Nutrient and spinosad concentrations influence liquid bait consumption and mortality in spinosad‐resistant melon flies (Diptera: Tephritidae)

Abstract BACKGROUND The success of insecticide‐laced liquid protein bait spray applications relies on attraction and consumption by the target pest. Commercial liquid baits often recommend a broad range of dilution rates, even though many insects adjust their intake of foods based on nutrient concentrations. This compensatory feeding response could influence the amount of toxin ingested, but this impact has not been thoroughly elucidated. We manipulated liquid diet and spinosad concentrations to reveal their effects on consumption and mortality in three lines of melon flies, Zeugodacus cucurbitae , that vary in levels of spinosad resistance. RESULTS Nutrient concentration had a non‐linear effect on consumption, which strongly influenced spinosad‐induced mortality. Spinosad‐susceptible flies suffered high mortality at the full nutrient concentration range when spinosad concentration was high. Resistant lines consumed less diet overall and hence suffered lower mortality on nutrient concentrations which were excessively dilute (22.5–45 g L −1 ) and excessively concentrated (720 g L −1 ). Similarly, the recommended label dilutions of a widely used commercial bait produced low levels of consumption. CONCLUSIONS We demonstrated that a high concentration of toxicant is effective for controlling a susceptible insect population regardless of consumption rate but maximizing consumption may be critical for managing insecticide‐resistant populations. © 2026 Society of Chemical Industry.