Effect of pesticides on the tobacco spider mite tetranychus evansi baker & pritchard on tomatoes in kenya.

Abstract

The objective of this study was to evaluate the efficacy of the pesticides commonly used by farmers in controlling the Tobacco spider mite Tetranychus evansi Baker & Pritchard. Laboratory and greenhouse experiments were carried out to determine the susceptibility of T. evansi to dicofol, bifenthrin, propargite, lambda-cyhalothrin, dimethoate and profenofos+cypermethrin using the manufacturer’s recommended rates. Ovicidal and adulticidal (contact and residual) assays were done in the laboratory. In the greenhouse, mite leaf damage scores were taken and mite population per cm2 was determined. Laboratory results showed egg mortalities due to contact effect to be 26.1% in dimethoate treatment and 100% in dicofol, bifenthrin, propargite, rofenofos+cypermethrin and lambda-cyhalothrin treatments. The adult mortality due to contact with the pesticides was 100% for dicofol, propargite, bifenthrin and profenofos+cypermethrin treatments whereas mortality for lambdacyhalothrin was 80%, dimethoate 4% and none in the control. Mortality due to residual effect was 100% for dicofol and was lower for all the other compounds although the mites escaped to the cotton barrier. Greenhouse studies showed that damage scores and mite populations reduced after the application of dicofol, propargite, bifenthrin, lambda-cyhalothrin and enofos+cypermethrin whereas for dimethoate and control, the scores continued increasing. There was gradual increase in leaf damage and mite populations per cm 2 in both Dimethoate treated plants and the control over time. Tobacco spider mites (T. evansi) collected from Loitoktok, Kibwezi, Athi-River and Subukia were evaluated for their susceptibility to bifenthrin, profenofos+cypermethrin, dicofol, dimethoate, lambda-cyhalothrin and propargite. 6 Bifenthrin, profenofos+cypermethrin, dicofol and propargite caused significant mortality in the mites from all the populations tested, however, mortality did not differ significantly between the populations for each of these pesticides (P<0.05>Pooled data also showed that mortality did not differ signicantly between dicofol, bifenthrin, propargite and dicofol with average contact adult mortalities of 100%, 99.67%, 99.83% and 99.83% respectively. Lambda-cyhalothrin caused significant mortality in the mites from Kibwezi. However, mites from Loitoktok, Subukia and Athi-River showed significant tolerance. There were significant levels of tolerance to dimethoate in all the populations sampled with the highest adult mortality of 42.5% for the Kibwezi population. Pooled data shows that there was significant difference (p<0.005> between lambda-cyhalothrin (65.33%), dimethoate (20.5%), the control (1%) and the rest of the pesticides tested. . Mites from most Kenyan farms respond well to bifenthrin, propargite, profenofos+cypermethrin and dicofol. Dimethoate and lambda-cyhalothrin should be avoided