Insecticide drift and reinvasion of spray blocks in aerial spraying experiments against Glossina Morsitans Centralis Machado (Diptera: Glossinidae)

Abstract

AbstractThe effect of insecticide drift on Glossina morsitans centralis Machado in Botswana was assessed using two-man fly-rounds downwind of the 1700–4000 km2 aerial spraying blocks. The blocks were subjected to sequential applications of endosulfan at between 6 and 12 g/ha with a droplet v.m.d. of 30 μm. Post-spray reinvasion of flies was assessed using two-man fly-round surveys across the edge of one spray block along ideal linear habitat. Each application had an effect on the flies outside the spray block, even at a distance of 20 km downwind. The first two applications of a series had a greater effect than later ones. Total reductions of G. m. centralis after a series of 5 or 6 applications were 92·5% 7 km outside a block sprayed in 1975, 98·3% 6–12 km outside a block sprayed in 1977 and 64·2% 10–20 km outside a block sprayed in 1978. Ageing studies showed that some females 10–20 km downwind of the sprayed area survived up to five applications, and three months after the final application the population there had recovered almost to pre-spray levels. The main front of fly encroachment after spraying across the downwind edge of the 1978 block travelled at about 0·8 km/month in the first 17 months after spraying, but small numbers of flies were found well ahead of the main front. It was concluded that the situation in the drift area could be ideal for the development of insecticide resistance and that adjacent blocks sprayed in consecutive years should overlap by 10 km.