Toxic Effects of Neonicotinoid Insecticides on a Field Strain of Cotton Leafworm, Spodoptera littoralis

Background and Objective: The potency and selectivity of neonicotinoid insecticides against agricultural pests has lead to increase their use in control programmes. This study aimed to assess the status of indirect exposure on feeding mortality of Spodoptera littoralis to the recommended field rates of neonicotinoids, thiamethoxam (TMX), thiacloprid (TCD), imidacloprid (ICD) and acetamiprid (AMD) against 2nd and 4th larval instars of S. littoralis in two types of feeding trial in the laboratory. Materials and Methods: In the first trial, three different concentrations representing -(HFR), 1-(FR) and 2-(DFR) fold the recommended field application rate (FR) of each insecticide were separately mixed with artificial larval diet. Second and 4th instar larvae of S. littoralis were exposed to the treated diets for 1 day without further feeding (direct effect), 5 days (accumulative effect) or 1 day then on untreated artificial diet until pupation (transitional effect). In the second trial, tomato plants were sprayed with the recommended FR of each insecticide under field conditions. Leaves with different residual ages of each insecticide, together with leaves from unsprayed control plants were picked daily and fed to 2nd instar larvae in the laboratory for 5 successive days. Results: The TCD showed the highest non-significant direct toxic effect on 2nd instar larvae of S. littoralis, with shorter LT 50 compared with other insecticides. The TMX was the most effective insecticide against 2nd instar (accumulative effect) and 4th instar (direct and accumulative effects) with a significantly shorter LT 50 in 4th instars. In all trials, AMD had the least toxic effect compared other insecticides tested. The TMX, TCD and ICD significantly reduced pupation percentage but did not affect adult emergence compared with control suggesting low transitional toxic effect. Feeding 2nd instar larvae on tomato leaves treated with insecticides showed significant direct and accumulative mortality effects and a shorter LT 50 with only TCD. Conclusion: Laboratory assays indicated that thiacloprid and thiamethoxam had the highest significant effects on S. littoralis which need to be confirmed in a larger-scale field trials.