Resistance of two strains of cotton aphid, Aphis gossypii Glover, to fenvalerate and imidacloprid were determined on cotton (Gossypium hirsutum L.) and cucumber (Cucumis sativa L.) after resistance selection of one strain to fenvalerate for 16 consecutive generations, and of a second strain to imidacloprid for 12 consecutive generations on cotton in greenhouses. Dose-response and activities of detoxication enzymes of the fenvalerate-resistant strain (R-fenvalerate), the imidacloprid-resistant strain (R-imidacloprid), and a susceptible strain (S) were determined. After 16 consecutive generations of selection, resistance of A. gossypii to fenvalerate increased >29,000-fold and to imidacloprid 8.1-fold. On cucumber. resistance of the R-fenvalerate strain to fenvalerate increased 700-fold and to imidacloprid 3.6-fold. However, the most significant finding in this study was that the R-imidacloprid strain exhibited cross-resistance to fenvalerate, with a resistance ratio of 108.9-fold on cotton and 3:3.5-fold on cucumber, whereas the R-fenvalerate strain did not show significant cross-resistance to imidacloprid on either plant species. Both resistant strains of A. gossypii were more resistant to fenvalerate on cotton than on cucumber, whereas their susceptibility to imidacloprid on otton and cucumber were not significantly different. The response of the S strain to fenvalerate and imidacloprid were similar on cotton and Cucumber. Activities of acetylcholinesterase (AChE) and alpha-naphthylacetate (alpha-NA) esterases of A. gossypii were significantly different among the three strains, with the R-fenvalerate strains having the highest, followed by the R-imidacloprid strain, and the S strain the lowest. The activities of the AChE and alpha-NA esterases for all three strains were also significantly higher on cotton than on cucumber. The resistance mechanism and resistance management strategies for the R-fenvalerate and R-imidacloprid strains of A. gossypii to fenvalerate and imidacloprid on cotton and cucumber are discussed.

Susceptibility of eggs, nymphs and adults of the greenhouse whitefly, Trialeurodes vaporariorum (Westwood) (Hom., Aleyrodidae), to six insecticides, abamectin, buprofezin, imidacloprid, acetamiprid, fenpropathrin and profenofos, was determined in the laboratory. All insecticides were most toxic to the 6-day-old eggs and the first instar nymphs, and decreased with successive stages. Of the three ages of the eggs, all insecticides were significantly more toxic to the older eggs (6-day old) than to the young (2-day old) and mid-aged eggs (4-day old). Abamectin was most toxic to eggs and nymphs, followed by imidacloprid, acetamiprid, buprofezin, fenpropathrin, and profenofos. Susceptibilities of the three stages of the fourth instar nymphs to the six insecticides varied greatly, and the first stage (17-day old) was most susceptible, followed by the second stage (20-day old), and the third stage (21-day old). Imidacloprid was the most toxic insecticide to T. vaporariorum adults, followed by acetamiprid, buprofezin, abamectin, fenpropathrin and profenofos. The results from this study can not only serve as baselines for resistance monitoring programmes, but also have significant implications for development T. vaporariorum management programmes, including accurate timing of the applications of insecticides that target the most susceptible late egg and early nymphal stages.