Abstract:
The two-spotted spider mite Tetranychus urticae is a pest of several plants including vegetables, fruits, ornamentals and field crops. Damage done to the host plant can cause leaf and fruit loss, complete defoliation and death of the host plant. They are mostly managed by the use of acaricides. Acaricides resistance has been reported in several countries, this is due to the fact that T. urticae fully becomes resistant to an insecticide after two to four years of use. The present study was initiated to monitor resistance by T. urticae to some selected insecticide in Accra. A survey carried out in the Accra suburbs revealed pattern and type of insecticide use for the control of pest, diseases and weeds. The survey also revealed that farmers have little knowledge about T. urticae and so do not specifically targets T. urticae in terms of control but that the insecticide used address the complex of pest affecting their crops. These insecticides are used in high dosages and at short intervals, this serves as a main cause of 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 sites in Accra (Opeibea, Ashaiman, University of Ghana farm and Department of Crop Science Sinna’s garden). These populations were used to raise colonies of spider mite in the screen house of the Department of Crop Science. Spray application was used to determine the resistance in T. urticae by application of different rates of acaricides against the different strains of the spider mite. Mortality was recorded 24 h after treatment. The LD50 values and slopes were estimated by probit analysis and resistance factor were calculated by dividing LD50 of field population by LD50 of susceptible strain. All field populations were quite resistant to Karate (up to 21.6-fold), the LD50 values for the susceptible strain was 0.007 ml/L and varied significantly from those of the field population which ranged from 0.084-0.151 ml/L. All field populations were susceptible to Levo®, Protect® and Sulphur®. The University of Ghana farms, Sinna’s garden and Ashaiman populations were susceptible to Imidacloprid except the Opeibea population which showed a low level of resistance (9-fold) to Imidacloprid. Optimal (non-lethal) concentration of the synergists (PBO and DEM) were also determined using the karate resistant population and a time series experiment was carried out to determine the time lapse between synergist and insecticide application. The optimal (non-lethal) PBO and DEM concentrations were 0.4 and 1.0 μl/ml respectively. Time series experiment showed significant difference between mortalities in mites exposed to karate immediately after pre-treatment with synergists and those with the miticide 1 h later. 1 h after pre-treatment with PBO gave a 94.8 and 94.7 % mortality in Opeibea and Ashaiman resistant population respectively. The 1 h pre-treatment with DEM also gave 88.2 and 85.05 % mortalities in Opeibea and Ashaiman resistant populations respectively. The synergist ratios obtained for Opeibea population was 4.3 and 2.9-fold for PBO and DEM where as that of Ashaiman is 2.2 and 2.6 for PBO and DEM respectively. The result of the synergist experiment suggests the involvement of glutathione-S-tranferases (GSTs) and cytochrome P450 monoxygenases as the possible mechanisms of resistance.
