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Stable Flies

Stable Flies on Leg
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Stable Flies on Leg
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The two principal fly pests of confined livestock are house flies and stable flies. The stable fly, Stomoxys calcitrans, is about the size of a house fly but is dark gray. Its abdomen has seven rounded dark spots on the upper surface. The adult’s piercing mouthparts protrude spearlike from under the head. Stable flies breed in wet straw and manure, spilled feeds, silage, grass clippings, and in various other types of decaying vegetation. Each female fly lives about 20 to 30 days and lays 200 to 400 eggs during her lifetime. Under optimum conditions, an egg develops to an adult in about 3 weeks.

Cattle are most irritated by these pests during the warm summer months. Both male and female stable flies feed on blood several times each day, taking one or two drops at each meal. Stomping of feet is a good indication that stable flies are present, since they normally attack legs and bellies. Production performance declines in infested herds because of the flies’ painful biting activity and animal fatigue from trying to dislodge flies.


Stable flies are monitored by counting flies on all four legs of about 15 animals in the herd. Treatment is warranted when counts reach an average of 10 flies per animal.


Cultural Control

A variety of cultural control practices can be used effectively to manage house flies and stable flies.

  • Practice sanitation.

    The fly life cycle requires that immature flies (eggs, larvae, pupae) live in manure, moist hay, spilled silage, wet grain, etc., for 10 to 21 days. Removing and spreading fly breeding materials weekly helps to break the cycle. Waste management is therefore the first line of defense in developing an effective fly management program. It is much easier and less costly to prevent a heavy fly buildup than to attempt to control large fly populations once they have become established.

    The prime fly sources in confinement areas are animal pens, especially those housing calves. The pack of manure and bedding under livestock should be cleaned out at least once a week. In free-stall barns the next most important fly breeding area is the stalls, which should be properly drained and designed to encourage complete manure removal. In stanchion barns, drops should be cleaned out daily. Wet feed remaining in the ends of the mangers, as well as green chop and other forage and feed accumulations around silos, breed flies and should be cleaned out at least weekly.

  • Use sticky tapes, paper, ribbons

    Sticky ribbons, especially the giant ones, are very effective for managing small to moderate fly populations. Their only disadvantage is that they need to be changed every 1 to 2 weeks because they dry out, get coated with dust, or get "saturated" with flies.

  • Maintain a fly-free zone in the milk room

    Sometimes fly location is more important than total fly numbers on the farm. Installing and maintaining tightly closed screen doors and windows to the milk room can greatly reduce fly numbers in this sensitive area. Keeping traffic in and out of the milk room to a minimum will help as well. The occasional flies that still get in can be controlled with sticky tapes, light traps, or careful use of insecticides (discussed below under Chemical Control).

  • Prevent flies from emigrating from the facility

    Again, fly location can be important, especially if housing and commercial developments have been built near the farm. Certain management practices can reduce fly breeding outdoors.

    Spreading manure and bedding as thinly as possible will help ensure that it dries out quickly. If practical, it should be disked under as well to help kill fly larvae and pupae that may be present, especially if cool or overcast weather will slow the drying process. Drainage problems that allow manure to mix with mud and accumulate along fence lines in exercise yards should be eliminated. Gaps under feed bunks where moist feed can accumulate should be sealed.

Biological Control

Female flies lay their eggs on manure, calf bedding, wet feed, or silage. The larvae hatch, and the maggots develop for about a week before they reach the pupal stage. Inside the pupa, which is protected by a hard reddish-brown shell, the developing fly goes through the metamorphosis from maggot to fly.

Flies have "natural enemies" that are commonly present in livestock barns. Beetles and mites devour fly eggs and larvae, adult flies are prone to diseases, and fly pupae are attacked by small parasitic wasps. Unnoticed and unaided by us, these natural biocontrol agents can take a heavy toll on the fly population.

Parasitic wasps are among the most important of these natural biocontrol agents. About a dozen species occur throughout the United States. Some species perform better in different climates, and some prefer different kinds of manure and other fly breeding materials. The species best adapted to dairy farms in the Northeast is Muscidifurax raptor. This versatile species attacks fly pupae inside barns as well as outside, and it accounts for most of the naturally occurring wasps on our dairy farms.

Parasitic wasps are like "smart bombs" that live only to find and to kill fly pupae. Although the female wasp has a stinger, she cannot use it for anything except killing flies. When she finds a fly pupa, she first stings and feeds on it. This kills the fly. She then uses her stinger to lay an egg inside the pupa. The egg hatches and the parasite larva feeds on the dead fly. The young adult parasite then chews its way out of the fly’s pupal case and resumes the search for new pupae to kill. Development from egg to adult parasite is completed in about 3 weeks.

Evolution has led to a natural balance that allows both the parasite and the fly to coexist. If we think of the fly and the parasite as competitors in a race each summer, the fly has certain advantages that help it to "win" unless we intercede to level the playing field. For example, the fly develops twice as fast from egg to adult, lives longer, and lays more eggs than Muscidifurax raptor parasites. As fly populations begin to grow in late May and early June, the parasite populations lag behind. The result is that the parasite population is usually behind that of the fly by several weeks.

