Resistance risk assessment to chlorpyrifos and cross-resistance to other insecticides in a field strain of Phenacoccus solenopsis Tinsley

The organophosphate insecticide chlorpyrifos is recommended for control of a number of insect pests, including cotton mealybug, Phenacoccus solenopsis Tinsley (Homoptera: Pseudococcidae) in Pakistan. This work assessed chlorpyrifos resistance evolution and cross-resistance to other insecticides. After 23 generations of selection, the P. solenopsis strain (Chlor-SEL) had a 26652-fold level of resistance to chlorpyrifos compared to a susceptible strain. Realized heritability (h2) of resistance to chlorpyrifos was 0.04. The Chlor-SEL strain also had a low level of cross resistance to lambda-cyhalothrin (14-fold) and a very low level cross-resistance to nitenpyram and profenofos after 23 generations of selection. The projected rate of resistance development indicated that if 50-90 percent of a P. solenopsis population were selected with chlorpyrifos, a ten-fold increase in the lethal concentration 50 (LC50) would occur in 22-10 generations (h2 = 0.04, Slope = 0.70). At a similar slope, if h2 = 0.14, then only 6-3 generations are required for a ten-fold increase in the LC50 at 50-90 percent selection intensity, respectively. Likewise, if h2 = 0.24, then the same would occur in 4-2 generations. This study showed that P. solenopsis has the ability to become resistant to chlorpyrifos but insect resistance management strategies such as rotation of different group of insecticides are needed to prolong the effectiveness of chlorpyrifos in controlling P. solenopsis.