Abstract

Background: Application of chemicals for the control of Helicoverpa armigera and Diparopsis castanea is based on egg threshold, while for Erias insulana and E. biplaga larval counts are used. In Zimbabwe the traditional farmer practice involves use of weekly spraying, fortnightly spraying and spraying at first sight of damaged squares. The methods may be less effective, costly, damaging to the environment and labour intensive. Experiments were conducted for three consecutive seasons from 2010/11 season up to 2012/13 season to evaluate the spray timing methods. Spraying at bollworm egg threshold level was the standard. The trial was laid as a randomised complete block design replicated four times at Cotton Research Institute, Dande and Umguza. Results: Analysis of variance was done on bollworm larval counts and seed cotton yield using Genstat 14th version of 2011. The standard method and weekly spraying produced comparably reduced larval counts. Fortnightly spraying and spraying at first sight of damaged squares was less effective in control of bollworms. The standard method significantly spared predators at most sites over the three seasons in contrast with weekly spraying. However predator counts for standard and the other methods were comparable. Results of cost-benefit analysis done only in 2012/13 showed that weekly spraying was most expensive than other methods. Conclusion: The standard method continued to be the best spray timing method for farmers.