Horse flies, members of the insect family Tabanidae, are large, robust insects recognized by their often iridescent or brightly colored eyes. Measuring from about half an inch to over an inch in length, these flies are larger than common houseflies. Female horse flies possess blade-like mouthparts to cut skin for blood, which they require to produce eggs. This blood-feeding behavior makes them a significant nuisance to humans and animals, as their bites are notoriously painful.
Horse Fly Seasonal Presence
Horse flies become active from late spring to early summer, with emergence varying by species and regional climate. Adult horse flies begin to appear around May or June. Their populations then build, reaching peak activity during mid-to-late summer, often in July and August. During these months, they are persistent and are attracted to movement, warmth, and carbon dioxide exhaled by hosts. Their presence is common in moist environments such as marshes, wetlands, and near bodies of water, where larvae develop.
Environmental Factors Leading to Decline
The decline of adult horse fly populations is linked to environmental changes as summer transitions into fall. A primary factor is the drop in temperature, which adult horse flies cannot survive, especially with the onset of frost. Insects are ectotherms, meaning their body temperature, metabolism, and lifespan are influenced by the environment. While horse flies can be active in warm conditions, their activity significantly decreases when temperatures fall below approximately 72°F. Reduced daylight hours also play a role, signaling the end of the breeding season.
Adult horse flies have a short lifespan, ranging from 30 to 60 days. Their short adult lifespan, combined with the physiological stress of decreasing temperatures, leads to their disappearance. Unlike some other insects that seek shelter to extend their adult lives, adult horse flies die as cold weather sets in, marking the end of their nuisance period. This seasonal mortality clears the adult population before winter.
The Horse Fly Life Cycle and Overwintering
While adult horse flies perish with the arrival of cold weather, the species itself survives the winter through its immature stages. Horse flies undergo complete metamorphosis, progressing through four distinct life stages: egg, larva, pupa, and adult. Female horse flies lay their eggs in clusters, typically on vegetation or objects overhanging water or moist soil.
Upon hatching, the larvae drop into the water or moist substrate where they develop. These predatory larvae feed on other small invertebrates and are well-adapted to survive the winter months, often overwintering in the last larval stage in mud, soil, or aquatic environments. As spring approaches and temperatures rise, the overwintering larvae transition into the pupal stage, usually in drier soil. This pupal stage lasts for a few weeks, after which a new generation of adult horse flies emerges, beginning the cycle anew in late spring or early summer. Most horse fly species complete one generation per year, though some larger species may require two to three years to develop.