Characterization of inheritance and preliminary biochemical mechanisms of spirotetramat resistance in Phenacoccus solenopsis Tinsley: An economic pest from Pakistan.

Phenacoccus solenopsis is an economically important insect pest of different agronomic and horticultural field crops. In Pakistan, the cotton crop was severely attacked by P. solenopsis during 2007 and since then a varied group of insecticides are used by farmers to manage this pest. As a result, insecticide resistance has become a barrier in control of P. solenopsis. The current study was designed to explore the basics of genetics, realized heritability and possible genetic mechanisms of resistance against spirotetramat in P. solenopsis. Before selection, the wild population (Wild-Pop) showed 5.97-fold resistance when compared with lab-reared susceptible strain (Susceptible Lab-Pop). The P. solenopsis was selected with spirotetramat to 21 generations, called Spiro-SEL Pop, which showed 463.21-fold resistance as compared with the Susceptible Lab-Pop. The values of LC50 for F1 (Spiro-SEL Pop ♂ × Susceptible Lab-Pop ♀) and F1 (Spiro-SEL Pop ♀ × Susceptible Lab-Pop ♂) populations were statistically similar and values of dominance level were 0.42 and 0.54, respectively. Reciprocal crosses between Susceptible Lab-Pop and Spiro-SEL Pop showed that resistance was of autosomal in nature with incomplete dominant traits. According to the fit test, monogenic model estimation of the number of genes, which are responsible for the development of spirotetramat resistance in a population of P. solenopsis, showed that multiple genes are involved in controlling the resistance levels in tested strains of P. solenopsis. The value of heritability for resistance against spirotetramat was 0.13 in P. solenopsis. Our results suggested the presence of a metabolic-based resistance mechanism associated with the monooxygenases in P. solenopsis, while testing the synergism mechanism. These results will provide the baseline to design an effective control strategy to manage P. solenopsis in the field.

Laboratory selection of chlorpyrifos resistance in an Invasive Pest, Phenacoccus solenopsis (Homoptera: Pseudococcidae): Cross-resistance, stability and fitness cost.

The cotton mealybug, Phenacoccus solenopsis is an important polyphagous sucking pest of ornamentals, horticultural and fiber crops worldwide. Some P. solenopsis populations have developed insecticide resistance. This study evaluated cross-resistance, stability of insecticide resistance and life history traits affected by chlorpyrifos resistance in P. solenopsis. After nine generations selected with chlorpyrifos, P. solenopsis exhibited a 539.76-fold resistance level compared to an unselected population (UNSEL Pop). Chlorpyrifos selected population (Chlor-SEL Pop) displayed moderate cross-resistance to profenofos, nitenpyram and high cross-resistance to lambda-cyhalothrin. Biological parameters of P. Solenopsis were affected by chlorpyrifos resistance. The Chlor-SEL Pop had a significant reduction in fitness (relative fitness=0.10), along with significant decreases in pupal weight, fecundity, egg hatching %, intrinsic rate of natural population increase, biotic potential, and mean relative growth rate. It is concluded that selection with chlorpyrifos had marked effect on resistance development in P. solenopsis and upon removal of selection pressure chlorpyrifos resistance declined significantly indicating unstable resistance. Development of resistance led to high fitness costs for the chlorpyrifos-selected strain. These findings should be helpful for better and more successful resistance management of P. solenopsis.

Spirotetramat Resistance Selected in the Phenacoccus solenopsis (Homoptera: Pseudococcidae): Cross-Resistance Patterns, Stability, and Fitness Costs Analysis.

The Phenacoccus solenopsis Tinsley (Homoptera: Pseudococcidae) is a major agricultural and horticultural pest of crops throughout the world. To develop a better resistance management strategy for P. solenopsis, we conducted a study on life history parameters of different populations of this pest, one selected with spirotetramat (Spiro-SEL), an unselected (UNSEL) population, and their reciprocal crosses. We also studied the cross-resistance and the stability of spirotetramat resistance. The Spiro-SEL of P. solenopsis exhibited a 328.69-fold resistance compared to the susceptible population (Lab-PK). The Spiro-SEL population also displayed a moderate level of cross-resistance to profenofos and bifenthrin and a high level of cross-resistance to abamectin. Resistance to spirotetramat in Spiro-SEL was unstable in the absence of selection. The study of life history parameters showed that there was a significant reduction in fitness parameters of Spiro-SEL population with a relative fitness value of 0.14. There was a significant decrease in survival rate, pupal weight, fecundity, egg hatching percentage, male and female generation time, intrinsic rate of population increase of males and females, biotic potential, and mean relative growth rate. It is concluded that selection with spirotetramat had marked effect on resistance development in P. solenopsis and upon removal of selection pressure spirotetramat resistance declined significantly, indicating unstable resistance. Development of resistance led to high fitness costs for the spirotetramat-selected population. Our study may provide the basic information on spirotetramat resistance and its mechanism to help develop the resistance management strategies.