Early in 1974 the Botswana Government approached the British Ministry of Overseas Development with a request for assistance in monitoring the potential side-effects of aerial spraying for tsetse fly control in the Okavango Delta. Unintended effects would be the killing of non-target animals or plants, the accumulation of residues of the more persistent compounds in the environment and the selection of strains of pest insects which are resistant to one or more pesticides.