Earlier studies of insecticides for fly control have shown that resistance is unequally distributed in California's numerous species of flies and that one chemical used for the control of the little house fly, Fannia canicularis, may not result in effective control of the house fly, Musca domestica. These differences in chemical effectiveness are particularly noticeable when poor control permits numerous adults to cause a serious nuisance problem on agricultural premises. In some cases, it may be profitable to spray for control of fly larvae in the manure and thereby prevent adult fly emergence.