Rapid evolution and genetic basis of spinetoram resistance in Phthorimaea absoluta: evidence from six years of monitoring and laboratory selection.

Background

Phthorimaea absoluta (Meyrick) (Lepidoptera: Gelechiidae) is one of the most destructive pests of tomato worldwide and has repeatedly evolved resistance to insecticides. Spinosyn insecticides, particularly spinetoram, are widely used for its control due to their high efficacy and favorable selectivity. This study investigated temporal changes in susceptibility to spinetoram in Brazilian populations of Phthorimaea absoluta over a 6-year monitoring period and characterized the genetic basis, metabolic contribution, and cross-resistance patterns of a resistant population.

Results

Diagnostic-dose monitoring repeatedly detected resistant phenotypes across geographically distinct populations during 6 years of surveillance, indicating that resistance alleles are established in Brazilian populations despite temporal variation in their apparent frequencies. Laboratory selection of a field-derived population (PTY-Sel) resulted in a high level of resistance to spinetoram (resistance ratio (RR50) ≈ 1145). Reciprocal crosses indicated autosomal inheritance, and dominance estimates (DD50 = -0.43 to -0.57) showed that resistance was incompletely recessive. Backcross analyses supported a monofactorial basis of resistance. Synergism assays revealed significant reductions in median lethal concentration (LC50) in the resistant strain after exposure to piperonyl butoxide (PBO), diethyl maleate (DEM), and S,S,S-tributyl phosphorotrithioate (DEF) (synergism ratio (SR50) = 7.59-8.78), indicating involvement of detoxification enzymes. However, resistance remained high under synergized conditions, suggesting that metabolic detoxification alone does not fully explain the resistance phenotype. Strong cross-resistance to spinosad (RR50 ≈ 683) was observed, whereas only minor differences in susceptibility occurred for the non-spinosyn insecticides; cyclaniliprole susceptibility was similar between strains, while PTY-Sel showed increased susceptibility to isocycloseram. Effective dominance declined with increasing concentration, and resistance became functionally recessive at ≥ 15 mg L-1.

Conclusion

Resistance to spinetoram in Phthorimaea absoluta can evolve rapidly and is primarily associated with a major recessive genetic factor, with metabolic detoxification contributing to the resistance phenotype. The cross-resistance profile provides important insights for designing resistance management strategies in tomato production systems. © 2026 The Author(s). Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.