Synergistic Control of <i>Megalurothrips usitatus</i> through Sequential RNAi Targeting Novel Lethal Genes and <i>Serratia marcescens</i> 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 ds MuaCOPI, ds MuNICE3, and ds MuSrp54k with HvSm-1. In adults, mortality ranged from 51.7% to 58.0%. Synergy occurred for ds MuaCOPI, ds MuSrp54k, and ds MuRop 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.