Synergistic Control of Megalurothrips usitatus through Sequential RNAi Targeting Novel Lethal Genes and Serratia marcescens HvSm-1

The phytophagous thrip, Megalurothrips usitatus is a destructive pest of cowpea. This study developed an integrated management strategy combining RNA interference (RNAi) and the entomopathogen Serratia marcescens HvSm-1. Seed feeding emerged as the optimal dsRNA delivery method. Screening identified four lethal target genes (MuαCOPI, MuNICE3, MuSrp54k, MuRop). Silencing significantly increased mortality (nymphs: 23.1-34.8%; adults: 25.5-33.3%). The bacterium HvSm-1 alone caused high, dose-dependent mortality. A sequential application of dsRNA followed by HvSm-1 induced synergistic or additive mortality, significantly enhancing control efficacy in both life stages. In nymphs, mortality reached 82.1-88.4%. Synergistic effects were observed for dsMuaCOPI, dsMuNICE3, and dsMuSrp54k with HvSm-1. In adults, mortality ranged from 51.7% to 58.0%. Synergy occurred for dsMuaCOPI, dsMuSrp54k, and dsMuRop with HvSm-1. However, direct mixing was ineffective as bacterial nucleases degraded dsRNA within hours. This work demonstrates the potential of sequential RNAi-microbial biocontrol while identifying dsRNA stability as a critical challenge for future formulation.