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The two-spotted spider mite Tetranychus urticae is a serious pest of vegetables, fruits and field crops in Ghana but hardly noticed by farmers. A recent survey in parts of Greater Accra Region revealed that farmers do not specifically target T. urticae in terms of control but use insecticides at high dosages and at short intervals to address the complex of pest affecting their crops. This practice is a recipe for resistance development in mite pest. Susceptibility studies were carried out using five miticides, lambda cyhalothrin, emamectin benzoate, prosular oxymatrine, imidacloprid and sulphur. Adult T. urticae were collected from four suburbs in Accra (Opeibea, Ashaiman, University of Ghana Farm and Department of Crop Science Sinna Garden). These populations were used to raise colonies on garden egg plants in separate laboratories. Infested leaves were taken from the plants and sprayed with serial dilutions of the miticides and mortality recorded after 24 h. The LD50 values and slopes were determined using probit analysis and resistance factors calculated. All field populations were quite resistant to Karate® (up to 21.6-fold) but were susceptible to the other four products where resistance factors were quite low. The University of Ghana Farms, Sinna Garden and Ashaiman populations were susceptible to Imidacloprid except the Opeibea population which showed a low level of resistance (9-fold). Synergism assays using piperonyl butoxide (0.4 μL/mL) and Diethyl maleate (1.0 μL/mL) on the Karate®-resistant population showed that the application of these products 1 h before treatment with Karate® reduce resistance in the Opeibea population from 21-fold to between 2.9 to 4.3-fold whereas that of Ashaiman was reduced from 19-fold to 2.6-fold suggesting the involvement of glutathione-S-tranferases (GSTs) and cytochrome P450 monooxygenases and esterases as the possible mechanisms of resistance.
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Keywords: Tetranychus urticea, resistance, glutathione-S-tranferases, cytochrome P450 monooxygenases, esterases, piperonyl butoxide, diethyl maleate.