The common perception of a “green fly” often refers to the harmless, iridescent Green Bottle Fly. This leads to confusion when people encounter a much larger, intensely painful bite from a different insect that also exhibits green coloration. The true biting green flies belong to a group of powerful, persistent insects that actively seek a blood meal. This distinction is important for understanding how to protect yourself from these summer pests.
Identifying the Biting Green Fly
The flies responsible for the painful bites are members of the family Tabanidae, commonly known as horse flies and deer flies, which includes species often referred to as “greenheads.” Many Tabanidae species possess large, brightly colored eyes that can be iridescent green or purple, or have a dark body with a striking green head. Deer flies (Chrysops) and horse flies (Tabanus) are known for their persistent pursuit of a host. Deer flies are typically smaller, while horse flies can be quite large, up to 1.25 inches in length. Only the female of the species bites humans and animals, as she requires the protein found in blood to successfully produce her eggs.
How the Bite Occurs
The bite mechanism is the primary reason for the immediate, sharp pain that sets it apart from a mosquito bite. Unlike a mosquito, which uses a delicate, needle-like stylet to probe for a blood vessel, the female horse and deer fly uses blade-like mouthparts. These mouthparts slash the host’s skin tissue, creating a large, open wound.
The fly then injects saliva containing an anticoagulant to prevent the blood from clotting, allowing it to pool on the surface of the skin. A sponge-like part of the mouth laps up this pool of blood for consumption. This method of feeding, known as pool feeding, is often interrupted by the host’s reaction to the pain, which means the fly may have to bite multiple times to obtain a full meal.
Immediate Effects and Associated Risks
The bite from a deer or horse fly is painful due to the lacerating action of its mouthparts, often described as a sharp pinch or a sting. This trauma is followed by a localized reaction, which includes swelling, redness, and a raised, itchy welt around the wound site. The swelling and irritation usually subside within a day or two.
A primary concern following a bite is the risk of a secondary bacterial infection caused by scratching the wound with unwashed hands. While rare in most areas, these flies are considered mechanical vectors, meaning they can transmit certain disease-causing agents. For example, they can transmit the bacterium that causes tularemia, also known as “deer fly fever” in parts of the western United States. A small number of people may also experience a severe allergic reaction, which could manifest as hives, dizziness, or difficulty breathing, requiring immediate medical attention.
Prevention and Treatment
To reduce the likelihood of a bite, modify your behavior and dress when in areas where these flies are prevalent, such as near wetlands or wooded edges. Female flies are attracted to movement, warmth, carbon dioxide, and large, dark objects, so wearing light-colored clothing can make you less appealing as a target. Standard insect repellents containing DEET may offer some protection, but treating clothing with a repellent like permethrin is a more effective preventative measure.
If a bite does occur, the first step is to clean the wound thoroughly with soap and water to minimize the chance of secondary infection. Applying an ice pack or cold compress to the site can help reduce the pain and swelling. Symptoms like itching and inflammation can be managed with over-the-counter treatments, such as an antihistamine cream or an oral antihistamine, which counteracts the body’s reaction to the fly’s salivary secretions.