the cuticle of gregarious larvae than in solitary individuals. When MsCP1 and MsCP2 were knocked down by star polycation (SPc) nanocarrier-mediated RNA interference (RNAi), the expression levels of the two genes were significantly downregulated in gregarious larvae as compared to dsGFP/SPc-treated gregarious larvae but were not significantly different from dsGFP/SPc-treated solitary larvae. Furthermore, silencing of MsCP1 and MsCP2 caused abnormal molting, alterations in ultrastructure, and reduced cuticle thickness in gregarious larvae. The mortality rates in dsMsCP1/SPc- and dsMsCP2/SPc-treated gregarious larvae were significantly higher after infection with the entomopathogenic fungus Beauveria bassiana as compared to the dsGFP/SPc-treated gregarious larva but were not significantly different from mortality rates in dsGFP/SPc-treated solitary larvae. Taken together, these findings suggest that upregulation of MsCP1 and MsCP2 are associated with resistance of gregarious M. separata to B. bassiana. These results broaden our understanding of insect DDP and are of great significance in the biological control of M. separata.
Li et al. (Wed,) studied this question.