Toxicity of Fipronil and Emamectin Benzoate and Their Mixtures Against Cotton Leafworm, Spodoptera littoralis (Lepidoptera: Noctuidae) With Relation to GABA Content

Abstract

Fipronil and emamectin benzoate (EMB) are effective insecticides for controlling cotton leafworm, Spodoptera littoralis. Fipronil works by blocking gamma-amino butyric acid (GABA) gated chloride. In contrast, EMB is activating GABA transporters. The objectives of our study were to assess relative toxicity of the technical fipronil and EMB alone and in mixture against S. littoralis. In addition, the GABA content was simultaneously determined using HPLC. Technical fipronil and EMB and their mixtures were applied topically to the fourth-instar larvae, and their LD50 values were estimated after 48 h. Results demonstrated that the LD50 for EMB applied alone was 0.751 ng/larva which was much less than for fipronil 7.271 ng/larva. Each of the two insecticides alone showed a significant decrease in GABA content at LD10, LD25, and LD50 doses, while their mixtures induced GABA levels. The highest potentiation was observed when both insecticides were in a mixture at the ratio of LD10:LD10 which was associated with higher increase in GABA levels. Moreover, the weight of the alive larvae was less than that was in the untreated control. However, all mixtures exhibited potentiation effect, except for the mixture of fipronil at LD50 with EMB at LD10, LD25, and LD50 that had antagonistic effect correlated with the lowest decrease in GABA level. Results suggested that lower doses of both insecticides in a binary mixture had potentiation effect against S. littoralis. This mixture could be used in combination as field application for successful and effective control of S. littoralis and would also help in managing insecticide resistance.

Keywords: GABA; cotton leafworm; emamectin benzoate; fipronil.