The parasite also lags behind the fly in developing resistance to insecticides. Many insecticide treatments for the fly therefore have the undesirable side effect of killing large numbers of parasites. If you use insecticides highly toxic to natural enemies in the early summer, you can get stuck on a "pesticide treadmill." Each subsequent insecticide treatment kills more beneficial insects and creates conditions that require repetitive treatments to keep flies in check. This also aggravates the problem of insecticide resistance in the flies.

Parasite populations can be conserved by using insecticides that are compatible with these important biocontrol agents. Methomyl scatter baits and pyrethrin space sprays are good examples of compatible insecticides. Residual premise sprays such as permethrin, dimethoate, and rabon are highly toxic to parasites and should be used only as a last resort for dealing with occasional fly outbreaks.

Parasite Releases

Along with conserving natural enemies, it is possible to go one step farther and make releases of parasites to "jump-start" their population growth in the early summer. Releases of parasites can be effective in managing fly populations if certain conditions are met:

  • Waste management is a must

    Parasite releases complement manure management but cannot replace it.

  • When insecticidal treatment is necessary for supplemental fly control, only those insecticides compatible with parasites (space sprays and baits) should be used.

  • Parasites are sent from suppliers as killed fly pupae containing immature parasites

    Local suppliers ship the parasites in cheesecloth bags. If most fly breeding on the farm occurs inside the barn, these bags should be stapled to posts and rafters near areas where fly breeding is a problem. If calves are housed in hutches, the bags should be opened and about 3 heaping teaspoons of pupae placed in each hutch weekly.

  • Many companies who sell parasites advertise their products in farm magazines, but not all of them sell the right species or provide parasites adapted for our northeastern climate

    Moreover, parasites from some commercial insectaries suffer from a debilitating disease that greatly reduces their effectiveness. Dairy farmers should look for Muscidifurax raptorand stay away from Nasonia vitripennis. Nasonia parasites are inexpensive but are inappropriate for use on our dairy farms. IPM Laboratories in Locke, NY (315-497-3129) is currently the only commercial insectary that produces and sells disease-free Muscidifurax parasites.

  • Releases should be started early, preferably in middle to late May, and continued weekly until the middle of August.

  • Weekly releases of either 200 parasites per milking cow or 1,000 parasites per calf have proven effective in research trials

    But every farm is different, and release rates and schedules may require adjustment to achieve a level both effective and affordable for an individual farm.

  • Prices vary, but they run at about $13.00 per batch of 10,000 parasites plus shipping charges

    At a release rate of 200 per cow (= 26 cents) per week, this means that total costs for the summer are between $2.60 and $4.70 per cow, depending on how long the releases are sustained.

    In research trials, parasite release costs have been more than offset by reductions in insecticide treatments. On average, dairy farmers who use biocontrol in fly IPM programs make 80 percent fewer insecticide treatments than farmers who rely on insecticides for fly control. In addition, fly populations on IPM farms are about 50 percent lower than on conventionally managed farms.

We are still at an early stage in understanding how to use biocontrol to full advantage in fly management programs. Please share your observations, successes, and disappointments so that we can all learn together. Call us (Don Rutz at 607-255-3251 or Charlie Pitts at 814-863-7789); or contact your local Cooperative Extension agent or regional specialist.

Chemical Control

Insecticides can play an important role in integrated fly management programs. Chemical control options include space sprays, baits, larvicides, residual premise sprays, and whole-animal sprays. Insecticides registered for fly control are listed in Table 1, enclosed in the pouch on the back cover.

Space sprays with synergized pyrethrins or a combination of dichlorvos and synergized pyrethrins provide a quick knockdown of adult flies in an enclosed air space. Because space sprays have very little residual activity, resistance to these insecticides is still relatively low in fly populations in the Northeast. Scatter baits containing the insecticide methomyl are also very useful for managing moderate fly populations. As indicated previously, space sprays and baits are compatible with fly parasites.

A number of insecticides are labeled for use as larvicides, either for direct treatment of manure or in controlled-release formulations. Direct application of insecticides to manure and bedding should be avoided in general, because of harmful effects on beneficial insects. The only exception is occasional spot treatment of breeding sites that are heavily infested with fly larvae but that cannot be cleaned out. Controlled-release larviciding options include boluses and feed additives that result in the insecticide’s being excreted with animal feces.

Treatment of building surfaces with residual sprays such as permethrin, dimethoate, naled, and rabon has been one of the most popular fly control strategies over the years. High levels of resistance to these insecticides are now very common. These materials should be used sparingly and only as a last resort to control fly outbreaks that cannot be managed with other techniques.

Whole-animal sprays can be made directly on the animals to manage stable fly problems. Although this approach can provide needed relief from biting fly pressure, the control is rather short-lived.

Fly Control in Milk Rooms

Milk rooms represent a special case. Because sanitary codes restrict insecticide use in milk rooms, the only chemical treatments recommended are space sprays with synergized pyrethrins (read label carefully regarding protection of milk and milk handling equipment) and placement of Vapona strips. Use of sticky ribbons and cultural practices that restrict fly entry into the milk room can greatly reduce the need for insecticidal treatment in the milk room.

Warning. Always read product labels carefully before applying any insecticide; mix and apply as directed, do not overdose, do not treat too often, and follow all precautions exactly. Remember that improper practices can lead to illegal residues even when correct materials are used. It is illegal to use an insecticide in any manner inconsistent with the label.