Abstract: An affordable means of protecting a barn against fly infestation by trapping in a novel trap design predatory arthropods such as Carcinops pumilio and Macrocheles muscaedomesticae from another barn having a mature predator population, and transferring the traps to accumulations of manure which are underpopulated with the predatory arthropods. The predators self-release onto the predator-free manure and establish themselves, preventing fly infestations which would normally occur. The trap of the invention is a box-like structure, having a filter surface on one side which has openings large enough to admit Hister beetles and Macrochelid mites, but small enough to prevent the entry of hide beetles and litter beetle pests. Inside the box is an appropriate bait material which will attract the desired predatory beetles or mites, such as a slurry of housefly eggs in water spread on absorbent material. The traps are spread across a mature manure pile.