Horsefly is the common English name for members of the family Tabanidae. Confusingly, ‘horse fly’ and ‘horse-fly’ are widely used and they are also regionally known as breeze flies, clegs or clags, deer flies or gadflies.
Adult horseflies are a seasonal pest to both livestock and their owners. The Tabanidae are true flies of the insect order Diptera. Tabanid species that habitually attack humans and livestock are widely regarded as pests because of the bites they inflict and the diseases and parasites that some species transmit.
Although adult flies normally feed on nectar and sometimes pollen, female flies require a blood meal before they are able to reproduce effectively and to aid in their egg development. Males do not, as they lack the same mouth parts (mandibles) that the females use in drawing the blood. The females’ primary sense for locating a suitable prey is by sight and they have large compound eyes that serve this purpose well. They are attracted to large, dark objects, to certain animal odours and carbon dioxide. They are also attracted by motion, their eyes being well adapted to its detection.
The bite from horseflies can be quite painful, especially considering the light, agile, and airborne nature of the fly. Unlike insects which surreptitiously puncture the skin with needle-like organs, female horseflies have specially adapted mouthparts which they use to rip and/or slice flesh apart. This causes the blood to seep out as the horsefly licks it up. The horsefly is secretive, with an annoying ability to land without being detected and escaping before the victim begins to experience any pain. The subsequent bite can be extremely irritating. The horse fly’s bite is considered more immediately painful than that of a mosquito. However the pain of a horse fly bite may mean that the victim is more concerned with assessing the wound and not swatting the culprit.
Horseflies are most active in hot weather, mostly in summer and autumn during the daylight hours. Most species also prefer a wet environment, which makes it easier for them to breed. Eggs are generally laid on stones close to water or on plant stems or leaves. On hatching, the maggots or larvae fall into water or moist earth, feeding on other invertebrates, such as snails and earthworms and even small vertebrates.
Effects and disease caused by Horsefly bites
Tabanidae are known vectors for some blood-borne diseases of animals and humans such as the equine infectious anaemia virus, as well as some Trypanosoma species. They have also been known to transmit anthrax among cattle and sheep and tularemia between rabbits and humans. Blood loss is a common problem in some animals, when large flies are abundant. Some animals have been known to lose up to 300 ml of blood in a single day from tabanid bites, which can severely weaken or even kill them.
The H-trap will also catch other biting insects that similarly respond to visual stimuli and are influenced by heat, movement and CO₂. Included in this list are mosquitoes and the stable fly (Stomoxys calcitrans). Stable flies typically appear mid-Spring, become severe in early summer and decrease in numbers throughout the remaining summer months. These flies are similar in appearance to house flies, except that stable flies have a bayonet like mouthpart (proboscis) protruding from the front of the head and they lack the four dark stripes on the thorax indicative of house flies.
Under optimal temperatures, the stable fly can develop from egg to adult in 12 days. Piles of moist, decaying plant material (e.g. grass clippings, hay, silage) should be considered potential sources of stable flies, especially when this material is mixed with animal manure and urine. Compost and piles of grass clippings are ideal breeding sites for stable fly larvae and may serve as the production source for an entire region.