Illinois Natural History Survey - University of Illinois

The Tree Squirrel Bot Fly


Susan Post


During May, as I looked out at our multitude of grey squirrels that we feed daily, I noticed a very “special” squirrel.  It had a golf-ball-sized lump on its jaw, giving it a chipmunk cheek!  I thought it odd, but then promptly forgot about it. A month later, as my spouse and I were checking our geraniums for water, we noticed a small, fury, beelike insect nestled in a bloom. We both said, “Wow look at that bee mimic,” and I said, “It’s so good we better get a photo.”  So my spouse got the camera and as he got close he discovered the fly had no mouthparts. He became quite excited, calling me over to take a look. “This is an insect family you just don’t see!” he exclaimed. What we had was a Cuterebridae, a robust bot fly.

Our Peterson Field Guide to Insects revealed that these are “large robust flies resembling bumble bees, and adults are rare.” Like any good entomologists, after photos we collected it and brought the specimen to share with our colleagues at work.

The tree squirrel bot fly, Cuterebra emasculator, is an obligate parasite of tree squirrels and chipmunks throughout eastern North America. A parasite is an organism that lives in another without killing it, and an obligate parasite is one that cannot live independently of its host. If the bot flies kill their hosts before they are fully developed, they will also die.

While this bot fly may resemble a bumble bee,  it is actually a fly and a member of the insect Order Diptera. It undergoes complete metamorphosis—egg, larva, pupa, and adult. There is only one generation per year.

As described above, these flies resemble bumblebees. They are about an inch long with black and straw-colored thoraxes and dark wings. While ours was on a flower, it was not visiting for food—tree squirrel bot flies do not have mouthparts, so they cannot eat. They also do not bite or sting.

These bot flies overwinter as pupae buried in the soil for anywhere from 8 to 10 months. They emerge in early summer seeking mates. The female lays her eggs on twigs, branches, or vegetation that is in the habitat of the hosts. They do not lay their eggs directly on their hosts. These off-white, oblong eggs resemble tiny grains of rice.

While the legless larvae of flies are called maggots, the larvae of bot flies are referred to as bots. The developing first instar larvae remain within their eggs until the body heat from a potential host stimulates them to emerge from the eggs. These first instar larvae are the infective stage. They are whitish and encircled with bands of black spines. If the instar contacts a potential host (squirrel or chipmunk), it will enter through one of the animal’s body openings (mouth, nostrils or anus) or a wound. Once it enters the host, the first instar travels through the body. This journey takes about a week.  The first instar then settles underneath the host's hide, molts to the second instar, and creates a hole to the exterior. This hole provides air and a route for the elimination of excrement.  The presence of the larvae stimulates a response in its host’s tissues; a pocket (warble) is formed that encloses the larva.

There are three larval instars and usually only one larva per pocket or warble. The larvae (instars) do not feed on blood, but instead ingest lymph fluid, cellular debris, and leukocytes of the host. Development in their hosts usually lasts three to four weeks. The mature larvae emerge from the hosts by backing out of the exterior holes, dropping to the ground, and burrowing into the soil where they pupate.

These are native parasites of chipmunks and squirrels, and unless the infestation is high, there is usually no detrimental effects on the hosts. The empty warbles usually heal after a week.

What happened to our bot fly? After show and tell and more photos, it was given to one of the entomology graduate students who is assembling an insect collection. After all, it is a rare family, and our squirrels are certainly grateful.

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