Controlling Ecto-parasites on Horses 1997-98
Guide B-706
L.M. English, Extension EntomologistR.L. Byford, Associate Professor
M.E. Craig, Sr. Research Assistant
College of Agriculture and Home Economics New Mexico State University
This publication is scheduled to be updated and reissued 12/02.In recent years there has been increasing interest in keeping horses for work, pleasure riding, show purposes, racing, or as breeding stock, and their numbers have grown rapidly in New Mexico. Many horses are still classified as farm or ranch horses, but ownership has been shifting towards city dwellers who keep them in riding stables or small paddocks near their homes. A New Mexico Department of Agriculture survey in 1987 estimated there were about 97,000 horses in New Mexico.
Horses, mules, and donkeys can be affected in several ways by arthropod pests. These parasites may annoy the animal and make them difficult to manage. Heavy infestations of blood-sucking parasites (biting flies, lice, and mites) and persistent pests such as house flies can cause a general unthriftiness in the animal. Pests such as mosquitoes can be important carriers of diseases such as encephalitis. Knowledge of these particular parasites and appropriate control measures can help the owner prevent injury to horses. Stings, wounds, or scabs resulting from ectoparasites are not acceptable, especially on show animals. Open wounds attract certain flies that lay their eggs, and these wounds can become sites for secondary infections.
Horse Bots
Three kinds of horse bot flies may infest horses in New Mexico: 1) the common horse bot fly, 2) the chin or throat bot fly, and 3) the lip or nose bot fly.
Horse bot adults are hairy, large flies. They do not have functional mouthparts; however, the buzzing sound and hovering behavior of female flies attempting to lay their eggs alarms horses.
These flies deposit their eggs in the autumn. Although the flies are seldom observed, the oval, light yellow eggs are easily detected because they are cemented individually to the horse's body hairs. Eggs of the common bot hatch only when the horse bites or licks the area and the small larvae enter the horse's mouth. The other species hatch and crawl directly into the soft mouth and tongue tissues. The injurious stage of all these flies is the bot or larval stage, when the insects live in the horse's digestive tract for about 10 months. These pests injure the tongue, irritate the lining of intestines and stomach, and interfere with digestive activities. A horse that is badly infested with bots appears generally run down and has a rough coat. Bots can also be detected by finding full-grown larvae in the feces during spring months; however, this occurs too late to apply effective control measures.
Ideally, bot control should begin between October 15 and December 15. This gives the bots time to hatch and enter the intestinal tract, but timely treatment prevents extensive damage.
Oral treatments containing trichlorofon or ivermectin effectively control bots attached to the intestines. Owners can buy paste formulations of these insecticides in disposable, individual applicators that deposit the desired dosage on the animal's tongue. The amount of paste administered depends on animal weight. Be sure not to overdose. These applicators ensure appropriate dosage and effective treatment.
Flies
Horses confined to stables or corrals will be pestered primarily by house and stable flies. Proper sanitation is the foundation of fly control and involves removing manure and managing water. Keep stalls dry and remove moist hay, straw, manure, and spilled grain once or twice a day. Fix leaky plumbing, watering troughs, and other water sources. Grade corrals and yards so water drains rapidly after a rain. Fill holes and low places where water may collect.
Residual Sprays
Residual sprays effectively control flies that evade sanitation efforts or that immigrate to animal facilities. Do not wait until flies become numerous to start a spray program. Apply sprays to the major resting surfaces for flies. Spray barns at the rate of 1 gallon per 300?500 square feet of surface. Do not spray horses directly and do not contaminate feed or water. On porous surfaces such as unfinished wood, brick, adobe, or concrete, wettable powder formulations usually give longer-lasting control than emulsifiable concentrates.
Fly Baits
Fly baits are an insecticide contained in an attractant. Place baits on flat surfaces such as window ledges, doorways, or alleyways where flies congregate. It is important that dry baits be scattered thinly and uniformly so they cannot be ingested by children or pets. It is also important to replace baits periodically, particularly during the first few days of baiting.
Although many prepared fly baits are available, liquid baits can be made by mixing the correct amount of insecticide with water, corn syrup, sugar, or molasses. Follow label directions on the container. Do not place baits where animals may contact them directly and do not contaminate feed, water, or utensils. Recommended rates for dry baits and liquid baits prepared by users are:
- Malathion: 1% prepared dry bait or prepare a 1% liquid bait.
- Dichlorphos: 0.5% prepared dry bait or prepare a 0.1% liquid bait.
- Ronnel (Korlan): 1% prepared dry bait or prepare a 2% liquid bait.
- Trichlorofon (Dipterex): 1% prepared dry bait or prepare a 1% liquid bait.
