Insect Pests of Horses (continued)

Extension Bulletin No. 55, June 1991

Nuisance Flies

Several types of nuisance flies may be associated with horses or their premises. These include the house fly, bottle flies, false stable flies and other species of barnyard flies. Face flies, usually a pest of cattle, may also affect horses, particularly when cattle are nearby.

Two major pest species which bother horses are the stable fly and the house fly, a non-biting species. A distinguishing feature, visible to the naked eye, that separates the two species is the distinct stiletto-like proboscis of the stable fly which extends forward beyond the head (Figure 1). This sharply pointed beak is used to pierce the skin and draw blood. The house fly cannot bite since it has sponging mouthparts.

House Fly

Adults: Both male and female house flies are grayish-brown with a black and grey striped thorax (Figure 6). The house fly is a medium sized fly ranging from about 1/4 to 1/3 inch long with sponging mouthparts. House flies do not bite but feed on a variety of plant and animal wastes and garbage, as well as other sources of carbohydrates and proteins. Eggs: House fly eggs are about 0.04 inch long, whitish and slightly curved. The females generally deposit eggs in batches of about 100 eggs at a time. Each female may deposit four to six batches of eggs during an average lifetime of two to four weeks during the summer. Larvae: The three larval stages are similar in appearance to stable fly larvae. The third stage reaches approximately 1/2 to 2/3 inch in length. Differentiation of the two species is based on the size and shape of the posterior spiracles (or respiratory tract openings). Pupae: Pupae are barrel shaped and are of the same approximate size and coloration as stable fly pupae.

Figure 6. Adult house fly.

House Fly Life History and Habits House fly females lay their eggs (Figure 7) in clusters, preferably in moist decaying organic material (Meyer and Peterson, 1983). Eggs hatch within eight to 40 hours, depending on temperature. Larvae feed on yeast, bacteria and decomposition products which occur in their development site. Larval development through three stages takes from three to eight days. Larvae crawl to drier areas to pupate when feeding is completed. The pupal stage lasts from three to 10 days, depending primarily on temperature. Adults emerge from the puparia and begin feeding within 24 hours. Males are ready to mate shortly after emergence and females begin mat-ing by the second or third day. Most females mate once and deposit eggs in batches every two to four days (Moon and Meyer, 1985; West, 1951). The flies feed on carbohydrates and proteins. Females require protein to produce viable eggs. Solid foods are first liquified with saliva and are then ingested using the sponging mouthparts. The entire life cycle from egg to adult can be completed in as little as 10 to 14 days during warm weather. Like the stable fly, house flies overwinter in sites where microbial fermentation heats the larval habitat, such as silage or manure piles. House flies may develop throughout the year in heated livestock facilities. They are active near sources of food during daylight hours and generally rest at night on stationary objects both indoors and outdoors. The flies prefer shaded areas during much of the day and commonly move inside structures where livestock are held. House Fly Management House fly management, like stable fly management, is based on a strong farm sanitation program. The methods for reducing house flies are the same as those discussed for the stable fly.

Figure 7. Life cycle of the house fly.

Face Fly

The face fly (Figure 8) is usually a pest of grazing cattle. However, when horses are pastured with or close to cattle or when face flies are numerous these flies will feed on secretions around the eyes of horses. Adult face flies look much like house flies. The face fly does not bite, but the persistent feeding behavior of the fly makes it a nuisance pest. In addition, the face fly can mechanically transmit parasites or pathogens to the horse. Control of face flies is difficult. Relief can be obtained by stabling horses during the daytime when the face fly feeds. In addition, since the face fly feeds predominantly on cattle, pasturing horses separately from cattle will lessen the incidence of these flies on the horses. Topical insec-ticide applications are usually not effec-tive because face flies spend little time on the vertebrate host. For additional information about the face fly see Extension Bulletin 36, The Face Fly, Musca autumnalis DeGeer in North Dakota.

Figure 8. Adult face fly.

 

Mosquitos

There are at least 43 species of mosquitoes known to occur in North Dakota. Fortunately, only a few species cause annoyance. Nevertheless, their presence affects people engaged in outdoor activities during the warm months of the year. Mosquitoes also annoy livestock causing weight loss, reduced milk production, and poor reproduction.

Besides the nuisance biting activities of various mosquito species, there are several species in the genera Culex and Aedes that can transmit diseases such as St. Louis and Western equine encephalitis to humans and horses, and heartworm to dogs. While Western equine encephalitis does occur occasionally in North Dakota, the disease is not common in the state. However, during the 1941 outbreak when encephalitis reached an all time high in the United States and Canada, 1,101 people and 2,552 horses contracted the disease in North Dakota with a mortality rate of 12.6 percent (139 deaths) and 21 percent (549 deaths), respectively. Practically all cases were the Western type enceph-alitis.

The danger of disease outbreaks such as encephalitis (sleeping sickness) in people and horses is always possible.

