Squirrels, Wolves, and Bot flies

Squirrels, Wolves, and Bot flies


Squirrels, wolves, and botflies represent a phenomenon of nature that is frequently not totally understood. Perhaps part of the confusion is due to the numerous species of botflies and the variation of hosts that they parasitize. This article intends to provide some clarification to that confusion.

Hunters frequently say that rabbits or squirrels like the featured image of this post are “full of wolves.” However and more accurately stated, the grotesque appearance of the pictured squirrel is due to an infestation of the parasitic larva of the botfly. Listening to the standard description and looking at the featured image, one might erroneously think that a wolf mauled the subject.  Accordingly, the pictured squirrel is suffering from a parasitic infestation of the botfly larva. 

Despite a large number of botfly species, they all do follow a general pattern of infestation with some minor variations. Typically the female botfly will deposit their eggs directly on the body of some mammal or on an intermediary vector that will later deposit the eggs on the host. Mosquitoes or houseflies are common examples of vectors that might be used.  After being deposited, the eggs will hatch and burrow into the skin where the will spend their entire larval stage of life.  Once the larva matures sufficiently, they will emerge and tumble to the ground as a pupa until they appear as a mature botfly.

Bot Fly Variations:

The botfly family of insects is represented by numerous species (around 30 that inhabit the United States) that follow a life pattern similar to that described above with a few variations.  Accordingly, the many species of botflies are often referred to as warble flies, heel flies or gadflies.  Technically they all belong to the insect family Oestridae.  The deer botfly hovers near the deer’s head where she deposits the eggs in the host’s nostrils.  Once hatched, the larva migrates to the base of the tongue where they will become attached and spend the larval stage of development

The Deer Botfly
The Deer Botfly

One species of botflies access horses as larval hosts.  The equine botfly usually deposits its eggs on the horse’s legs. Later when the horse licks the area or brushes it with their nostrils, the eggs are transferred to the mouth and later to the intestinal tract.  There they become attached to the wall of the stomach where they remain until maturity when passed with feces.

Yes! That is the answer to the obvious question: There is a bot fly species that will parasitize humans with their larva development.  It is called the human bot fly (Dermatobia hominis), and it inhabits South America northward through most of Mexico.  Its range seldom extends into the United States but can become an issue for visitors who frequent the inhabited zone during the season that the egg-laying season.  The only physical threat that the noted infestations present is that the tumor should not be mashed to expel the infected larva.  If the larva is mashed the exposed fly fluids could create a reaction that would resemble shock.  Medical personnel familiar with bot fly infestations should be used for removal.

The majority of botfly infestations, including those that afflict small mammals and humans, are deposited as eggs to the host’s skin via another insect vector or directly.  When hatched, they will burrow into the skin and spend the larval stage of development.

Botfly Larva
This is the larva of the deer botfly.

Squirrel :

Now for the star of the bot fly species that parasitize the gray squirrel: the tree squirrel bot fly (Cuterebra emasculator).  The tree squirrel bot fly inhabits around 30 states in the United States and a couple of provinces in Canada.  The fly is shaped like the deer bot fly in the featured photograph, except it is black in color and without the hair of the deer bot fly.  It is similar in size, shape, and appearance to a bumble fly, but with fly features.

The physical appearance of the bot fly infestation presents a grotesque appearance, which is probably an exaggeration of its threat to the squirrel’s health.  The infestations should present discomfort and if the squirrel host is stressed with age and other illness the overall impact could make the host vulnerable to predation.  For the most part, however, the squirrel will fully recover after the larva is expressed to the ground in the pupa stage.

Small mammals follow the typical botfly life cycle pattern.  As such the area where the larva imbeds itself produces a large lump that enlarges as the larva grows.  The feature photo, taken during mid to late summer, well illustrates the larval stage of development.  However, it is not uncommon for the larva to still be present in-game shot during the hunting season. 


Photo of deer bot fly:
Karsten Heinrich (& G. Kothe-Heinrich): Licensed under Creative Commons: By Attribution 3.0 License; http://creativecommons.org/licenses/by/3.0/; Alterations: resized to fit; Author assumes no endorsement for conditions of use

Photograph of Botfly Larva:
Kalumet, selbst erstellt Datum: 22.12.2005 (& G. Kothe-Heinrich): Licensed under Creative Commons: By Attribution 3.0 License; http://creativecommons.org/licenses/by/3.0/; Alterations: resized to fit; Author assumes no endorsement for conditions of use

Jordan Taheri, Phillip Kaufman and Frank Slansky (Retired), University of Florida. Featured Creatures: entomology and Nematology; University of Florida. Web. November 2007.

Bot Fly. Wikipedia. Web. 2019

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