Assessment of the level of resistance to bifenthrin, cypermethrin and methomyl insecticides in bemisia tabaci (gennadius) and trialeurodes vaporariorum (westwood) whitefly populations from selected sites in Kenya
Njuguna, Thomas Njaramba
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Despite heavy use of insecticides against whiteflies in Kenya, no information was available on their resistance to insecticides. In this study, the level of resistance of Bemisia tabaci (Gennadius) and Trialeurodes vaporariorum (Westwood) populations to methomyl (carbamate), cypermethrin and bifenthrin (phrethroids) was assessed using leaf dip and glass vial bioassay methods. Whitefly samples were obtained from a laboratory culture and five field sites in kenya: Kibwezi, Kitui , Mwea, Kihara and Nguruman. The samples were collected from tomatoes, eggplants (brinjals) squash (corgettes) Dolichos, French beans and Soya beans. The resistance levels of the field populations were determined using the probit analysis method. Resistance levels of the populations dominated by the two whitefly species as well as the expression of resistance from the two bioassay methods were compared. Cypermethrin had the highest lethal concentration (LC) values at every site while bifenthrin had the lowest. The whiteflies from Nguruman, consisting of 94% T. vaporariorum, were the most resistant to cypermethrin with resistance factors (RF) of 87.971 and 10.085 for the leaf dip and glass vial methods respectively. Whiteflies from Kihara (98%) T. vaporariorum) were the most resistant to bifenthrin (RF 6.821), and to methomyl with RF 8.087 and 7.593 for leaf dip and glass vial methods respectively. Resistance levels of the Kibwezi field population (88% B.tabaci) were moderately high for cypermethrin (RF 5.646) and methomyl (RF 4.168) in the glass vial bioassay. The whiteflies from Kitui (96% B.tabaci) were the most susceptible to the insecticides tested. Comparison of the expression of resistance through the two bioassay methods showed the leaf dip method to be better in distinguishing the susceptible from resistant whitefly populations. On average, T.vaporariorum dominated populations were more resistant than those dominated by B. tabaci. The result indicated some positive relationship between insecticide usage at different sites and resistance to the chemicals. Nevertheless, resistance to insecticides that had never been used at some sites was also noted.