Spray droplet assessment and insecticide drift studies during the large scale aerial application of endosulfan to controlGlossina morsitansin Botswana∗

Abstract
Tsetse control in the Okavango Delta of Botswana is being conducted using a night‐time drift spraying technique, the effectiveness of which is dependent on the production of the correct droplet size spectrum and on suitable meteorological conditions. A 35 e.c. formulation of endosulfan was sprayed at 5 l/min with an atomiser rotation speed of 10,000–12,000 rev/min and flying speed of approximately 70 m/s. Droplet size measurements were made by sampling on to magnesium oxide coated glass slides, and chemical assessments were made by collecting droplets on filter papers or strands of synthetic angora wool for analysis. The results showed that the method was effective, producing an aerosol cloud of 20–30 μm diam. droplets, but that airborne spray concentrations varied considerably with meteorological parameters. Insecticide drift decayed exponentially with distance downwind from the nearest spray point, but amounts recovered up to 12.5 km downwind were sufficient to kill tsetse. The wool sampler was found to be the most practical and effective method for the assessment of low levels of long range insecticide drift.

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