Insect Pests of Horses (continued)
Extension Bulletin No. 55, June 1991
Ticks and Mites
Ticks
Ticks and mites are arthropods related to insects that may become numerous and damage horses in some years. Several species of ticks may occasionally become pests on horses in North Dakota. Dermacenter variabilis, the American dog tick, and Dermacenter albipictus, the winter tick (Figure 13), cause the most frequent problems. Both species are similar in appearance. Unfed stages are very flat and hard. Before ticks feed they are about 1/4 inch long. Ticks attach to the vertebrate host to obtain a blood meal; heavy populations may cause the death of animals from excessive blood loss. Engorged ticks are about 1/2 inch long and look inflated. The adults have eight legs. The engorged female tick drops from the host and lays large numbers of eggs (up to 6000) on the ground.
The winter tick is a one host tick; this means that all stages in the developmental cycle can parasitize the same type of host animal. The preferred host for the winter tick is the moose; however, horses are frequently fed upon. Occasionally cattle, sheep, bison and other large animals are attacked. The winter tick is most abundant during the fall through the winter months. Heavy infestations cause loss of appetite, depression and debilitation of horses.
The American dog tick is a three host tick. Larvae and nymphs feed on rodents and small mammals. Adult dog ticks feed on a variety of larger mammals including horses, cattle and humans. Grooming of horses should reveal ticks which are present. These ticks can be removed by hand, or topical applications of acaricides can be used to prevent infestations.
Figure 13.
The American dog tick (A) and winter tick (B).
Mites
Several types of tiny mites cause skin conditions, collectively called mange, in horses. Generally the symptoms of mange include areas of cracked, dry skin and formation of scabs. Mite activity is extremely irritating and results in scratching, rubbing and lick-ing at infested areas. Secondary infections of the infested areas are possible. Sarcoptic, chorioptic and psoroptic manges of horses are caused by separate species of small mites which invade the skin of the horse. All of these mites are very difficult to see without magnification. A sarcoptic mange mite, Sarcoptes scabiei equi, is shown in Figure 14. Confirmation of a mite infestation is usually by examination of skin scrapings or excised mites under a microscope. Early detection is important in limiting the spread of the mites to other horses and is equally important to prevent severe mange on an animal. These mites spread from horse to horse by direct contact, or by the use of common grooming tools and tack. Frequent grooming is important for early detection; when symptoms appear, consult a veterinarian for confirmation and treatment.
Figure 14. Sarcoptic mange mite (Sarcoptes scabiei equi).
Horse Bots
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Three species of bots parasitize horses, mules and donkeys. These are the common bot, Gasterophilus intestinalis (DeGeer), the throat bot, G. nasalis (Linnaeus), and the nose bot, G. haemorrhoidalis (Linnaeus). Adults of these three species are nearly the same size as bees and are hairy bodied; however, they do not bite and do not feed at all. Mouthparts are greatly reduced and non-functional. Bot flies are active in the warm weather months in the north central plains until the first frost. Description of Stages Common bot. Eggs of the horse bot are stalkless and are generally glued near the end of the hairs. The eggs are greyish yellow to yellow in color and about 0.05 inch long. Two flanges along the lower half of the egg encircle the hair and serve to attach the egg to the hair. The non-flanged half extends from the hair at about a 30 degree angle. Throat bot. Eggs of the throat bot are also stalkless and are usually laid near the skin. For this reason, they are often obscured by overlying hair. The flanges which attach the egg to the hair extend almost the entire length of the egg. The color is whitish-yellow and the egg is approximately 0.05 inch long. The long axis of the egg extends parallel to the hair. Nose bot. Nose bot eggs are stalked and the general shape is barnacle-like. The connecting flange extends from the stalk upward toward the top of the egg. The general color is brownish-black and the egg is about 0.06 inch long. Larvae. Horse bot larvae are well adapted to life in the digestive tract of the horse. Larvae are equipped with mouth hooks, setae and spines to damage and irritate submucosal tissues in the mouth of the horse and to attach to the lining in the stomach and intestines of the horse. The last stage larva (Figure 16) is robust and yellowish in color. The feeding larva is the overwintering stage. Larvae of all three species are similar in appearance. Pupae. The pupae of horse bots are all similar. Pupation takes place on the ground, after the last stage larvae have left the horse. The pupal period lasts approximately one month before the emergence of the adult fly. Adults. The adults of all three species are similar in appearance and superficially resemble honey bees (Figure 17); they are hairy-bodied and about the same size as honey bees. Adult flies have non-functional mouthparts and do not bite. |
Figure 18. Horse bot fly life cycle. |
Horse Bot Life Histories and Habits
The life histories of the three species of bots share a number of similarities
(Figure 18). All species have one generation per year in the north central plains.
