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HOLEY SQUIRRELS

We know the lazy days of summer are waning at Hilton Pond Center when Eastern Chipmunks start climbing understory trees to harvest berries from our Flowering Dogwoods. Meanwhile, up in the canopy, Eastern Gray Squirrels are doing their best to gather fruit from Black Walnuts, Pecans, and Shagbark Hickories--even though the nuts aren't quite ripe by human standards. Regardless, the nutmeats must taste good, for squirrels spend all day scouting out yet another nut; the only ones that go uneaten are those at the ends of branches too thin to support the squirrels. Black Walnuts appear to be the squirrels' finest prize; when they manage to grab one and strip away the green husk, they haul the nut to a nearby branch and spend long stretches of time gnawing the thick shell. This seems like a lot of effort for the amount of nutmeat they get in return, but a couple of the squirrels we've been watching have an apparent health problem and may need all the nutrients they can find.

All text & photos © Hilton Pond Center

Every September we see a few Eastern Gray Squirrels at Hilton Pond Center with one or more gaping wounds on their sides or shoulders (above). They look like bullet holes, but since we don't have a gun and wouldn't shoot the squirrels in the first place, that can't be the cause. Although we sometimes catch squirrels in our bird traps, none that have been snared have had the wounds; however, after observing them through binoculars and telephoto camera lens, we finally figured out the cause: they've got to be Bot Flies. Even if you're a bit squeamish we hope you'll read on about these interesting little parasites that cause our Hilton Pond squirrels to become holey hosts.

Bot Flies are found almost worldwide, but especially in the tropics. Depending on their species, they may parasitize livestock, human beings, rabbits, or--in the case at hand--rodents such as Eastern Gray Squirrels, Sciurus carolinensis. Our squirrels at Hilton Pond are hosts for larvae of the Tree Squirrel Bot Fly, Cuterebra emasculator, whose species name may be a bit over-dramatic; i.e., Bot Flies can cause problems for their hosts, but probably not to the point of completely incapacitating the males. This particular species is the only one infesting squirrels in South Carolina; it occurs from the Atlantic coast westward to just beyond of the Mississippi River, and from southern Canada to the Gulf Coast.

The typical Bot Fly is a squat, hairy insect with two wings, a short abdomen and large thorax, and a big head with prominent eyes; one Cuterebra that parasitizes rabbits (above right) looks to us like some sort of cartoon character rather than a naturally occurring insect. One species--the Human Bot Fly, Dermatobia hominis--occurs in Mexico and South America and sometimes comes home aboard human visitors to the tropics, embedded as larvae beneath the skin of the unsuspecting host. Our local Tree Squirrel Bot Fly (left) is a little less exotic in appearance--superficially it resembles a small bumblebee--and fortunately very rarely parasitizes people or pets. Instead, the female lays her eggs (below right) on twigs or in nests or other locations frequented by rodents. When the potential host bumps into a Bot Fly egg, its body warmth apparently causes the egg to hatch, after which the larva climbs onto its new host. Eventually the tiny larva enters the squirrel's body--perhaps through the eye or mouth--and wanders about internally for a week or so before finally burrowing beneath the host's skin and forming the sores we observed at Hilton Pond Center. These wounds are also called "warbles" or "wolves" and are frequently encountered in fall by squirrel hunters--one good reason to wait until winter to go hunting for "bushy-tailed tree rats."

Each warble contains a single Bot Fly larva that chews through the host's skin with dual mouth hooks, creating a pore (below left) through which it can breathe and excrete wastes while dining on the host animal's tissues and bodily fluids. Understandably, this opening oozes and irritates the squirrel, which often scratches the area until large patches of skin become red and bare. After several weeks of parasitizing the host, the Bot Fly larva exits its chamber, drops to the substrate, and forms a pupa that emerges the following year as an adult Bot Fly. A photo (below right) of the larva of a Human Bot Fly gives you an idea of what was maturing inside that warble on the squirrel.

We got particularly interested in Bot Flies a couple of years ago after receiving an E-mail from Frank Slansky, entomologist at the University of Florida. He and colleague Lou Rea Kenyon have been studying Tree Squirrel Bot Flies and their environmental significance. After looking at distribution maps for Cuterebra emasculator, Frank was curious why the scientific literature listed this species as occurring across North Carolina and Georgia but not South Carolina. Frank knew of our natural history work in the Piedmont Region and contacted us through the Center's Web site to inquire if we had ever encountered squirrels with warbles. We just happened to have on file the top and bottom photos on this page and ended up collaborating with Frank in a statewide Bot Fly survey.

Together, over a 16-month period, we contacted wildlife biologists, veterinarians, wildlife rehabilitators, hunters, and nature enthusiasts across South Carolina to inquire about the prevalence of Tree Squirrel Bot Flies. In the end we received at least one report from each of South Carolina's 46 counties, indicating the scientific literature had been inaccurate in failing to list the species as occurring throughout the Palmetto State.

All text & photos © Hilton Pond Center

Respondents to our survey suggested that Eastern Gray Squirrels and Fox Squirrels, Sciurus carolinensis and S. niger, may be the most common hosts, followed by Eastern Chipmunks, Tamias striatus, and Southern Flying Squirrels, Glaucomys volans. It is not known conclusively how seriously Bot Flies affect these host organisms. Some authorities suggest an infestation may cause anemia or interfere with reproductive functions, but it may be that having a few Bot Flies in your warbles isn't all that devastating for a typical host. Just the same, we wouldn't want to trade places with the Eastern Gray Squirrel above, photographed while it dined on Pickerelweed stalks outside the old farmhouse at Hilton Pond Center. This animal had two developing warbles on its left side and another on the right--a rodent host made holey by a larval trio of Tree Squirrel Bot Flies.

All text & photos © Hilton Pond Center
Photos of Tree Squirrel Bot Fly, warble & eggs courtesy F. Slansky & L. R. Kenyon,
Dept. of Entomology & Nematology, University of Florida
Photo of Human Bot Fly & larva courtesy of Jim Kalisch, Dept. of Entomology, University of Nebraska

NOTE #1: Results of the South Carolina Bot Fly survey mentioned above have been published, as follows:

Slansky, F. & B. Hilton Jr. 2003. Distribution of the bot fly Cuterebra emasculator (Diptera: Cuterebridae) in South Carolina. Journal of Agricultural & Urban Entomology 20(2):83-91.

(Click on the title above for a downloadable PDF of the paper.)

NOTE # 2: Although a viral disease called "squirrel fibromatosis" or "squirrel pox" produces lesions that can be confused initially with Bot Fly infestation, this disease differs from warbles in appearance, intensity, and location on the host. Squirrels with fibromatosis typically have a few to many lesions--up to 100!--on torso, legs, toes, tail, and/or head (especially around the eyes). Wildlife rehablitators reported that squirrel pox is almost never seen in South Carolina.

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OVENBIRD

This Neotropical migrant is among our more commonly captured wood warblers, primarily because it is a ground-dweller and flies at the level of our 3-meter-tall mist nets. We've banded 204 Ovenbirds at Hilton Pond, the first in Autumn 1983; our best year was 42 in 1994.

* = New species for 2004

WEEKLY BANDING TOTAL
5 species
25 individuals

YEARLY BANDING TOTAL
(2004)
53 species
1,755 individuals

BANDING GRAND TOTAL
(since 28 June 1982)
123 species
45,060 individuals