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Mystery insect is tree squirrel bot fly

Tree Squirrel Bot Fly Cuterebra emasculator4By Bob Bowles -- Any live bug in Simcoe County in late March -- other than the occasional woolly bear, snow flea, or winter stonefly – is, of course, extremely interesting. Therefore, I was intrigued to receive a message from an Orillia resident with a report and photo of a yellow and black bumblebee-like creature she discovered crawling over a neatly folded pile of clean laundry inside her house. She felt this was unusual and asked if I could identify it. 

The photo did look like a bumblebee and my first step was to confirm it was an insect. The next step was to confirm it to one of the 31 possible orders. Wings and antennae indicated it was not in the order hymenoptera which includes ants, wasps, and bees but the order diptera which includes all the flies. This took me to my dipteral references in my office library using the volume reference of Manual of Nearctic Diptera and the recently published Flies – The Natural History and Diversity of Diptera by Dr. Stephen Marshall. I could see it was in the higher Diptera Suborder - Brachycera or Eremoneura in the subgroup - Calyptratae which include Louse and Tsetse Flies, House Flies and Relatives and Blow Flies, and Parasitic Flies and Relatives. I then took it to the Superfamily – Oestroidea (Blow Flies, Parasitic Flies and Relatives). The next step was to take it down to family level.

I first thought it was in the Tachinidae Family – Parasitic Flies in the Genus Belvosia which lay their eggs on butterflies and moths. However, I could not see all the details in the attached photo so asked if I could receive the specimen, which was then dropped off at my house.

Once I saw the large lower calypters and features in the microscope and photos, I realized it was not in the Family Tachinidae (Parasitic Flies) but in the Family - Oestridae (Bots and Warbles).

I found it in my reference books in the Subfamily – Cuterebridae in the Genus Cuterebra (Rodent Bots). The species is Cuterebra emasculator – tree squirrel bot fly. The genus resembles bumblebees and they lay their eggs in host mammals such as squirrels, chipmunks, mice, dogs, cats -- and even humans, for one species.

The tree squirrel bot fly lays its eggs during the summer in Eastern Gray Squirrels and Eastern Chipmunks. Another closely related species, Cuterebra fontinella, which has a yellow back as an adult, lays its eggs in deer mice and white-footed mice. The tree squirrel bot fly is univoltine, having one generation per year throughout its range. The pupal stage overwinters buried in soil and requires about eight to 10 months before emerging as an adult. Adult Cuterebra flies are rarely seen and live only a few weeks, during which they mate and lay eggs.

So, why was one alive in the house in winter? Females lay eggs in groups of five to 15 on grass stems, wood chips, and bark along narrow trails or rodent runs in areas of rodent burrows. Eggs hatch in response to sudden increases in temperature and the first instar larva moves onto the fur of passing rodents. The parasite enters the host through natural body openings, not by penetration through the skin. It forms a subcutaneous cyst and grows rapidly within the host. It enlarges the pore from which it will exit three to six weeks after initial infection, depending on the species. After emerging, the bot burrows into the soil to pupate, then emerges as an adult after 10 months.

This adult was inside the house in late March and probably recently hatched from the pupal bot stage, which lives for 10 months in this stage, buried in the ground under soil. This pupating bot may have been in soil in a flower pot or on cloths or gloves that were used in the garden last summer and brought into the house last fall, then hatched as an adult in March. This is not a common insect and is at the north part of its range here in Simcoe County. It is not usually observed as an adult due to its short life span in that stage, spending most of its time in as a pupa buried in the ground or as a larva buried the body of a squirrel or chipmunk. This was a learning experience for me since I have never seen one and was unaware that squirrels and chipmunks in our area have these parasitic flies living under their skin in bot-sized growths.

Bob Bowles is an award-winning writer, artist, photographer and naturalist, founder and coordinator of the Ontario Master Naturalist Certificate Program at Lakehead University.

He is also the Lake Simcoe Nature Detective who writes a nature column in every issue of Lake Simcoe Living. Send your questions about any insects or other creatures that you find in the Lake Simcoe area that you want to have identified and to learn its function in nature. His email is This email address is being protected from spambots. You need JavaScript enabled to view it.

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