Many factors influence the host preferences of entomophagous insects and have an important role in determining the fate of parasites in the field. Some of the factors influence the ability of parasites to locate their hosts. Others determine the suitability of the host as a medium for larval development (7, 15, 18, 26, 27, 31). These factors together determine the range of hosts attacked and control the degree to which nonpreferred hosts are selected.

The Lygus complex was treated as a number of genera and subgenera in a recent paper (Kelton, 1955). The genus Liocoris Fieber now contains the economically important species, the pratensis group, referred to collectively in the literature as the “lygus” bugs.

In the Prairie Provinces of Canada, several species of the pratensis group are of economic importance mainly in the growing of alfalfa seed, especially in the north-central agricultural region and in the irrigated areas of the southwest. A review of the names used in dealing with these pests on the Canadian prairies showed that most of them were based on misidentifications, although fortunately the use of the names has been fairly consistent. Such widely recorded and used names of Lygus pratensis (L.) and L. elisus Van D. have each been used for specimens of another species, and L. hesperus Kngt. for those of three other species.