It's nearly impossible to find and remove a horsefly breeding location. In most cases, horse flies breed in wetlands, so it's hard to kill them with insecticides because of potential collateral damage. The horsefly is a strong flying insect and can easily move into a home from long distances. In most situations, horsefly infestations are located a substantial distance away from a home. A typical horsefly breeding site is extensive, so it's best to call a pest control expert.