Horseflies are large biting insects known for their painful bites. Only female horseflies bite, as they require a blood meal to produce eggs. These flies can be a nuisance to humans and animals.
The Horsefly Life Cycle
Horseflies undergo a complete metamorphosis, a four-stage life cycle including egg, larva, pupa, and adult stages. Female horseflies lay their eggs in a single mass, containing 100 to 800 eggs, on vegetation overhanging water or on wet soil near water sources.
Eggs are often dark and spindle-shaped, hatching within two to twelve days, depending on temperature. Once hatched, larvae drop into the water or saturated soil below. Horsefly larvae are aquatic or semi-aquatic predators, feeding on small invertebrates such as insect larvae, worms, and sometimes small vertebrates like tadpoles. This larval stage is the longest, lasting several months to a year, though some species can remain in this stage for up to three years, overwintering in the mud.
After completing larval development, they move to drier soil near the water’s edge to pupate. The pupal stage is a non-feeding transition where the larva transforms into an adult. This stage lasts one to three weeks. The adult horsefly emerges from the pupal case.
Preferred Habitats and Breeding Grounds
Horseflies are found globally, but they have specific habitat preferences important for their life cycle. Many species are associated with aquatic or semi-aquatic areas, which are important for egg-laying and larval development. These include wetlands, marshes, bogs, the edges of ponds, lakes, slow-moving streams, and even salt marshes.
The moisture in these habitats is necessary for the larvae to feed and develop, as they burrow into the mud, sand, or gravel substrate. Some species of horsefly larvae can also be found in moist forest soils or decomposing wood. Adult horseflies, particularly females seeking blood meals, are found in open grassy areas, meadows, pastures, and along forest edges near these wet breeding sites. These environments provide both the necessary moisture for immature stages and the presence of large mammalian hosts for adult females.
When Horseflies Are Most Active
Adult horseflies are most active during the warmer months, from late spring through summer, with peak activity in mid-summer. Their emergence and activity are influenced by temperature, with warmer conditions accelerating development.
Daily, horseflies are diurnal. They are active during warm, sunny days, especially around dawn and dusk. Though present throughout the day, their activity intensifies in the early morning and late afternoon. Female horseflies seek blood meals during these active periods to obtain the protein necessary for egg production.