Spinosad resistance selected in the laboratory strain of Phenacoccus solenopsis Tinsley (Hemiptera: Pseudococcidae): studies on risk assessment and cross-resistance patterns

Cotton is a major cash crop of Pakistan and susceptible to a variety of insect attacks including the cotton mealybug, Phenacoccus solenopsis Tinsley (Hemiptera: Pseudococcidae). In this study we determined the rate of risk of resistance development in P. solenopsis against spinosad by selection experiments and also evaluated the cross-resistance of spinosad selected population with spinetoram, profenofos, and deltamethrin. The spinosad selected population developed 539.49 and 15,233.84-fold resistance after 22 generations of selection compared with field and laboratory susceptible populations, respectively. Cross-resistance induced due to selection with spinosad was very low, medium and very high to profenofos, spinetoram, and deltamethrin, respectively in P. solenopsis. Realized heritability (h(2)) of spinosad resistance was 0.06. At constant slope = 1.10, ten-fold increase in resistance to spinosad would require 8.27, 3.1, and 1.91 generations for their corresponding h(2) = 0.06, 0.16 and 0.26 at 95% selection intensity. At constant h(2) = 0.06, the number of generations needed for ten-fold increase in spinosad resistance would be 8.27, 15.78, and 23.30 for their corresponding slope values 1.10, 2.10 and 3.10, respectively if selection intensity is 95%. The development of spinosad resistance in P. solenopsis can be reduced by adopting IPM strategies such as insecticide rotation schemes, i.e. discontinue the use of the same insecticide for as long as possible, use of refuges and adoption of suitable cultural and biological control practices.