Resin Strips
These fly control devices can be used in addition to sprays or baits. They are not effective in rooms or buildings where a constant exchange of inside and outside air occurs. Hang one 10-inch, 20% dichlorvos resin strip from the ceiling for each 1,000 cubic feet of space. Do not place resin strips over feed or water troughs, and keep them out of reach of children.
Space Sprays
Space sprays can be used for fogging or can be applied from aerosol or hand-operated mist applicators in barns. Space sprays leave no residual but kill flies only on contact, so they must be applied daily.
Control of Fly Larvae (Maggots) in Manure Piles
Flies often breed in manure piles. If manure cannot be removed and scattered on fields, larvacides will decrease fly populations. Larvacides should be applied as coarse sprays to manure accumulations. Flies will continue to lay eggs on sprayed manure, but a regular insecticide application will kill most maggots before they become adult flies. Do not apply where animals will contact treated manure. Apply one of the following insecticides at a rate of one gallon per 100 square feet of surface:
- Dimethoate (Cygon): mix 1 pt of 23.4% emulsifiable concentrate in 2 1/2 gal of water.
- Stirofos (Rabon): mix 1/2 lb of 50% wettable powder in 3 gal of water.
Control of Screw-worms and Other Blow Fly Larvae
Collect samples of the larvae from wounds for identification by your county agent. Treat the wound twice the first week and once a week thereafter until the wound is healed. Use coumaphos (Co-Ral) 5% dust or 3% spray foam; or use ronnel (Korlan) 5% smear, dust or pressurized 2 1/2% spray aerosol. Apply the material locally to wounds and scratches. Follow all instructions and precautions on the label.
Biological Controls
Several species of parasites and predators are commercially available to use in biological control programs for flies. However, results from biological control programs using these beneficial insects in commercial animal production facilities have been variable. There is not yet enough scientific research on the practical and cost-efficient use of parasites and predators under various field conditions to recommend them to New Mexico owners. Before considering biological control of flies in your operation, consider the following.
Good sanitation is essential to any fly control program. Pesticides may damage a biological control effort. Usually thousands of parasites or predators must be released in a well-timed and planned program. One release will not be sufficient.
Such a program can be time consuming and costly. The producer must decide what control level is acceptable to be cost effective.
Flies Pestering Horses Kept Outside
Horn flies, stable flies, horse flies, house flies, black flies, and mosquitoes usually pester horses kept in an outside paddock. Pesticides applied to the horse's neck, withers, shoulders, and around the face give best control of these flies.
Lice
Horses can be infested with biting or sucking lice. Sucking lice are more damaging because they feed on the animal's blood. They are most often found on the head, neck, and mane, but can spread as the horse's winter coat develops. Lice infestations usually increase during winter and early spring. Lousy horses may become thin, suffer hair loss, lose body condition, and become more susceptible to diseases.
Lice eggs, commonly called nits, are white and are usually attached to the horse's hair. Nymphs and adults are brown to gray, similar in appearance, and often found in dense, localized clumps. Full-grown lice are wingless, soft-bodied, and about 1/8" long. Lice can be carried from one animal to another on harnesses, blankets, saddles, brushes, or curry combs, or they may move directly from a lousy animal to a lice-free one.
Eggs hatch after incubating for 1?2 weeks. Nymphs develop in about 2 weeks and adults live 4?5 weeks. Egg to adult development takes about 2 months. Each female will lay from 50 to 100 eggs. Inspect horses regularly for lice during late fall and winter.
Effective louse control requires covering the horse completely with an insecticide. The insecticide must penetrate the hair coat and reach the skin. Repeat insecticide sprays every 14-18 days to kill lice hatching from nits or eggs not destroyed by earlier applications.
Mange Mites
Horses, mules, and donkeys can be infested by mange mites similar to those that affect cattle. Mangy animals scratch and lose their hair. The damage resembles louse injury, but if no lice are present, suspect mange. As mite numbers increase, the skin oozes and becomes crusty and leathery. If scabies is suspected, skin scrapings should be taken to the county agent or veterinarian for identification. Mange mites are nearly microscopic, spider-like arthropods. Adults and nymphs have eight legs, but newly hatched larvae have only six legs. Females lay one to two eggs per day during their 2-week life span. Eggs hatch in about 4 days and nymphs develop into adults in 1?2 weeks. These mites burrow in and under the skin and cause a scabby condition. These mites can be eradicated by spraying or dipping the infested animals.
Ticks
Several species of ticks can attack horses, mules, or donkeys. Thorough coverage of the belly, sides, and back with an effective insecticide is essential to control ticks on horses. Make sure to leave no dry areas. Start treatment as soon as ticks are noticed in spring and repeat every 2?4 weeks as needed. Start treatments in fall to control winter ticks. To control black-legged ticks, applications must be repeated every 30 days.
Please refer to PDF version of this guide to view chart containing information on insects and their treatments.
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Revised December 1997Electronic Distribution December 1997