Are Helicoverpa armigera and Tetranychus spp. populations still susceptible to pesticides? A Zimbabwean study.

The African bollworm,Helicoverpa armigera(Hübner), and the cotton red spider mite,Tetranychusspp., are important pests of cotton in Zimbabwe. A study to assessH. armigeraresistance to fenvalerate 20 EC (Cyano (3-phenoxyphenyl) methyl 4-chloro-α-(1-methylethyl)benzene acetate) andTetranychusspp. resistance to amitraz 20 EC (n’-(2,4-dimethylphenyl)-n-[[(2,4-dimethylphenyl)imino]methyl]-n-) was conducted at the Cotton Research Institute (CRI) during the 2005/06 cotton-growing season. Field populations ofH. armigeraandTetranychusspp. were collected from some of Zimbabwe’s major cotton-growing areas of Sanyati, Umguza, Chisumbanje, Chinhoyi and the CRI in Kadoma and exposed to bioassays. The African bollworm leaf disc technique and the red spider mite attached leaf-dipping technique were used to assess responses of the African bollworm and red spider mite to the pesticides. Susceptible laboratory populations served as the standard populations and their responses were compared with those of the field populations. The graphical method and MSTAT-C probit analysis computer program were used to calculate LC50values. Although the CRI field population, used as a reference population for the registration ofH. armigerainsecticides, had the highest LC50value (graphical = 0.000100000; MSTAT-C = 0.000088195) compared with all the other field populations, overall log-dose probit bioassays on all field-collected strains of the bollworm showed no resistance to the pyrethroid (RFs = 0.04–0.54-fold).Tetranychusspp. showed very low levels of resistance (RFs = 1.26–2.00-fold). Continuous monitoring of major cotton pests, especiallyH. armigeraandTetranychusspp., from all cotton districts of Zimbabwe is vital for early detection of resistance development. [ABSTRACT FROM AUTHOR]