Spray-collecting Area of Locusts and their Susceptibility to Insecticides

Abstract

LOCUST swarms may be sprayed with insecticides when the insects are settled or in flight. Techniques have been developed for the estimation of the amount of spray collected by individual locusts^1–4, and fundamental to their interpretation is the spraycollecting area of a locust. Two areas have been used for calculations: the plan area for settled locusts^{1 2}, reduced to allow for the less vulnerable parts, and the ‘horizontal equivalent area’ for flying locusts^3,4, defined as the horizontal plane area which, when passed through a given spray of droplets with the same velocity as a locust, collects the same number of droplets as the locust. The plan area of the desert locust, Schistocerca gregaria (Forsk.), is about 6½ cm.², and the horizontal equivalent area has been found experimentally to vary greatly with drop size, increasing from 30 cm.² for droplets 300µ in diameter to 450 cm.² for 50µ drops⁵. The collecting surface of a settled locust approximates to the plan area when the drops are falling nearly vertically, but at shallow angles of approach the quantity of spray deposited on the locust is many times that estimated from area dosage and plan area.