There are many kinds of insecticides on the market that can be applied as space sprays inside closed barns. Most available products for this use contain active ingredients that are botanical pyrethrums or pyrethroids. These chemicals kill the adult flies that are exposed directly to the spray. In general, these chemicals can be used safely inside horse barns, because they are relatively non-toxic to animals and people, and their active ingredients break down rapidly. Some product labels state clearly that application to animals or their feeds should be avoided, while others are less restrictive. Whichever material you choose, you should read the package label carefully and follow its instructions. Look for statements about how to apply the material, and for statements that would restrict the kinds of places where the product can be used safely. You should avoid direct application to feeds and animals, unless the labels state otherwise.
Benefits of fogging closed barns are likely to be temporary. Time required for fly numbers to return will depend on how much fly breeding is occurring inside the barn, and how well the barn is screened to exclude flies from outside.
By Roger Moon, PhD, University of Minnesota