Confirmation of Attraction of the Mixture of a Sugar–Vinegar–Alcohol Lure and Methyl Eugenol for Sexually Immature Males of Bactrocera dorsalis

The male annihilation technique for Bactrocera dorsalis relies primarily on methyl eugenol (ME), which exclusively targets mature males and consequently misses the critical window for managing sexually immature males (SIM). To address this operational gap, we cross-validated male testicular development with ME responsiveness across a 1–12-day age gradient, improved a conventional sugar–vinegar–alcohol lure (SL), and evaluated the combinations of the optimized sugar–vinegar–alcohol lure (OSL) and ME using a 2 × 2 factorial field design. Laboratory assays revealed that ME capture rates and off-white-to-yellow testicular transitions were highly synchronized and strongly correlated; notably, males aged 1–4 days exhibited entirely off-white testes and negligible ME responsiveness, establishing testicular coloration as a robust physiological hallmark of immaturity. In field trials, OSL demonstrated stable attractancy across diurnal intervals, outperforming water controls but yielding captures statistically comparable to the conventional SL. Furthermore, incorporating ME into OSL significantly enhanced SIM captures; however, factorial analysis confirmed that their interaction was not statistically significant. Ultimately, these findings indicate that OSL and ME exert a significant, independent additive effect rather than a synergistic interaction on SIM attraction. This study provides a reliable physiological–behavioral diagnostic framework and a practical, low-cost direct-mixing strategy to extend the operational window of B. dorsalis management.