Stable fly

Stable fly is closely associated with human activity and can be a serious pest of livestock around animal enclosures, stables, feedlots and paddocks or pastures.

It is sometimes known as biting fly and is a problem for cattle, horses, goats, dogs and humans because of its painful bite as it draws blood during feeding.

Cattle and horses are most affected. Animals will try to avoid the fly by stamping their feet, tail switching, throwing their heads down toward their front legs, and kicking sand up onto their legs and body.

Horses are seen to stamp when stable flies bite their legs.

This can lead to reduced weight gain from continual movement and allergic reactions on their skin from the flies' feeding. When stable flies are present in large numbers (more than 20 to 30 per animal), cattle will often bunch together to get to the centre of the group and avoid the fly, or they may stand in open water to avoid being bitten.

This continual agitation reduces the animal’s normal grazing time, and they may move to feeding at night when the fly is not active. Bunching together by cattle is particularly hazardous in summer, when animals can be at risk of heat stroke.

Stable fly numbers can be monitored by counting the flies on all four legs of about 10 animals. When the average number is more than 25 flies per animal (treatment threshold), action is needed. At more than 25 flies per animal, measurable reductions in weight gain and condition occur and numbers above 50 flies per animal can reduce weight gain by 25% and milk production by 40 to 60%.

• Slightly smaller than a house fly and slightly larger than a bushfly
• Has a checkerboard of dark spots on the back of the abdomen
• Has a prominent black proboscis that is used to pierce the skin and draw blood.

The life cycle from egg to adult is about 13 to 18 days in temperatures ranging from 24°C to 30°C. At lower temperatures (10 to 20°C), such as those likely to be experienced over winter, development can take 3 to 5 months.

After ingesting a blood meal, the female fly lays around 90 eggs in 4 or 5 suitable locations, such as rotting vegetable matter or animal manure. She may lay up to 600 eggs over a lifetime and in warmer areas may breed year-round.

In summer, the eggs hatch in as little as 20 hours and the active larval stages, which are extremely heat-tolerant, begin feeding for up to a week before pupating. The red-brown to black pupae are the size of a grain of rice, and can take as little as 5 days to emerge at high temperatures, or up to 30 days at lower temperatures.

After emergence, the adult flies disperse, sometimes covering tens of kilometres in search of a host animal. Both male and female adult flies feed on blood, as soon as 6 hours after emergence.

The adult flies live for about 3 to 4 weeks, with extreme heat (over 35°C) reducing their lifespan to just 2 weeks.

The flies mostly feed in the morning and again in the late afternoon when they can extract 5 times as much blood as a mosquito with each meal.

The fly will make several attempts to feed, adding to the distress of its host. Once settled on an animal, it takes 2 to 5 minutes to complete the blood meal, after which it seeks a shady place to digest it.

This adaptable fly will breed in any rotting or decaying organic matter where there is a high degree of bacterial activity. Key breeding sites include:

• ageing manure mixed with organic material, such as straw
• rotting vegetable crop residues left after harvest, including reject produce
• straw bedding mixed with urine and faeces
• reject vegetables fed to livestock in large piles
• rotting hay, straw, or sawdust, fermenting feed, and piles of grass clippings.

The key to stable fly control is managing its larval habitats, ideally by their removal or drying out so they are less attractive.

Treating livestock or pets with insecticides and repellents can aid in control, however the impact is often short-lived (hours to days).

If treating livestock with insecticide, application to the lower legs and underbelly of the animals is critical. Using commercial or home-made fly traps can help to reduce fly numbers in local situations.

Eliminating breeding sites

There are several management practices that will remove breeding sites: 

• Mulching and burying vegetable crop residues and reject produce soon after harvest, then compacting the soil above the residues using a land roller.
• If feeding waste vegetables to livestock, do so in long thin lines or in a trough, or box to prevent mixing with the soil. 
• Regular removal of accumulations of spilled grain feed or other organic material in pens and yards.
• Regular (weekly) removal of animal manure and soiled straw accumulating in pens and yards, around water troughs and under fences, and gates.
• Animal manures should only be stockpiled for a short period (less than 3 days) before covering with plastic to protect them from getting wet.
• Covering animal feed and bedding waste with a fly-proof barrier and keeping it completely dry with a tarp or plastic sheet.
• Spreading animal manure and grass clippings into thin layers on the ground to dry out.
• Manure can also be used as a blend for compost or sprayed with insecticide to prevent fly development.

The less effective method of slashing the residue, leaving it on the surface, spraying with pesticide, and turning water off is no longer recommended.

Protein-based traps put out to catch houseflies, blowflies, and bushflies will not catch stable flies because they are not attracted to rotting protein.

White boards with a sticky surface will attract them, as they are likely to rest on a cool, vertical surface after a blood meal.

The Williams trap is the simplest form and uses a white alsynite panel painted with a non-drying glue, such as Stikem®, to catch the flies. These traps are specific to stable flies and catch very little else.

How to make your own trap

To make your own trap, heat the non-drying glue on a hot plate or over hot water so it thins out and can be used more easily. Cover both sides of the alsynite board with the glue. Secure the white board to a star picket about 1 m from the ground to avoid it being covered in dirt and dust.

One litre will suffice for up to 15 traps. When the white board is covered with flies, remove the glue with a paint scraper before re-applying.

Visit your local hardware store to enquire about alsynite panels and non-drying glues.

Chemical methods

Insecticides and repellents can be used to keep stable flies away from livestock, animal yards, and places where the fly rests. Numerous products on the market range from residual sprays to animal backline pour-ons and sprays, to insecticide-impregnated ear tags.

The relative effectiveness of these products has not been tested but it is thought the flies quickly overcome any initial repellent action. The best option may be to use 3 or 4 different repellent sprays, in regular rotation.

Residual sprays should be applied where the flies rest (for example, shady surfaces, fences, walls) so the insecticide residue can be absorbed, thus killing them. These are effective for about one or 2 weeks, but rain, high temperatures, and sunlight reduce the residual effect.

For current information on chemical products, refer to the Australian Pesticides and Veterinary Medicines Authority (APVMA).

Australian Pesticides and Veterinary Medicines Authority (apvma.gov.au) 

The Biosecurity and Agriculture Management (Stable Fly) Management Plan 2019 was published in the Western Australian Government Gazette No. 127 (3 September 2019) and replaced the 2016 Management Plan. This current plan outlines the measures to be taken to control stable fly in the gazetted local government areas.

The current management plan is due to be replaced in 2024 with an updated plan.

Western Australian Government Gazette No. 127 (3 September 2019)