Cuterebra emasculator

Geographic Range

Cuterebra emasculator, or tree squirrel bot flies, are prevalent throughout eastern North America, and occupy areas west of the Mississippi River to the Atlantic coast. (Jacobson, et al., 1981; Slanksy and Kenyon, 2002; Slanksy, 2006)


They are a temperate zone species that resides in forested areas. More specifically, Cuterebra emasculator is more prevalent in secondary growth mixed forests than in low mature coniferous forests. In Mississippi, the species resides in bottomland or flatland regions as well as hardwood habitats.

Adult Cuterebra emasculator are free-living, while the larvae are subcutaneous parasites that live in encapsulated pockets called warbles. The species’ typical hosts include tree squirrels (Sciurus, including eastern gray squirrels (Sciurus carolinensis) and fox squirrels (Sciurus niger) and eastern chipmunks (Tamias striatus). In squirrels, larvae develop in the axillary and back regions, while in chipmunks, larvae are prevalent in the genital or groin region. Each host is observed to have between one to three larvae. Some atypical hosts of other Cuterebra species include raccoons, cats, dogs, and humans; however, there is no evidence that Cuterebra emasculator utilizes these hosts. ("Cuterebra emasculator Fitch (Insecta: Diptera: Oestridae)", 2007; Jacobson, et al., 1981; Slanksy, 2006)

Physical Description

The white oblong eggs of squirrel bot flies are approximately 1.5 millimeters long. The legless larvae, called bots, are 2 to 4 millimeters long. The first larval instar is grayish-white and encircled by black spiked bands. The second larval instar is cream-colored. As the larva develops into the third instar, it eventually becomes sclerotized and the cream color darkens into brown. It possesses two black mouth hooks and is covered with cuticular platelets. It reaches the length of 20 to 42 millimeters long during the pre-pupal stage. The pupal stage is defined by hardened puparium. It remains the dark brown color and possesses two yellow anterior spiracles. Adult bot flies are relatively large, with broad black bodies measuring 16 to 22 millimeters long. They possess black wings and yellow thorax, thus they visually resemble bumblebees. This species exhibits no sexual dimorphism. ("Cuterebra emasculator Fitch (Insecta: Diptera: Oestridae)", 2007; Bennett, 1955)

  • Sexual Dimorphism
  • sexes alike
  • Range length
    16 to 22 mm
    0.63 to 0.87 in


Cuterebra emasculator females deposit eggs on objects such as twigs, branches, or vegetation. After larvae emerge from their eggs, they are only stimulated by the body heat, carbon dioxide, and moisture from the presence of a potential host. Larvae exit the egg through a trap door called the operculum and wait for a passing host. Once the larvae have transferred onto their hosts, they search for an opening into the host's body. They may enter the host through the mouth, nasal openings, anus, or external wounds, and once inside will settle under the host's hide. After a week, the larvae then molt to the second instar and create an opening in the skin known as a "warble pore" which is used to breathe and excrete waste. The host's tissues surrounding the larvae begin to swell and form a pocket (known as a "warble") that encapsulates the larva. After another week, the larvae molt to the third instar. After reaching this instar, larvae leave the host through the warble pore and drop to the ground. They burrow into the detritus and loose soil where pupation occurs. During spring, adults emerge from pupae by using hemolymph-inflated structures called "ptilinum" which form on their heads. After emerging, these ptilinum permanently retract into the head. ("Cuterebra emasculator Fitch (Insecta: Diptera: Oestridae)", 2007; Bennett, 1955)


There is little information on the mating systems of Cuterebra emasculator.

Cuterebra emasculator is a semelparous species that only breeds once during its short lifespan. Breeding takes place in the spring after the adults hatch emerge from their pupal case. The adults then breed over the following weeks, and die when winter comes. Males of other Cuterebra species wait at distinctive locations in the habitat, such as stems of vegetation, and wait for females. It is unknown whether or not C. emasculator also exhibits this behavior. Females presumably deposit eggs in the environment frequented by hosts, such as the entrances of burrows on branches and vegetation. Bot flies in general have relatively high fecundity, often producing more than 1,000 eggs per female. ("Cuterebra emasculator Fitch (Insecta: Diptera: Oestridae)", 2007; Bennett, 1955)

  • Breeding interval
    Cuterebra emasculator breeds once yearly.
  • Breeding season
    Cuterebra emasculator breeds during spring, which varies geographically.
  • Average eggs per season

After females oviposit their eggs, there is no further parental investment by either adult.

  • Parental Investment
  • no parental involvement


Cuterebra life spans are generally short, as adult bot flies' only purpose is to reproduce. Adults hatch during the spring after overwintering as pupae, and will mate and lay eggs only once before dying shortly afterward. ("Cuterebra emasculator Fitch (Insecta: Diptera: Oestridae)", 2007)

  • Average lifespan
    Status: wild
    1 years


Cuterebra emasculator is a fairly sedentary, parasitic species that performs most of its movements within its hosts' bodies. The fly larvae are capable of very limited movements upon hatching and can only sway back and forth while attached to their hatching substrate. The larvae attach themselves to suitable, passing hosts and will travel within the hosts' bodies until they settle under the skin. Larvae pass through three instars at this location, after which they exit the skin and drop to the forest floor to pupate. When winged adults emerge in the spring, they are highly mobile and will begin their search for mates. ("Cuterebra emasculator Fitch (Insecta: Diptera: Oestridae)", 2007; Bennett, 1955)

Home Range

Exact home range for Cuterebra emasculator is unknown.

