Abstract:
Laboratory studies were performed on tobacco spider mites (Tetranychus evansi) collected from Chinamhora and Mutoko to investigate grower claims of resistance to commonly used acaricides. Bioassays of T evansi collected from tomatoes showed presence of resistance to dimethotae, malathion, abamectin and amitraz. The RF values for Chinamhora were 1.2 for abamectin, 1.0 for malathion and 1.4 for an1itraz and dimethoate. For Mutoko RF values were 1.2 for aban1ectin and malathion, 1.4 for amitraz while it was 12.1 for dimethoate. The farmers could have used these chemicals for a long time without changing them especially
dimethoate. In addition, their water quality could have been compromising control, as pesticides are effective with clean water. In the field, there was no evidence of resistance although populations increased as the season progressed. Damage leaf indices for site one ranged from 0-3 for the 4 weeks while those of site two were between 0-4 in the 7 weeks data was collected. At both sites, malathion and dimethoate had the highest indices showing that they were not as effective as the other acaricides. Dimethoate however perfonned better than malathion. From a visual assessment, aban1ectin was the most effective followed by an1itraz. Using the risk category for the natural enemies, bioassays showed that all the
acaricides were harmful. Malathion and dimethoate were slightly harmful to the ladybird beetle but moderately harmful to the other three natural enemies. Amitraz was moderately harmful to all natural enemies. Abamectin was moderately ham1ful to the ladybird beetle and aphid wasp parasitoid while harmful to the predatory mite. As a result of this experiment control options and recommendations as part of an insecticide resistance management strategy were discussed briefly. These, it is envisaged, would delay or slow down development of insecticides resistance (IR).
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