Observing a squirrel with an unusual lump or growth can be concerning. These signs are often due to an infestation by bot flies, a natural occurrence for squirrels. While the appearance can be alarming, understanding the relationship between bot flies and squirrels reveals these infestations are typically not as harmful as they might seem.
Understanding Bot Flies and Warbles
Bot flies (Cuterebra genus) are obligate parasites that commonly affect squirrels and other small mammals. Their life cycle involves a larval stage developing within a living host. Adult female bot flies lay eggs in areas squirrels frequent, like pathways or near burrows. When a squirrel contacts these eggs, its body warmth triggers the larvae to hatch.
The tiny larvae enter the squirrel’s body, often through natural openings or small wounds. They migrate beneath the skin to develop, forming a noticeable lump called a “warble,” which is a pocket around the growing larva with a small hole for breathing and waste excretion. Warbles are most common in late summer and early fall, typically appearing on the front shoulders and chest. The larva grows significantly, up to 1.5 inches long and 1 inch wide, over three to seven weeks. After maturation, the larva exits the warble, drops to the ground, and pupates in the soil, emerging as an adult bot fly the following year.
The Impact on Squirrel Health
A common concern is whether bot fly infestations are fatal to squirrels. In most cases, bot fly larvae are not lethal to their squirrel hosts. As obligate parasites, bot flies require a living host to complete their life cycle; killing the host would also result in the larva’s death. This dynamic ensures the parasite generally does not cause severe harm to its host.
While a warble can be unsightly and cause temporary discomfort, healthy adult squirrels typically recover without lasting effects. They may scratch or groom the affected area more. Rarely, if an infestation is severe, or if the squirrel is already weakened, very young, or injured, bot flies could contribute to its decline. Secondary bacterial infections at the warble site might occur, complicating recovery. However, once the larva exits, the wound usually heals quickly, often within about a week, and the fur typically grows back.
What to Do if You See an Infected Squirrel
When encountering a squirrel with a bot fly warble, it is generally recommended to allow nature to take its course. Direct intervention, such as attempting to remove the warble yourself, is usually unnecessary and can be harmful. Untrained attempts to remove the larva can cause complications, including injury or the release of toxins if accidentally ruptured.
Wildlife rehabilitators typically do not intervene with bot fly infestations in otherwise healthy squirrels, as these are natural occurrences. Intervention may be considered if the squirrel is orphaned, severely debilitated, or if the warble site shows signs of a serious secondary infection requiring antibiotics. In such rare cases, a trained wildlife rehabilitator or veterinarian might remove the larva using specialized tools and techniques, followed by wound care. For the average person, observing from a distance and allowing the squirrel to complete its natural healing process is the most appropriate action.