09/16/2025
Rehabbers get a lot of calls about lumpy squirrels, especially in late summer and early fall! There are two common causes of lumps on squirrels: squirrelpox, a viral infection we’ll discuss in a separate post, and botflies, which are insects that develop in animals’ skin in the larval stage. When people find lumpy squirrels, they almost always feel that they need to capture the squirrel for treatment, or worse, they’ll rush to killing the critter out of the idea that they’re “ending its suffering.” Please don’t do either of those things!
Botflies (also called warbles, wolfworms, or heelflies) may give you the heebie-jeebies, but they’re native insects, just like bumblebees and monarchs. They play an important role in our ecosystem, and we never kill native animals— no matter how small or unimportant they may seem— just because their life cycle is uncomfortable to see.
The species of botfly that typically affects North American squirrels is a small, unassuming little bug that resembles a bee (but doesn’t sting). A mother botfly lays her eggs in areas frequented by rodents, like tree hollows and brush piles. When the a squirrel bumps into the eggs, they cling to the fur and hatch, and the baby botfly grows under the squirrel’s skin. It may be a yucky-looking process, but it’s not really as bad for the squirrel as it seems. The larvae will grow just under the squirrel’s skin until maturity, then will fly away, leaving a tiny hole in the skin that heals quickly.
An otherwise-healthy squirrel will do just fine even with many botfly larvae growing under the skin. Parasites like fleas, roundworms, and botflies are just part of life for wild animals, and squirrels and their native botflies have coexisted for millions of years without issue. The stress of capturing a squirrel for treatment would be much greater than the risk of the botflies simply running their course, which is usually completed by late fall.
There’s no need to panic about botflies passing to you or your pets. Botflies tend to be species-specific and it’s very rare for them to grow in the skin of any animal besides their preferred host. If you live in North America, you’ve probably already encountered squirrel botfly eggs many times while climbing trees as a kid or doing yard work as an adult, and of course there was no harm done. You don’t have to kill or relocate squirrels, or use pesticides in your yard, to keep yourself safe.
Please let nature be nature, even when it looks icky! Both squirrels and native insects deserve to live in peace.
