The alfalfa plant bug, Adelphocoris lineolatus (Goeze), a well-known pest of alfalfa in Europe and the United States, was first recorded in Saskatchewan in 1947, in the Hudson Bay district about 16 miles west of the Manitoba border. By 1959 it had spread westward through the northern agricultural area of the province to within 35 miles of the Alberta border. Economic infestations appeared within two to four years of its initial occurrence in each district.

The migratory behaviour of the adult, which is apparently essential for maximum reproduction, effects rapid dispersal of the species. Also, because of this migratory behaviour control measures may have to be undertaken each year, and seedling stands may be infested in the first crop year.

The seasonal development of the insect is described. Apparently in the spread of the insect northward a univoltine population is being selected. Eggs that have overwintered in stems hatch in late May and early June. Development proceeds through five nymphal instars, and adults begin to appear toward the end of June. These adults lay eggs during the latter part of July. Most of the eggs enter diapause: less than five per cent of them hatch the same year, and any second-generation adults that do occur do not have time to mate and lay eggs before all activity is halted by freezing weather.

The main host plants of A. lineolatus are two cultivated legumes, alfalfa, Medicago media Pers., and red clover, Trifolium pratense L. No native host plants have been found in Saskatchewan.

About three per cent of field-collected nymphs from the Nipawin district were parasitized by braconid larvae tentatively identified as Euphorus pallipes (Curtis).