Optimization of Antimicrobial Concentrations in an Agar-Free, Low-Cost, Wheat Germ-Based Diet for Mass Rearing of Drosophila suzukii

The bottleneck for the mass production of Trichopria drosophilae Perkins (Hymenoptera: Diapriidae), the principal parasitoid of the Drosophila suzukii (Matsumura) (Diptera: Drosophilidae), is the lack of efficient mass-rearing methods. Here, we optimized the concentrations of five antimicrobials (each at three concentration levels) in an agar-free, low-cost, wheat germ-based diet using a Taguchi orthogonal array design (L27 = 35). The optimal formulation (as determined indirectly through insect-rearing performance) consisted of 1.2 g L−1 of sodium benzoate, 1.2 g L−1 of propyl paraben, 0.3 g L−1 of potassium sorbate, 0 g L−1 of sodium propionate, and 0 g L−1 of triclosan. In the Taguchi screening, the best-performing treatment produced approximately 4.9 times as many pupae as the antimicrobial-free control. Subsequent validation confirmed that both the best-performing treatment and the optimized formulation significantly increased pupal production relative to the internal antimicrobial-free control. Furthermore, the formulation reduced the number of antimicrobial compounds required, simplifying diet preparation and lowering production costs. Additionally, because the diet does not contain agar and requires only basic equipment and simple procedures, it represents a practical and scalable alternative for the mass production of D. suzukii and its parasitoid T. drosophilae. This optimized, low-cost formulation is expected to facilitate the scalable mass rearing of T. drosophilae, thereby promoting its adoption as a practical and accessible biological control strategy against D. suzukii, especially in resource-limited berry-producing regions.