The insect family Tabanidae includes the common horse-fly and deer-fly. These true flies belong to the order Diptera and are found worldwide, except for some polar regions and isolated islands. With approximately 4,300 described species, they are known for their robust bodies and powerful flight. While often considered pests, they also contribute to ecosystems as pollinators.
Physical Characteristics and Life Cycle
Tabanids are stout-bodied flies ranging from 5 to 25 millimeters, distinguished by their large compound eyes that are often brightly colored. In females, these prominent eyes are widely separated, but in males, they are close enough to touch. Horse-flies are larger, reaching up to 25 millimeters, and are typically dark brown or black. Deer-flies are smaller, usually under 10 millimeters, and often have dark patches or patterns on their wings.
The tabanid life cycle has four stages: egg, larva, pupa, and adult. Females lay their eggs in clusters on vegetation near water. The larvae are aquatic or semi-aquatic, living in damp soil or at the bottom of ponds and streams. These larvae are carnivorous, feeding on small invertebrates, and after one to two years of growth, enter the pupal stage before emerging as a winged adult.
Habitat and Feeding Behavior
These flies are frequently encountered in sunny areas near water sources like marshes, streams, ponds, and adjacent forests. Tabanids are primarily active during the daytime, avoiding dark and shady areas. They are drawn to large, dark-colored moving objects, as well as the carbon dioxide exhaled by potential hosts. This attraction often leads them to humans, livestock, and even vehicles.
A significant distinction exists in the feeding habits between the sexes. Male tabanids have weaker mouthparts and feed on nectar and plant exudates, contributing to pollination. In contrast, females require a blood meal for egg development. They obtain this by feeding on a wide range of vertebrate hosts, including livestock, wildlife, and humans.
The Painful Bite and Health Risks
The bite of a female tabanid is painful because of her mouthparts. Unlike the piercing-sucking mouthparts of a mosquito, a female tabanid’s mouthparts function like sharp, serrated blades that slash the skin. This action creates a small wound from which blood pools, which the fly then laps up with a sponge-like part of its mouth. The reaction to a bite is a sharp pain, followed by a welt, swelling, and persistent itching.
While tabanids are an annoyance, the risk of them transmitting diseases to humans is relatively low. They are known mechanical vectors for the bacteria that causes tularemia, but such instances are not common. The more significant health risk is to livestock. In horses, they are known to transmit the virus that causes Equine Infectious Anemia (EIA), and they can also spread anaplasmosis and anthrax among cattle and sheep. Heavy infestations can lead to reduced growth rates and lower milk production.
Avoiding and Controlling Tabanids
For personal protection, wearing light-colored clothing is recommended. Covering up with long sleeves, pants, and a hat can also provide a physical barrier against bites. Chemical repellents containing DEET may offer some protection but are often not fully effective against these determined flies.
For controlling populations, trapping can be a useful method. Commercial traps, such as the Manitoba trap, exploit the flies’ hunting behaviors. These traps use a large, dark-colored object, like a black ball, to attract the flies into a collection chamber. Managing their breeding grounds by altering aquatic habitats is impractical due to their widespread nature.