Toxicity and Residual Activity of Insecticides against Diadegma insulare, a Parasitoid of the Diamondback Moth

SIMPLE SUMMARY: The diamondback moth is an insect pest that feeds on broccoli, cauliflower, cabbage, and other related plants. When its population increases, it can cause significant damage to those crops and economic losses to farmers. The fastest way to control this pest is by applying insecticides; however, the pest has become resistant to many of them. Therefore, it has been necessary to use higher doses of insecticides to decrease its population over time. At the same time, insecticides affect the pest’s natural enemies, which are essential to keeping pest populations low. Diadegma insulare is a tiny wasp that lays its eggs and develops inside the larvae of the diamondback moth, and kills it. This wasp is an important parasitoid of the pest and is found naturally in crop fields; it has also been introduced in some countries worldwide. In this research, we identified insecticides that are least toxic to D. insulare. If farmers have this information, they can choose products to kill the pest and reduce the impact of those products on natural field populations of D. insulare. By using the least toxic or residual insecticides, farmers will keep and increase the role of D. insulare as a natural enemy of the diamondback moth. ABSTRACT: Plutella xylostella is the main pest of cruciferous crops worldwide. To reduce P. xylostella populations, better integration of natural control and chemical control (dominant tactic used) is needed. This work analyzed the compatibility of nine insecticides with the parasitoid Diadegma insulare, outlining them as complementary tools in an integrated pest management strategy. The acute toxicity of spinosad, imidacloprid, indoxacarb, flonicamid, naled, pyridalyl, emamectin benzoate, and spinetoram against the parasitoid was assessed. Residual activity (persistence) was also evaluated over time; the mortality of the parasitoid in contact with leaf tissue of plants treated with insecticides was analyzed. According to the International Organization of Biological Control, all nine insecticides were toxic to D. insulare; the lowest mortality was recorded with spirotetramat (64%) and pyridalyl (48%), while the rest of the insecticides caused 100% mortality at 72 h after application. In terms of persistence, by days 14, 16, 16, 17, 17, 21, and 22 after application, flonicamid, naled, spirotetramat, spinosad, piridalyl, imidacloprid, and indoxacarb caused mortality of less than 25%, respectively, so they were considered harmless (Category 1). Nonetheless, some insecticide toxicity and residual activity must be regarded within integrated pest management programs for conserving the role of D. insulare field populations.