Lytta nuttalli Say, a large purple and green blister beetle, readily attacks rapeseed in western Canada (Church 1975; Church and Gerber 1976, 1977). As L. nuttalli commonly feeds on caragana (Caragana arborescens Lam.) on the Canadian prairies, attacks on rapeseed crops often occur near caragana hedges. A typical attack was observed near Vanscoy, Saskatchewan in July 1981. A large number of L. nuttalli, from an adjacent caragana hedge where they were abundant, were feeding on experimental plots of a canola variety of Brassica napus L. rapeseed, and rapidly devouring leaves, stems, and flowering heads. Often they chewed off the stems and fed on the severed plant parts on the ground. There were such large clusters of the beetles on some plants that stems were bent to the ground. The gregarious behaviour of these beetles, and their habit of assembling in clusters on their food plants, are well known (Selander 1960; Church 1975; Church and Gerber 1976, 1977). In the experimental plots at Vanscoy, insecticide application was necessary for survival of the canola plants. Fortunately, attacks by L. nuttalli on commercial canola crops are not frequent enough to constitute a major problem. Increased canola production itself is not likely to cause major increases in L. nuttalli populations, as populations are probably limited by the availability of ground-nesting leafcutter bee cells, which are necessary for larval development (Church 1975; Church and Gerber 1976).