Horse flies do not construct traditional nests like birds or social insects. Instead of a centralized nest structure, their reproductive cycle involves specific environmental conditions for egg laying and larval development. Control strategies must focus on understanding their life cycle and targeting their breeding environments.
How Horse Flies Develop
Horse flies undergo a complete metamorphosis, transitioning through four distinct stages: egg, larva, pupa, and adult. Female horse flies lay egg masses, containing 100 to 800 eggs. These egg clusters are typically deposited on vegetation that overhangs water or directly onto moist soil near aquatic environments. Common sites include the undersides of leaves or stems of emergent plants in wetlands, marshes, ponds, or streams.
After two to three days, eggs hatch, and the newly emerged larvae drop into the water or saturated soil below. These worm-like larvae are predatory, feeding on small invertebrates like insects, worms, and tiny fish. The larval stage is the longest part of their life cycle, lasting from several months to one to three years, depending on the specific species and environmental conditions.
Larvae typically spend winter in their moist habitats, continuing to develop. In the spring, they migrate to drier soil areas to enter the pupal stage. This non-feeding pupal phase lasts six to twelve days before the adult horse fly emerges. Adult horse flies appear from late spring through summer, with only the females seeking a blood meal for egg development.
Managing Horse Fly Populations
Since horse flies do not build traditional nests, control efforts cannot focus on locating and destroying a central breeding site. Management strategies instead concentrate on disrupting their life cycle and reducing adult populations. One effective approach involves modifying their habitat by draining standing water, improving drainage in moist areas, and removing woody plants that provide egg-laying surfaces. Minimizing wet, marshy areas on a property can significantly reduce potential breeding grounds.
Physical barriers offer protection for both animals and humans. Installing screens on windows and doors prevents adult flies from entering structures. For livestock, fly sheets and masks provide a direct physical shield against bites. These measures create environments less accessible to biting flies.
Traps are a useful tool in reducing horse fly numbers. Commercial traps, such as H-traps, are designed to mimic the heat and visual cues of a warm-blooded animal, luring the flies away from hosts. Some traps utilize specific baits or scents to attract flies, and when strategically placed along flight paths from damp areas to animal gathering spots, they can achieve significant reductions in fly populations, sometimes between 70% and 95%.
Personal repellents containing ingredients like Icaridin, DEET, or essential oils such as citronella and peppermint oil offer protection from bites. Maintaining clean facilities by regularly removing manure and soiled bedding eliminates potential breeding sites. Allowing animals to graze during nighttime hours provides relief, as horse flies are primarily active during the day.