Help Your Horse Survive the Winged Pests of Summer

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If possible use properly installed fans to blow air over stalled horses as this will deter flies and mosquitoes. Circulating air makes it harder for flies to navigate and land on the animals.
Consider using fly traps such sticky traps and replace them frequently as they quickly become clogged with dead flies and dust.
If possible make your barn flypredator friendly by encouraging bats and birds to nest in the rafters. Barn swallows are aerial insectivores that feed on flying insects. This group of birds which includes swifts swallows flycatchers and goatsuckers has declined alarmingly in population across Canada. According to the Government of Ontario barn swallows declined 65 percent between 1966 and 2009 where they are now listed as a threatened species. Bird Studies Canada reports that the Canadian population of barn swallows has declined 80 percent since 1970. Reasons include pesticides migration hazards and loss of nesting habitat when old barns are removed for development. In fact right across North America the barn swallow has actually seen a 95 percent drop in population in the last 40 years a statistic mirrored by many species of song birds.
Products such as topical sprays wipes and rollons work temporarily to stop insects landing on the horses skin and to augment other control methods and debris management.
Pyrethrum or Resmethrin fogs and space sprays can be used to kill adult flies indoors but relief will be temporary because these insecticides break down quickly says Moon. Pyrethrum contains pyrethrins that are extracted from certain cultivated chrysanthemums. Resmethrin is a synthetic pyrethrin. Longer lived pyrethroid and organophosphate residual premise sprays can be applied indoors and outdoors and will be most effective if applied to fly perching areas.
Many horse owners prefer natural products to chemical insecticides for their horses. Garlic apple cider vinegar peppermint and citronella are bases for some success with fly repellents. In 2014 Health Canada HC made a decision to ban the use of oil of citronella despite the fact that its own expert panel argued that fly sprays with citronella were safe and that the science research that HC was relying on was flawed. In the wake of a furious pushback HC reversed its ban in 2015 allowing for the use of the very popular oil. 
Physical barriers for horse protection from flies include fly sheets fly masks made of seethrough mesh to protect horses eyes ear nets and mesh leg coverings. Interestingly white
or whitebased summerfly sheets seem to work better than dark materials. And anecdotally some people notice that white horses are less bothered by flies than dark bay and black horses.