Toxicities of 4 neonicotinoid insecticides to Phenacoccus solenopsis by 3 bioassay methods

【Objective】 To study the chemical control of cotton mealybug Phenacoccus solenopsis Tinsley and to screen its efficient pesticides.【Method】The toxicities of 4 neonicotinoid insecticides (imidacloprid, acetamiprid, thiamethoxam, nitenpyram) to P. solenopsis were determined by three methods: leaf dipping, nymph and leaf dipping, and the residual film method under the laboratory conditions. 【Result and conclusion】 The results showed that the mortalities of the 3^rd instar mealybug nymphs treated with 40 mg·L^-1 thiamethoxam after 24 and 48 h were 90.0% and 98.8% by using leaf dipping method, 98.9% and 100.0% by using nymph and leaf dipping method, 85.8% and 85.8% by using the residual film method. Relative toxicities of four insecticides from high to low to P. solenopsis 3^rd instar nymphs through leaf dipping method were thiamethoxam, acetamiprid, nitenpyram, and imidacloprid. Relative toxicities from high to low tested by nymph and leaf dipping method were thiamethoxam, imidacloprid, acetamiprid, nitenpyram, and those for the residual film method were thiamethoxam, acetamiprid, imidacloprid, and nitenpyram. Thiamethoxam was proved to be the most efficient to P. solenopsis among these four insecticides. The relative toxicity of imidacloprid was the highest by nymph and leaf dipping method; the secondary was residual film method, and the lowest was leaf dipping method. For acetamiprid, the relative toxicities to the mealybug by the nymph and leaf dipping method and the residual film method were similar, while the relative toxicity of leaf dipping method was lower. The toxicity of thiamethoxam to the mealybug with the nymph and leaf dipping method was higher, and those were lower with leaf dipping method and the residual film method. The same results for nitenpyram were presented with the above bioassay methods. Nymph and leaf dipping method was proved to give the comprehensive toxicity of contact effect and stomach poisonous effect for neonicotinoid insecticides in a better way.