Cotton pest control in tropical Africa: advances in techniques and results

Within the nine major cotton producing countries of subsaharan French-speaking Africa, seed cotton production reached 1.3 million tonnes in 1988. Indeed, almost exclusively produced by smallholders, the average yield has increased from 400 kg to over a tonne of seed cotton per hectare in the last fifteen years. Phytosanitary protection is one of the main factors which have contributed to this improvement and 78 percent of the areas under cotton benefit from insecticide protection. Pests can be grouped in four categories: pests attacking the reproductive organs, mainly Lepidoptera larvae, leaf-eating caterpillars, sucking insects, mites. The principles of chemical control are based on spraying programmes which are pre-established according to the cotton plant's physiological evolution. According to the country, 4 to 6, exceptionally 8 sprayings, are carried out every 10 to 14 days. The evaluation of this type of protection is positive and insecticide consumption limited (15 litres of commercial formulating per hectare). Pesticide active ingredients are essentially pyrethroid-organophosphorous associations. The pyrethroids are used against Lepidoptera while the organophosphates are used against mites and sucking insects. In 98 percent of the cases, application techniques use spinning disc sprayers ULV with a 1 to 3 liters per hectare of ready to use formulation opposed to the former technique of 80 to 120 liters per hectare emulsion in water, applied with a knapack sprayer fitted with a horizontal boom. This technological transfer can be considered to be the prime reason for the increase in protected surfaces and consequently for the spectacular rise in production. Coverage is perfect at the upper part of the canopy; however, despite more numerous and smaller droplets, the lower parts of the cotton plant and the under surface of the leaves is never reached by this spraying method. This furthers the development of pests such as mites or sucking insects. At present some countries such as Cameroon apply the VLV (Very Low Volume) technique using the same equipment with emulsifiable concentrate (E.C.) formulation and 10 1 water per hactare. Besides, new spraying programs with less A.I. are developed for their saving potential. Chemical control constitutes the essential element of cotton crop production but biological assistance makes it possible to envisage new developments: resistance traits of the cotton variety, biological agents such as entomophagous insects, entomopathogens and pheromones