The Collection of Insecticide by Low Flying Locusts

Abstract

Adults of the Australian plague locust, Chortoiceies termini/era (Walker), were towed at about their Right velocity directly through a cloud of insecticide produced by an aircraft in a light wind. The amount of insecticide acquired by the insects entering the cloud at different times after spray release was measured and compared with the characteristics of the cloud. IntroductionThe collection of insecticide by individual desert locusts, Schistocerca gregaria (Forskal), has been studied theoretically and in wind tunnels (Sawyer 1950; Wootten and Sawyer 1954;MacCuaig 1962) but not in the field. Spillman (1976) has shown that most flying insects preferentially collect very small droplets such as are produced by current methods of control of the Australian plague locust, Chortoicetes terminljera (Walker).Swarms of C . terminijera fly only on warm days when the speed of the low-level wind does not greatly exceed the locusts' flight velocity of ca 3 mjs (Clark 1969); even then the majority of the locusts are settled. Individuals rarely rise higher than 15 m but fly in spells of at least several minutes, usually maintaining a constant orientation (Hunter 1979).A swarm is treated by making a series of crosswind runs working upwind to cover the whole target. A flying locust will accumulate insecticide if it is within the spray cloud; the amount accumulated will depend on the volume concentration and the droplet composition of the cloud, both of which change with time after spray release. Wind speed may influence the characteristics of the spray cloud slightly but otherwise will not affect the amount of insecticide collected since every locust which passes through the cloud will do so at its flight velocity. The total amount of insecticide acquired by an individual locust from a single cloud will depend on the time it remains within the cloud and the integral of the rate of acquisition over that period. A locust oriented at right angles to the wind directly beneath the spray aircraft should remain within the spray throughout the cloud life. A locust traversing the cloud by flying either up or down wind will spend the least time within it. This paper describes estimates of the amount of insecticide collected by such a locust in a light wind entering the cloud at different times after release of the spray; the paper also includes information on the change in the characteristics of the spray cloud.