Population Dynamic of Phenacoccus solenopsis Tinsley on Cotton Plants and Its Susceptibility to Some Insecticides in Relation to the Exposure Method

The cotton mealybug, Phenacoccus solenopsis Tinsley (Hemiptera: Pseudococcidae) was first recorded during 2014 as an invasive cotton pest in Egypt. Thus, field experiments were conducted during the seasons 2015 and 2016 to study the population dynamic of P. solenopsis on cotton in Kafr El-Sheikh governorate, Egypt. Also, chlorpyrifos, methomyl, deltamethrin, lufenuron, acetamiprid, thiamethoxam and imidacloprid were tested for their toxicity to the third instar nymphs of P. solenopsis under laboratory conditions using leaf-dip and insect-spray methods. In both seasons, the infestation of P. solenopsis started during the early June when the cotton plants aged about two months. The highest population densities of 142.9+-15.6 and 191.5+-18.1 insects per twig were recorded on September 6th 2015 and September 11th 2016, respectively. The infestation was positively correlated with the maximum (r = 0.77) and minimum (r = 0.71) temperature and the relative humidity (r = 0.49). Based on the multiple regression analysis and the coefficient of determination values (R^2), the maximum and minimum temperature and the relative humidity were responsible for the changes in the insect population by 54.2 -- 63.7%. Thiamethoxam was the most effective insecticide (LC50 = 23.6 mg AI L^-1) against the third instar nymphs of P. solenopsis using the leaf-dip method, while lufenuron was the least toxic one with LC50 value of 556.1 mg AI L^-1. When the insect-spray method was adopted, the toxicity of deltamethrin and lufenuron increased, whereas the toxicity of the others decreased comparing to that obtained in the leaf-dip method.