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

摘要

The African bollworm, Helicoverpa armigera (Hubner), and the cotton red spider mite, Tetranychus spp., are important pests of cotton in Zimbabwe. A study to assess H. armigera resistance to fenvalerate 20 EC (Cyano (3-phenoxyphenyl) methyl 4-chloro- alpha -(1-methylethyl)benzene acetate) and Tetranychus spp. 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 of H. armigera and Tetranychus spp. 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 LC₅₀ values. Although the CRI field population, used as a reference population for the registration of H. armigera insecticides, had the highest LC₅₀ value (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). Tetranychus spp. showed very low levels of resistance (RFs=1.26-2.00-fold). Continuous monitoring of major cotton pests, especially H. armigera and Tetranychus spp., from all cotton districts of Zimbabwe is vital for early detection of resistance development.