From time to time, all backyard wildlife watchers see something that they cannot explain. For example, if gray squirrels frequently visit your feeders, chances are you will eventually spot one that appears to have lumps somewhere on its body. When this happens, it is only human nature to wonder what causes these mysterious lumps. A vast majority of the time, hidden inside each lump is the larva of a parasitic fly known as the squirrel bot fly.
Squirrel bot flies parasitize animals such as gray and fox squirrels, and rabbits. The squirrel bot fly looks much like a bumblebee. However, if you have the rare opportunity to examine one, you will see that it has one set of wings instead of two like the bumblebee.
Adult bot flies live for only two weeks or so. During this brief time, a female must lay her fertilized eggs on the branches of trees and other surfaces. If a squirrel happens by and touches an egg, it rapidly hatches and the larva tries to hitch a ride on the squirrel. The tiny larva must then make its way into the squirrel’s body via a body opening. This often occurs when a squirrel grooms itself and unwittingly swallows a bot fly larva. A larva can also gain access to the body through the bushytail’s eyes.
Once inside, the larva travels through the squirrel’s body before eventually ending up between the animal’s skin and flesh. Here it rapidly grows. As the larva increases in size, it creates a noticeable lump. Many people call these lumps warbles or wolves.
Over time, the liquid-filled lump swells. A bot larva can reach a size of 1.5-inches long and an inch wide. During this period in its life, a larva chews an exit hole in the squirrel’s skin through which it eventually wiggles out and drops to the ground.
Once on the ground it burrows into the soil and pupates. It will remain there until spring when it sheds the covering that protected it throughout the winter and emerges as an adult.
Meanwhile, once a larva leaves a squirrel, the open wound it left behind heals and any hair that was lost while the larva was living just below its skin will regrow.
Fortunately, we, as well as dogs and cats, cannot be infected by coming into contact with a squirrel parasitized by a bot fly.
The amazing drama involving the bot fly and the gray squirrel is played out in countless backyards across Georgia each year. However, I had never seen a “lumpy” squirrel in my yard until this year. Have you ever seen one in your yard? If you have, you now know the answer to a mystery that has confounded many folks for generations.