Effect of manual topping on insect pest incidence and cotton yield

Bollworms and sap-sucking insect pests are a significant constraint to cotton production in Africa. Manual topping of cotton plants, which consists of cutting off the top of the plant, i.e. the terminal bud of the main stem, is a promising alternative to chemical control by removing resources for certain pests or inducing plant defences. In this study, we evaluated the level of protection by topping against bollworms and sap-sucking insect pests and the effect of topping on seed cotton yield. Six experiments combining insecticide protection (sprayed vs. unsprayed plots) and topping (topped vs. non-topped plots) were conducted in Mali and Senegal. Topping by itself conferred weak protection against bollworms with an 11% reduction in the proportion of damaged shed squares and bolls, compared to insecticide protection (−55%). Topping had a low to moderate effectiveness in reducing the incidence of sap-sucking pests (−20 to −48%), but equivalent to insecticide treatments. Although topping significantly reduced the proportion of damaged bolls (−31%) at harvest in the absence of insecticide protection, this did not substantially affect seed cotton yield (+4%), unlike insecticide protection (+52%). However, this technique could be advantageously combined with threshold-based interventions integrating biopesticides to reduce the environmental impact of crop protection in cotton.