The work described here is an attempt to assess the effect of insecticides, particularly DDT, on the more prominent elements of the insect fauna of alsike clover Trifolium hybridum L., and on the yield of seed harvested. The decline in legume seed production, over a period of years, in districts which were once major seed producing areas is a familiar story in North America. For instance, Sorenson (1939) records that in 1925 Utah produced nearly 40 per cent of the total alfalfa seed in the United States. Five years later this state produced less than 4 per cent, the drop being largely the result of a decline in yield from an average of 6.4 to 1.2 bushels per acre. A similar history has been experienced in the production of alsike clover seed in Southern Ontario in the last twenty years. Haldimand County once produced heavy and profitable yields of over 5 bushels per acre but now yield is so poor and unprofitable that it is difficult even to find fields left for Experimental work. No doubt many causes are at work in this reduction but there is good reason to believe that the successful establishment oi specific insect pests, as a large and permanent part of the fauna, has been partly responsible in both these cases.