Ceratapion basicorne is a univoltine weevil that feeds and oviposits within invasive yellow starthistle's leaf petiole and mesophyll cells. The leaf pH level is a plant functional trait that attracts and deters female weevils. The topical application of methoprene and 20-hydroxyecdysone terminates the reproductive diapause of C. basicorne. To assess the preference of hormone-treated female C. basicorne, weevils were placed on a control leaf in tap water alongside a pH-adjusted leaf that simulates the rosette-stage yellow starthistle, with both leaves sourced from the same plant. Between days 0 and 16, hormone-treated weevils preferred the pH-adjusted leaf over the control. Gravid weevils initiated oviposition within the leaves on day 8, depositing 200% more eggs in the pH-adjusted leaves compared to the control from days 8 to 16. Combining hormones and pH adjustment boosts egg production to mass-rear C. basicorne for field release. The finding also applies to other mass-rearing programs for biological control agents in agricultural, forest, and rangeland ecosystems in California & beyond.
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