Communication and Perception

Cuterebra emasculator eggs hatch when increasing temperature in the environment, carbon dioxide, and moisture from a potential host are detected. Some bot flies are attracted to pheromones produced by hosts, which stimulates their sensor neurons. Ceterebra emasculator may also communicate with chemical signals. (Bennett, 1955; Tommeras, et al., 1993)

Food Habits

While within their hosts, larvae consume interstitial fluid and possibly cellular debris and leukocytes. Cuterebra emasculator adults are free-living but do not bite nor feed; in fact, they lack functional mouths. ("Cuterebra emasculator Fitch (Insecta: Diptera: Oestridae)", 2007; Slanksy, 2007)

  • Animal Foods
  • body fluids


Cuterebra emasculator adults are most likely the prey of spiders, birds, frogs, and other natural predators of large flies. Currently, no studies have been conducted on specific predators of Cuterebra emasculator. (Slanksy, 2007)

Ecosystem Roles

Cuterebra emasculator are obligate parasites of tree squirrels, including eastern gray squirrels and fox squirrels and eastern chipmunks.

Although infection by C. emasculator larvae may cause anemia in their host and secondary bacterial infection of open wounds may occur, their presence causes little negative impact on the host population as a whole. ("Cuterebra emasculator Fitch (Insecta: Diptera: Oestridae)", 2007; Bennett, 1955; Jacobson, et al., 1981; Slanksy, 2006)

Species Used as Host

Economic Importance for Humans: Positive

There are no known positive effects of Cuterebra emasculator on humans.

Economic Importance for Humans: Negative

Cuterebra emasculator has very little proven effect on humans. The only direct influence the species has on humans is the timing of squirrel hunting season. In some states, because hunters would discard squirrels with warbles (although the meat is safe for consumption) the hunting seasons were established after the main period of infestations of C. emasculator. ("Cuterebra emasculator Fitch (Insecta: Diptera: Oestridae)", 2007; Slanksy, 2006)

Conservation Status

Currently, there are no conservation concerns regarding Cuterebra emasculator.


U-Bin Li (author), University of Michigan-Ann Arbor, Heidi Liere (editor), University of Michigan-Ann Arbor, John Marino (editor), University of Michigan-Ann Arbor, Barry OConnor (editor), University of Michigan-Ann Arbor, Rachelle Sterling (editor), Special Projects.



living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.

World Map

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.


an animal that mainly eats meat


uses smells or other chemicals to communicate


animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature


forest biomes are dominated by trees, otherwise forest biomes can vary widely in amount of precipitation and seasonality.


(as keyword in perception channel section) This animal has a special ability to detect heat from other organisms in its environment.


A large change in the shape or structure of an animal that happens as the animal grows. In insects, "incomplete metamorphosis" is when young animals are similar to adults and change gradually into the adult form, and "complete metamorphosis" is when there is a profound change between larval and adult forms. Butterflies have complete metamorphosis, grasshoppers have incomplete metamorphosis.


having the capacity to move from one place to another.

native range

the area in which the animal is naturally found, the region in which it is endemic.


reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.


an organism that obtains nutrients from other organisms in a harmful way that doesn't cause immediate death

seasonal breeding

breeding is confined to a particular season


offspring are all produced in a single group (litter, clutch, etc.), after which the parent usually dies. Semelparous organisms often only live through a single season/year (or other periodic change in conditions) but may live for many seasons. In both cases reproduction occurs as a single investment of energy in offspring, with no future chance for investment in reproduction.


reproduction that includes combining the genetic contribution of two individuals, a male and a female


living in residential areas on the outskirts of large cities or towns.


that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).


Living on the ground.


uses sight to communicate


UF/IFAS. Cuterebra emasculator Fitch (Insecta: Diptera: Oestridae). EENY-401. Lauderdale, Florida: UF/IFAS. 2007. Accessed April 01, 2010 at

Bennett, G. 1972. Further studies on the chipmunk warble, Cuterebra emasculator (Diptera: Cuterebridae). Canadian Journal of Zoology, 50/6: 861-864.

Bennett, G. 1972. Observations on the pupal and adult stages of Cuterebra emasculator Fitch (Diptera: Cuterebridae). Canadian Journal of Zoology, 50/11: 1367-1372.

Bennett, G. 1973. Some effects of Cuterebra emasculator Fitch (Dipter: Cuterebridae) on the blood and activity of its host, the Eastern chipmunk. Journal of Wildlife Diseases, 9: 89-93.

Bennett, G. 1955. Studies on Cuterebra emasculator Fitch 1856 (Diptera: Cuterebridae) and a discussion of the status of the genus Cephenemyia. Canadian Journal of Zoology, 33: 75-98.

Jacobson, H., M. Hetrick, D. Gyunn. 1981. Prevalence of Cuterebra emasculator in squirrels in Mississippi. Journal of Wildlife Diseases, 17/1: 78/87.

Slanksy, F. 2006. Cuterebra bot flies (Diptera: Oestridae) and their indigenous hosts and potential hosts in Florida. Florida Entomologist, 89/2: 152-159.

Slanksy, F. 2007. Insect/Mammal Associations: Effects of Cuterebrid Bot Fly Parasites on Their Hosts. Annual Review of Entomology, 52: 17-36.

Slanksy, F., L. Kenyon. 2002. Bot fly (Diptera: Cuterebridae) infestation of nest-bound infant eastern gray squirrels. Florida Entomologist, 85/2: 369-371.

Tommeras, B., A. Wibe, A. Nilssen, J. Anderson. 1993. the olfactory response of the reindeer nose bot fly, Cephenemyia trompe (Oestridae), to components form interdigital pheromone gland and urine form the host reindeer, Rangifer tarandus. Chemoecology, 4/2: 115-119.