Evaluation of nano- and hydrogel formulations of some selected synthetic insecticides against fall armyworm Spodoptera frugiperda (Lepidoptera: Noctuidae)

The fall armyworm Spodoptera frugiperda (Lepidoptera: Noctuidae) has been a challenging pest of maize crops around the world. Novel, effective, eco-friendly and target-oriented approaches are needed to mitigate the damage of S. frugiperda. Farmers rely predominantly on extensive and repetitive use of synthetic insecticides, but they have unsatisfactory control because of the cryptic feeding style of the pest’s larvae and rapid decomposition of the applied insecticides. This study aimed to screen out available synthetic insecticides being used against different lepidopterous pests including S. frugiperda under field conditions, and then the two most effective insecticides were further formulated with silver nanoparticles and with sodium alginate hydrogel beads and bioassayed against the third larval instar of S. frugiperda under laboratory conditions using the standard leaf-dip method. Treatments included the following: T₁ = emamectin benzoate and chlorantraniliprole formulation at the recommended dose (90 mL/acre); T₂ = nanoformulated emamectin benzoate and chlorantraniliprole formulation; T₃ = nanoformulated emamectin benzoate and chlorantraniliprole at the recommended dose (90 mL/acre); T₄ = emamectin benzoate and lufenuron formulation at the recommended dose (400 mL/acre); T₅ = nanoformulated emamectin benzoate and lufenuron formulation; T₆ = nanoformulated emamectin benzoate and lufenuron formulation at the recommended dose (400 mL/acre); T₇ = nanoparticles alone (control); and T₈ = water (control). According to the results, the nanoformulated recommended dose of the emamectin benzoate and lufenuron formulation manifested the highest mortality (100%) at 24 h, followed by the nanoformulated recommended dose of the emamectin benzoate and chlorantraniliprole formulation (100%) at 48 h post-treatment. In other bioassays, alginate beads loaded with the recommended dose of emamectin benzoate and lufenuron formulations caused up to 50% more larval mortality compared to beads loaded with emamectin benzoate and chlorantraniliprole formulations, which resulted in 40% higher larval mortality than the control treatments. Overall, the findings of this laboratory study suggest further evaluation of such formulated application of synthetic insecticides combating S. frugiperda under semi-field and field conditions.