All three species attach their eggs directly to the hair on horses. Larvae of
all species are internal parasites of the gasterointestinal tract. Species differ
in egg laying sites on the host and there are some differences in the sites
of infestation within the horse.
Common bot females lay their eggs along the forelegs and flanks where they can be reached by the horse as it rubs its muzzle and tongue over the area. The heat of friction supplied by the rough tongue of the horse causes the larvae to emerge from the eggs. The larvae are picked up on the tongue where they invade the mucous membrane. Larvae remain there for several weeks before they migrate to the cardiac portion of the stomach. Here the larvae attach to the lining until spring or summer. Mature larvae pass out with the feces to pupate in dry soil or the dried feces. Adults emerge in approximately one month. Mating takes place soon after emergance and the female lays about 500 eggs over a period of about a week.
Throat bot females deposit their eggs under the jaw or throat area by hovering in midair, then darting in to attach eggs. The fly makes repeated attacks until it has laid many eggs. One female can lay about 500 eggs. Larvae hatch within three to five days and crawl along the jaw, enter the mouth and penetrate the gumline. Formation of pus pockets is common and irritates the horse. Larvae remain in these locations for a month or more. Development continues in the distil part of the stomach during the winter and spring. Grown larvae pass out with the feces and pupate on the ground.
Nose bot females oviposit on the very fine hairs around the lips, particularly the upper lip, close to the mouth. This species and the throat bot cause distress to the horse as they dart in to deposit a single egg at a time. The ovi-positing fly makes repeated attacks in this manner to lay its full complement of eggs. Each female can deposit about 160 eggs. The eggs hatch in as little as two days and penetrate the lip and tongue membranes where they develop for five to six weeks. They migrate to the stomach where they remain for the duration of the winter. In the spring they detach and migrate to the rectum; here they reattach near the anus before dropping to the ground to pupate. Adults emerge about a month later.
Effects on the Horse
Egg laying activity causes indirect damage to horses by making them unruly and
difficult to manage. Irritated and frightened horses may damage themselves,
equipment or fences.
Direct damage results from larval infestations in the horse. Newly hatched larvae produce irritation as they tunnel into the gum line, tongue or lips. Horses may show signs of this irritation by rubbing their lips and noses on the ground or against fences, stock tanks or other equipment.
Bot infestations in the digestive tract of the horse impair digestion and occasionally larvae are numerous enough to obstruct passage of material from the stomach and cause colic. In extreme cases rupture of the stomach wall could cause death.
Management of Horse Bots
To control horse bots you must break the insect's life cycle and stop its development.
Bot eggs, particularly of the common bot, can remain viable long after flies
have disappeared. Sponging the horse with warm water frequently will induce
eggs to hatch and the exposed first stage larvae will die quickly. (This treatment
is most effective if it is used on cool days). Treatment of egg laying sites
with insecticidal washes will also reduce the numbers of larvae which can be
injested by the horse. These treatments should be applied weekly for common
bot control and more frequently for both the throat and nose bots. Treatment
of the horse's face is best applied using a wipe; a spray may irritate the eyes,
nose and mouth of the horse and could startle and frighten the animal.
A variety of direct animal treatments are available to rid horses of bot infestations in the stomach and intestines. These medications are available as pastes, gels, pellets, liquids, powders and boluses. All of these product dosages are based on animal weight, so you should be able to estimate your animal's weight with some accuracy. Many of these products are best applied by a veterinarian familiar with horses. Several of these products are also effective treatments for other internal parasites. These types of chemical control are most effective when the bots are all in the stomach or intestines, generally from November through January.
