Factors Affecting the Aggregation of the Adult Cabbage Root Fly (Erioischia brassicae (Bouche)) at Hedges

Members of various groups of insects, including weak fliers such as Cecidomyiidae and stronger flying Staphylinidae, accumulate on the leeward side of artificial and natural windbreaks (e.g. Lewis 1965, 1969, 1970; Lewis & Stephenson 1966). Many factors affect this accumulation, including the height and permeability of windbreaks and the wind speed, but it has been suggested (Lewis & Dibley 1970) that insects accumulate because they are carried on the wind and are caught in the turbulent circulating air mass to the leeward of the windbreaks. Hawkes (1 972a) showed that adult Erioischia brassicae accumulated at hedges and this behaviour was associated with feeding from hedgerow flowers. There was a diurnal population movement of flies between hedges and a brassica crop and this appeared to be incompatible with accumulation at windbreaks as a result of the insects being carried on the wind. This paper describes experiments to observe whether wind causes aggregation of E. brassicae at natural hedges and at a lath barrier representing an artificial windbreak.