POULTRYAND
LIVESTOCK
POULTRY
The information and recommendations in this newsletter are applicable to
North Carolina
and may not apply in other areas.
Deer flies and horse flies are a summer nuisance in North Carolina. The cool, rainy weather of late has slowed these flies somewhat, but expect the onslaught to increase again with hot weather. Deer flies (also called yellow flies) are the first to appear in significant numbers. In Sampson County, North Carolina and areas to the east, swarms of deer flies make life miserable from the first of May until mid-June. This roughly coincides with the average life span of three to four weeks for deer flies. The swarms simply diminish as the season's first and heaviest emergence of adults dies off. Make no mistake, however, they never completely go away. Deer flies continue to harass until the first killing frost. In areas where there is significant breeding habitat for the flies, such as in and around wetlands, the numbers can remain quite high throughout the summer. Their larger cousins, horse flies, are generally less numerous and seldom attack in swarms. They also show a greater preference for livestock than humans and small animals. I do not know if their common name originates from the fact that they seem to bother horses, or because a number of species are as big as a horse.
Tabanidae, the family containing these flies, is large and diverse. There have been over 3,000 species identified to date. Unfortunately, specific information about the life cycle and habits of many of these species is sketchy. It is known that tabanids are heavily influenced by weather conditions and light. As a general rule, they like it hot. Many species are also most active when it is bright and sunny. Both adult males and females feed on nectar, but females also drink blood. After mating, most female tabanids must have blood to nourish their developing eggs. To get blood they inflict vicious bites for which the family is so well known and despised.
Tabanid species around the world transmit a number of diseases and parasites. For example, they are vectors of equine infectious anemia and hog cholera. Since they feed indiscriminately and take relatively large amounts of blood from their hosts, severe infestations of tabanids can affect animal performance and cause anemia from blood loss. They can also harry livestock and people to such an extent that some areas are rendered nearly useless for daytime activities during the summer.
The life cycle of most tabanids requires a year, but some species need several years to develop from egg to new adult. Eggs are deposited in a variety of habitats, but most species prefer one that is moist. Some species lay eggs in wet areas, others in moist soil near water, and still others in soil away from water. Larvae go through six to nine instars (larval stages) before they pupate. The last larval instar is thought to be the overwintering stage in some species. In the spring, these larvae resume development, pupate and emerge as adults.
Management of deer and horse flies is difficult. In other words, this is not a pest that you can beat with current technologies. It is impractical to attempt habitat modification in an effort to eliminate tabanid breeding sites; their breeding habitat is too diverse and widespread. At best it may be possible to restrict access to areas where deer and horse flies congregate. For example, a buffer zone between woodland and pasture fences of 50 to 100 feet may reduce attacks on livestock. Tabanids typically rest and hide on low hanging vegetation such as brushy shrubs and tree limbs. They find a host over some distance by way of odor. Once a host is within a few meters of their hiding place, the hungry female tabanids use visual cues. They launch themselves toward high contrast targets (cattle, horses and humans) moving through open areas (pastures or trails). When hosts are stationary, the flies use a combination of olfactory and visual cues. Carbon dioxide is a particularly strong attractant.
The use of insecticides to control this pest is of marginal value. Unlike horn flies and face flies, tabanids do not remain on their hosts long enough to be killed by insecticides. Use pyrethroids (pyrethrins or permethrin) on animals. Formulations that include natural pyrethrins combined with a synergist like piperonyl butoxide are effective since they act quickly. Permethrins do not have the quick knockdown effect of synergized pyrethrins but can be used as a repellant rather than a toxicant. Neither active ingredient remains effective for more than a few days, so frequent applications (sprays, oilers or wipes) will be needed. People can apply standard insect repellents before going out during deer and horse fly season. Fogging or spraying wooded perimeters around pastures, yards and other areas may be helpful, but is expensive and time consuming. Pyrethroids and other insecticides, such as malathion, may be used. The insecticide used, weather conditions, the density of tabanid populations, and other factors determine how often such treatments must be made. Most area fogs and sprays will be effective for no more than about one week. This approach also has a major drawback of potential harm to non-target organisms if used indiscriminately.
One approach that has some value for horse owners is to change the
grazing habits of their horses. Keep horses stalled during the day
and let them out to graze at night. Most tabanids are not active
at night. Few horse or deer flies will enter any but the most
open-sided barns during the day, allowing the animals welcome
relief from their predation. There is some risk that horses will
become a target for hungry mosquitoes at night, but this may be
acceptable if tabanid pressure is severe during the day. It is not
necessary to give up daytime rides either. Simply use an
insecticide as outlined above before taking out the horse.
Web page last updated on 3 June 1996 by Stephen J. Toth, Jr..