Blister Beetles
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Blister beetles (Figure 19) in the insect family Meloidae have a toxic secretion, cantharidin, in their blood. Cantharidin is a defensive chemical produced by blister beetles that causes severe dermal irritation to the skin and mucous membrane of warm blooded animals. This defensive chemical serves blister beetles well, since once a warm blooded animal crushes a blister beetle on its skin, it will take conscious steps to avoid future contact with blister beetles. If whole or crushed parts of blister beetles are ingested by a grazing animal, the cantharidin can cause irritation and hemorrhages in the stomach. The amount of cantharidin produced varies from male to female and among species of blister beetles (Capinera et al., 1985). Sufficient cantharidin is produced by any of the common North Dakota species to be of veterinary concern. Among domestic grazing animals, horses are most susceptible to this toxin. Only a few beetles, eaten with hay, can cause severe illness or even death to a horse. Affected horses exhibit signs of colic, frequently void small amounts of blood tinged urine, and at times have muscle tremors. If blister beetle poisoning is suspected, immediately consult your local veterinarian so treatments can be started. Blister beetles commonly feed on alfalfa and the flowers of a number of plants that frequently grow in hay fields. The problem occurs when the hay harvesting process crushes or grinds up blister beetles and toxic parts remain in hay that is fed to horses. North Dakota has over 20 species of blister beetles, and the three most abundant species feed on alfalfa and other flowering plants in hay fields and meadows. Thus, the potential for a problem always exists, if the beetles are accidently incorporated into hay. Some steps can be taken to reduce the possibility of incorporating blister beetles in hay. First cutting hay seldom has blister beetles present, since if it is cut in early to mid-June, that is before the adult blister beetles are present in alfalfa. Blister beetle poisonings have increased since the advent of swather-conditioning equipment which runs hay between rollers or crimpers. Separate cutting followed by windrowing allows the beetles to find their way out of windrows while the hay is drying and prior to baling. Hay conditioning equipment will kill many beetles as they pass through the rollers, contaminating several feet of windrow with crushed beetle parts. For additional information about blister beetles see Extension Service circular E-1002, Blister Beetles. |
Figure 19. |
Chemical Controls for Insect Pests of Horses
Always follow the label recommendations for proper rate and method for application of insecticides. Use only formulations approved for use on horses. For information on specific products or recommendations for a particular pest control problem consult your county extension agent, an extension entomologist or your veterinarian. Control recommendations for the current year are contained in the North Dakota Insect Control Guide. Mention of any proprietary product does not imply endorsement of a particular brand.
References Cited
Capinera, J.L., D.R. Gardner and F.R. Stermitz. 1985. Cantharidin levels in blister beetles (Coleoptera: Meloidae) associated with alfalfa in Colorado. J. Econ. Entomol. 78: 1052-1055.
Guyer, G.E., H.L. King, R.L. Fischer, and W.A. Drew. 1956. The emergence of flies reared from grass silage in Michigan. J. Econ. Entomol. 49: 619-622.
Harris, R.L., E.D. Frazer and R.L. Younger. 1973. Horn flies, stable flies and house flies: Development in feces of bovines treated orally with juvenile hormone analogues. J. Econ. Entomol. 66: 1099-1102.
McBride, D.K., A.W. Anderson and W.D. Valovage. 1988. Mosquito management. NDSU Extension Publication E-472. 10 pp.
Meyer, J.A. and J.J. Peterson. 1983. Characterization and seasonal distribution of breeding sites of stable flies and house flies (Diptera: Muscidae) on Eastern Nebraska feedlots and dairies. J. Econ. Entomol. 76: 103-8.
Miller, R.W. 1970. Larvicides for fly control — a review. Bull. Entomol. Soc. Amer. 16:154-158.
Moon, R.D. and H.J. Meyer. 1985. Non-biting flies. In Williams, R.E., R.D. Hall,A.B. Broce, and P.J. Scholl (Ed.) Livestock Entomology. John Wiley and Sons. Chapter 5. pp. 65-82.
Morgan, P.B. 1980. Sustained releases of Spalangia endius Walker (Hymenoptera:Pteromalidae) for the control of Musca domestica L. and Stomoxys calcitrans L. (Diptera: Muscidae). J. Kansas Entomol. Soc. 53: 367-372.
Patterson, R.S., G.C. LaBrecque, D.F. Williams and D.E. Weidhaas. 1981. Control of stable fly, Stomoxys calcitrans (Diptera: Muscidae) on St. Croix U.S. Virgin Islands using integrated pest management measures. J. Med. Entomol. 18: 203-210.
Peterson, J.J. and J.A. Meyer. 1983. Host preference and seasonal distribution of pteromalid parasites (Hymenoptera: Pteromalidae) of stable flies and house flies (Diptera: Muscidae) associated with confined livestock in eastern Nebraska. Environ. Entomol. 12: 567-571.
Peterson, J.J. and J.A. Meyer, D.A. Stage and P.B. Morgan. 1983. Evaluation of sequential releases of Spalangia endius (Hymenoptera: Pteromalidae) for control of house flies and stable flies (Diptera: Muscidae) associated with confined livestock in eastern Nebraska. J.Econ. Entomol. 76: 283-286.
Scholl, Phillip J. and James J. Peterson. 1985. Biting flies. In Williams, R.E., R.D. Hall, A.B. Broce, and P.J. Scholl (Ed.) Livestock Entomology. John Wiley and Sons. Chapter 4. pp. 49-63.
Scholl, P.J., J.J. Peterson, D.A. Stage and J.A. Meyer. 1981. Open silage as an overwintering site for immature stable flies in Eastern Nebraska. Southwestern Entomol. 6: 253-258.
West, L.C. 1951. The House Fly. Its Natural History, Medical Importance and Control. Cornell Publishing Company, Ithaca, NY, 584 pp.
Acknowledgements
The authors thank Albin Anderson, Wayne Boland and Bob Johnson for critical reviews of the manuscript.
Extension Bulletin No. 55, June 1991
