It sounds simple but overall sanitation is still the most important and effective way to control flies. A little extra work “neating up” can yield a big fly reduction.
All outside trash and manure areas should be kept clean. Use garbage cans with tight fitting lids, lined inside with plastic bags. By placing garbage in secure plastic bags, odors will be reduced, therefore attracting fewer egg-laying flies. Locate the garbage cans as far away from the house or buildings as possible.
Dispose of garbage or manure weekly as House Flies take a minimum of 8 days to emerge. Sending your fly problem to the dump or back forty is much preferred to keeping it at your barn.
Eliminate other potential breeding materials. This includes rotting round bales, loose straw, mulch, leaves, manure, garbage, and moist soil with animal manure. Compost or manure piles that are not being added to frequently can be covered with black plastic which will increase the heat of the decomposing organic matter, destroying fly larvae. Avoid leaving moist pet foods outdoors for several days. Remove fallen, fermenting, or over-ripe fruits from the ground.
Any potential breeding material should be spread thinly in the field and allowed to dry to prevent fly development. The goal is to get the manure or other matter dry within five days as that is the minimum time for a House Fly to pupate. While forming their cocoon (pupate) if they are in a medium that is drier (or wetter) than 40-60% moisture by weight, the pupae does not form properly and that particular fly never completes its development.
Dispose of road kill animal carcasses quickly during the summer. Otherwise, they will quickly become “fly factories” within 24-48 hours. A small animal (i.e., dead cat, ground squirrel, tree squirrel, rabbit, etc.) can easily produce more than a thousand Blow Flies if left by the side of the road. Dead deer, left to rot in a ditch, can yield tens of thousands of flies. You should report such problems to your county road department or other responsible authority and ensure that the carcass is promptly removed.