Studies of the physical and biological factors involved in the application of concentrated involatile oil solutions of a persistent insecticide (dieldrin) in the form of a fine spray to the low-density vegetation typical of the breeding areas of the desert locust, Schistocerca gregaria (Forsk.), have led to a technique of control of the locust hoppers with a dosage as low as 0·30 litres per hectare (approximately ⅕ pint per acre). A simple device for producing such a spray has been designed to work off the exhaust system of a light four-wheel-drive motor vehicle. Successful control of hoppers has been obtained under a wide variety of weather conditions in both desert and bush typical of locust breeding areas, using ‘target’ and ‘barrier’ spraying techniques. Both techniques involve the spraying of vegetation which it is judged will be subsequently eaten by the hoppers, but target spraying is done on or near a known hopper band, whereas barrier spraying is done, not necessarily in sight of hoppers, across an area known to be highly infested. Examples of such successful attacks are described from the Eritrean coast, Tripolitania and Ethiopia. The methods can replace the well established technique of poison-baiting individual bands, and have the additional advantage of producing highly persistent toxic swathes which can be exploited by the barrier technique for treating locust egg-fields and for creating widely spaced barriers for the control of extensive infestations.