of Rhode Island GreenShare Factsheets
& Horse Flies
flies and deer flies belong to the fly family Tabanidae. Horse flies
(genus Tabanus) are considerably larger than deer flies (genus
Chrysops). Both horse flies and deer flies have large heads
and large eyes. Horse flies can range from 20 mm (3/4 inch) to more
than 25 mm (1 inch) in length. A common species typically found
near beaches and saltmarshes is Tabanus nigrovittatus, or "greenhead fly." This
species has brilliant green eyes which are sometimes crossed with
red or purplish bands. Larger species of
horse flies are brown to black and have varying stripes or triangles
on their abdomens. The larger species of horse flies are less common,
but inflict a painful bite similar to other species of horse flies.
The antennae of horse flies are thick and lengthen into 5 thinner
segments; the antennae of deer flies are long and thin. The wings
of horse flies are usually clear or completely dark, whereas deer
fly wings have varying patterns. Deer flies are comparable in size
to house flies and are mostly yellow or black with varying stripes
and shapes on their abdomens. The wings of deer flies are usually
marked with dark patterns. Deer flies also have brilliantly-colored
eyes, ranging from gold to green, with large brightly-colored stripes.
female horse flies and deer flies feed on blood. Both flies are
vicious, painful biters which feed on the blood of cattle, horses,
mules, hogs, dogs, deer and other mammals, including humans. These
flies cut through the skin using razor-sharp mouthparts that are
shaped like a knife or razor. The flies will then suck the blood
up from the wound for several minutes. This process makes these
flies potential mechanical vectors of such diseases as anthrax,
tularemia, anaplasmosis, hog cholera, equine infectious anemia and
filariasis. Deer flies and horse flies are also suspected of transmitting
Lyme disease (New England Journal of Medicine 322:1752, 1990). The
open wound left by the fly bite also permits secondary feeding sites
for other nuisance insects. Biting deer flies frequently attack
humans along beaches, streams, ponds, lakes and around woods and
dirt roads near large bodies of water. Some people suffer severe
lesions, high fever and even general disability when bitten. Allergic
reactions may occur from the saliva, which is poured into the wound
to prevent clotting while the fly is feeding. A person can become
increasingly sensitive to repeated bites. However, horse flies and
deer flies are generally thought of as primarily nuisance pests.
lifecycles of both horse and deer flies are similar. The first
stage of development is the egg stage. The eggs are dark, shiny
shaped. They are layered in masses ("tiers") which contain a few
to several hundred eggs. These masses are laid on vegetation which
hangs over water. Eggs hatch within five to twelve days, and small
larvae drop down and burrow into moist soil. Suitable habitats
saltmarshes, swamps, bogs and areas along the edges of ponds, lakes
and streams. Deer fly larvae feed on organic debris and other small
organisms. Horse fly larva will feed on organic debris, insects,
small crustaceans, snails, earthworms and other organisms. Horse
fly larvae are also cannibalistic and will eat other larvae.
Larvae overwinter in muddy soils, maturing in late spring. In some
cases, larvae take one to three years to complete development. In
late spring, the larvae migrate towards dryer soils and develop
into pupae. The pupal period varies between species and may range
from six to twelve days, depending on temperature. Adult flies emerge
from pupae and immediately begin mating and blood feeding. Adults
are strong fliers, searching visually for hosts and mates. The females
require a blood meal for their eggs, but also feed on nectar and
plant juices for flight energy. Males also require nectar and plant
juices for flight.
Unfortunately, there are no satisfactory methods for control of
horse flies and deer flies. It is impractical in most regions to
eliminate their breeding areas, especially along endangered wetlands,
where these flies are commonly found. Larval control is equally
impractical, especially in recreation areas or reservoirs. Adulticiding
is not practical because it requires the use of broad-spectrum insecticides,
which can be toxic to fish, birds and mammals. Traditional repellants
are not effective in keeping away horse and deer flies. Repellants
containing DEET (N-diethyl-meta-toluamide) do not prevent flies
from landing, although they may inhibit the flies from biting.
greatest horse and deer fly activity occurs on warm, sunny days
when there is little or no wind. A slight drop in temperature or
a sudden breeze reduces biting attacks. Horse and deer flies are
visual insects, locating hosts by movement. Dark, moving objects
and shapes are most attractive to the flies. They are also attracted
to carbon dioxide that is released from their hosts. To reduce exposure
to bites, it is best to wear light-colored clothing, including a
light baseball cap, especially on warm, sunny days when flies are
devices have been designed to catch flies attracted to moving objects.
One device is the trolling deer fly trap. This device consists of
a blue cup coated with sticky material; the cup is placed outside
of a moving vehicle, attracting horse and deer flies with its movement
and color. Another device is the Tred-not Deer Fly Patch, which
is a new, non-chemical sticky patch for controlling horse and deer
flies. Some testers have reported good results from these odorless,
non-chemical, adhesive patches. The patches are 7.5 cm (3 inches)
wide by 15 cm (6 inches) long, and are worn on the back of a baseball
cap to trap and hold biting deer flies.
number of fly traps have been developed which attract these flies
using dark, moving objects, as well as carbon dioxide and other
attractants. Although these traps will not completely eliminate
all the flies, they will reduce the populations to a more tolerable
level. Examples of trap designs include canopy traps, box traps,
malaise traps and light traps.
available horse and deer fly repellants are available for use on
animals only. Most of these repellants contain permethrin, which
may be harmful to humans. One must take precautions to use these
repellants only as instructed. Repellants safe for horses are not
necessarily safe for other animals. Always follow label directions.
Kristen Bartlett, 1999
are poisonous! Read and follow all safety precautions on labels.
Handle carefully and store in original containers out of reach
of children, pets or livestock. Dispose of empty containers
immediately, in a safe manner and place. Pesticides should never
be stored with foods or in areas where people eat.
When trade names are used for identification, no product endorsement
is implied, nor is discrimination intended against similar materials.
Be sure that the pesticide you intend to use is registered for
the state of use.
The user of this information assumes all risk for personal injury
or property damage.
information, call the URI CE Gardening and Food Safety Hotline
at 1-800-448-1011 or (401)874-2929 from outside Rhode Island;
Monday-Thursday between 9 am and 2 pm.
of Rhode Island Cooperative Extension provides equal program