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

The phytophagous thrip, <i>Megalurothrips usitatus</i> is a destructive pest of cowpea. This study developed an integrated management strategy combining RNA interference (RNAi) and the entomopathogen <i>Serratia marcescens</i> HvSm-1. Seed feeding emerged as the optimal dsRNA delivery method. Screening identified four lethal target genes (<i>MuαCOPI</i>, <i>MuNICE3</i>, <i>MuSrp54k</i>, <i>MuRop</i>). 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<i>MuaCOPI</i>, ds<i>MuNICE3</i>, and ds<i>MuSrp54k</i> with HvSm-1. In adults, mortality ranged from 51.7% to 58.0%. Synergy occurred for ds<i>MuaCOPI</i>, ds<i>MuSrp54k</i>, and ds<i>MuRop</i> 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.