Insecticide mixtures reduce the selectivity enhanced by pyrethroid resistance in a predatory lady beetle

Insecticide mixtures provide various modes of action in a ready-to-use formulation, broadening the range of managed pest species and delaying the development of insecticide resistance. Nonetheless, the insecticides used in the mixture may change the selectivity status for natural enemies obtained when using an individual formulation. Eriopis connexa (Germar), a lady beetle resistant to the broad-spectrum pyrethroid λ-cyhalothrin (EcViR), was exposed to insecticides in both individual and mixture formulations. The insecticides λ-cyhalothrin, chlorantraniliprole, sulfoxaflor, λ-cyhalothrin+dinotefuran, λ-cyhalothrin+thiamethoxam, and λ-chlorantraniliprole+thiamethoxam were tested. Survival of larvae and survival and reproduction of adults were assessed when enclosed with dry insecticide residues obtained at the maximum insecticide recommended rate. Furthermore, concentration-mortality curves were also established for larvae and adults exposed to insecticides with mortality rates exceeding 40%. For 30 days, the number of adults who survived exposure at the larval and adult stages and were still living and reproducing was recorded. The toxicity of the neonicotinoids thiamethoxam and dinotefuran present in the mixture prevailed over the physiological selectivity obtained through the resistance to λ-cyhalothrin. The combination index revealed that adding neonicotinoids to λ-cyhalothrin or chlorantraniliprole was very harmful to the lady beetle. On the other hand, the combination of λ-cyhalothrin with chlorantraniliprole or sulfoxaflor allowed EcViR larvae and adults to survive more than 80%. Therefore, incorporating neonicotinoids into the insecticide formulation nullified the physiological selectivity achieved by E. connexa resistance to λ-cyhalothrin, reducing the potential effectiveness of E. connexa in pest management as an augmentative biological control agent.