Life Cycle and Breeding Habits
The most abundant mosquitoes in North Dakota are the Aedes (Figure 9) temporary pool water breeders (also sometimes known as flood-water mosquitoes). They lay their eggs singly on damp soil near water. Like all mosquitoes, they pass through four life stages: egg, larva (four stages or instars), pupa and adult (Figure 10). In North Dakota, they overwinter in the egg stage. All mosquitoes live in water continuously from the time the eggs hatch through the larval (wiggler) and pupal (tumbler) stage until the adults emerge. Multiple generations are possible. They are found in shallow water with abundant vegetation above and/or on the water surface and where there is a fluctuation of water level and they are protected from wave action. Roadside ditches are common breeding sites. They do not live in running water or deep, open waters of lakes and ponds. Mosquito eggs, if not exposed to water, can survive for several years until they are flooded.

Other types of mosquitoes occurring in North Dakota are permanent water breeders. These permanent water types (Culex, Culiseta and Anopheles) lay their eggs on the water surface (Figure 9). Several generations are produced each summer. The adults overwinter in protected areas.

The adults emerge from pupal cases, their wings expand, and after a few hours the exoskeleton becomes hardened enough for flight. Because blood is necessary for egg development, the female then seeks a blood meal from human or animal. Adults often rest in weeds, tall grass or other vegetation but never reproduce there. After a few days the females return to suitable pools to deposit eggs and the cycle begins again. Depending on the amount of light and temperature, the cycle from egg to adult may take one to four weeks.

Adult mosquitoes are strong fliers. They can fly (or be blown) long distances from their breeding sites, although they usually go only far enough to find a blood meal.

Figure 9. Life cycle of permanent water breeders (Culex).
(larger version of Figure 9 -- 12KB)

 

Figure 10. Life cycle of temporary water breeders (Aedes).
(larger version of Figure 10 -- 12KB)

Mosquito Management
Mosquito reduction on an area-wide basis is a complex problem which should be based on established principles of good mosquito management. A number of techniques are available, depending on the target species involv-ed and the priorities which have been established. For example, the control of species implicated as disease vectors can be quite a different problem from that of species which are strictly nuisance biters.

An effective mosquito management program cannot be planned or implemented until a survey is made to locate the major breeding places of problem mosquitoes. Mosquito surveys take a great deal of time and work but are well worth the effort. Though mosquitoes usually require standing water for breeding, it is not true that mosquitoes will be produced in every body of standing water. A survey will identify breeding sites which must be eliminated or treated. This will avoid unnecessary intrusion upon areas which need not be treated, thereby preserving the environment. Since the most efficient management programs concentrate on control of mosquito larvae rather than adults, the survey is an essential prerequisite.

The following practices may be used to reduce mosquito breeding sites:

1. Ditch and clean stagnant streams to ensure a continuous flow of water to eliminate border vegetation which provides habitat for mosquito development.
2. Drain or fill back-water pools and swamps where stagnant water accumulates. Sanitary landfills, which can often be used in such locations, will eliminate mosquito breeding sites and improve the value of the land. Before considering establishing such landfills contact the North Dakota State Health Department.
3. Since all mosquitoes breed in shallow, quiet water, remove vegetation and debris from along the shores of lakes and ponds to discourage mosquito breeding. Such bodies of water should have a steep, clean shoreline with as little vegetation as possible. Approved weed killers may be used in some cases to eliminate or prevent emergent plant growth.

Chemical control is, at best, a temporary expedient which should be limited to situations which offer no other alternatives. In general, chemical control can be divided into two major operations. The first, control of larvae, is the most efficient and effective and should be the backbone of any good chemical program. The second, control of adults, is less efficient and should be used strictly for supplemental or emergency purposes. The detection of active transmission of a mosquito-borne disease is an example of such an emergency.

A number of insecticides are registered for mosquito control. The relative value of chemical control varies with the mosquito species and environmental conditions at the location where control is to be applied. Because each situation differs, care must be taken to select the proper insecticide for your particular situation. Some factors to consider include: effectiveness against target species (resistance problems); relative toxicity to humans and domestic animals (impact on non-target organisms); contamination of garden or fruit; cost; availability in quantities needed; need for residual action in some situations; chemical stability; flammability; ease of preparation; corrosiveness; and offensive odor, staining, etc..

Resistance can be a problem in mosquito control, especially when using some of the carbamate and organo-phosphate compounds. However, before assuming that resistance is the cause of poor control, it must be established that poor control is not caused by other factors such as improper identification of mosquitoes, spray techniques, lack of knowledge about insect habits, or faulty source reduction procedures. Any decrease in susceptibility should be substantiated in carefully controlled tests before changing either the toxicant or the application procedure.

You can reduce numbers of mosquitoes on horses by treating individual animals using sprays or wipe-on insec-ticides. In stables, sprays, fogs and insecticide impregnated strips provide useful methods of control.

Lice

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Extension Bulletin No. 55, June 1991