The Distribution of Insects at Low Levels in the Air

Abstract

Migrating insects are often recorded as flying at specific heights, usually a few feet above the ground; migrant butterflies are specially apt to give the impression of flying at a selected level. But the same impression is also gained by observers of other kinds of insects. How much the judgment with small insects is affected by the height of the observer, 5-6 ft, is a matter for conjecture. In contrast to such impressions, many small insects have been caught at heights up to 15 000 ft and such insects can hardly be supposed to have chosen to fly at any specific height. The distinction between a selected height of flight and one imposed to some extent by forces outside the control of the insect is important in developing a rational view of insect movement, including the distances travelled. Agricultural entomologists studying crop infestations have collected most of the existing information on low-level flight, so the species studied are few and all are small. Nevertheless, workers have suggested that given species mostly fly at specific heights and that different species, even of such a physically homogeneous group as aphids, fly at different heights. If this suggestion is correct, it implies a different kind of distribution of the same insects at low and at high levels. Johnson (1957a) has shown from trapping records at high levels that insect density diminishes continuously and regularly as height increases, which suggests that distribution is determined by factors outside the immediate control of the insect. The ranges of height he dealt with were: 200 to 5000 ft (data from Glick 1939); 80 to 1500 ft (data from Hardy & Milne 1938); 10, 177 and 277 ft (data from Freeman 1945); 50 to 2000 ft and a small amount of material at 10, 50, 500 and 1000 ft (data of Johnson 1957a). Although these records often extend up to very high levels, they are relatively few near to the ground. Small insects must frequently fly in wind speeds well above their own low flight speeds even at low levels. For example, the flight speed of thrips (T. Lewis, unpublished), aphids and psocopterons (Taylor, unpublished), and probably many other small insects, is less than 1 m/sec. The air speed at Kew, a sheltered inland station in England, in 1937 exceeded 1 m/sec at 10 m for almost 90 % of the time. At 2 m it would exceed 0*78 m/sec for 90 % of the time (see Gregory 1945); in other words, the wind above 2 m is higher than the flight speed of many small insects for most of the time. Johnson (1954) showed conclusively that migratory aphids fly mainly in windy weather. This might be expected to lead to some kind of continuity of density distribution at all levels. I have therefore re-examined the