Flies are a major cause of eye infections in equines

Flies are not just a nuisance – they are a major cause of disease, suffering and economic hardship around the world. It is estimated that flies may be involved in the transmission of more than 65 diseases to humans alone, including typhoid fever, dysentery, cholera, leprosy and tuberculosis. They are also responsible for significant reductions in the production of meat and dairy products, as well as reproductive failure and eye infections in equines.

Intensive livestock production often provides an ideal breeding environment for flies, making control a major challenge. In the past, many relied on the application of a single broad spectrum adulticide to control flies. However, there have been major advances in the understanding of this pest in recent years, and this is no longer considered to be the most efficient approach.

The modern approach to controlling flies is through the use of an integrated pest management programme. In this approach to fly control, use of insecticides are minimised and management is emphasised, specifically, reducing fly-breeding materials, improving general sanitation, using traps to kill the adult flies, and using parasitic wasps to kill the immature flies.

According to Dr Bennie van der Merwe of Biofly, the four major aspects of an integrated pest management for flies are as follows:

A.Reduction of fly breeding sources (the most important part of fly control)

• Remove all soiled bedding, spilled feed, manure, lawn clippings and garbage once or twice a week. Then these should be stacked compactly, spread thinly on fields, buried, burned, or put into fly-tight pits or boxes
• If manure or haylage is stacked, it must be packed compactly with steep sloping sides so moisture does not penetrate the sides. The areas around the base of the sides must also drain to prevent moisture accumulation
• Fill potholes to prevent water accu­mulation
• Slope drainage areas at 3% for rapid disposal of water
• If at all possible, straw should not be used for animal bedding because it is one of the best fly-breeding materials
• Coarse sawdust should be used for bedding
• Piles of straw should be spread out to dry, be burned or plowed under after harvest
• Inspect round bales or stacks of hay which are placed in pastures for ad libitum feeding and remove any loose hay accumulated at the base on a weekly basis – this is where biting flies breed
• Buildings equipped with pits for collecting manure, have little problem with stable flies, but may breed house flies. It is suggested that the level of waste should be kept low enough to prevent crusting.

B. Use of outdoor and indoor traps

The use of traps can help reduce adult fly population, if a sufficient number of traps are placed in the correct locations and kept functional. Traps that rely on the flies walking upwards, are best.

C. Parasitic wasps (also called parasitoids)

• These wasps kill the immature stages of the flies and parasitise the fly pupae for their own reproduction. They are harmless to humans and livestock
• Wasps should not be the sole control method because they are not fast acting or 100% effective
• Several species of parasitic wasps are available from commercial sources
• The species of parasitic wasps should be suited for the locality and released at the recommended places and times
• Do not use insecticides, especially synthetic pyrethroids, in areas where wasps are released.

D. Chemical insecticides

• The number of flies may increase rapidly and it may be necessary to use a chemical insecticide to quickly reduce the population
• If flies are a problem indoors, often all that is needed is a space spray or fog with pyrethrin-based insecticide. This insecticide has no residual action and must be repeated whenever flies reach an unacceptable level.

Fly abundance

According to guidelines by Novartis Animal Health fly numbers are determined by abiotic factors (environmental factors such as temperature, moisture of breeding habitat and humidity) and biotic factors (natural enemies including parasites, predators and pathogens).

Theoretically, a house fly population would grow to astronomical numbers if there were no factors limiting production. However, the abiotic and biotic factors prevent this from happening. Fly populations can reach unacceptably high levels in and around confined animal production facilities when the livestock management system includes practices which cancel out the abiotic and biotic factors. The climate of an area provides an overall indicator of the potential level that fly populations may reach. However, facilities for confined-animal production greatly alter the environment.

In nature, predators have a significant role in regulating fly population levels. However, the concentration of animals and manure in confined-animal production facilities, once again creates an unnatural situation.

The overall temperature range and prevailing summer temperatures in an area are crude indicators of how fast flies will develop and how many generations per year there will be.

Because flies develop rapidly and generations overlap, large fly populations develop quickly and are sustained as long as temperatures are high. However, confined animal housing alters the situation substantially, because it creates an artificially protected environment.

For this reason, even more flies and generations per year are likely to occur in and around confined-animal housing than the climatic zones would suggest. The temperature maintained in the housing system will determine the fly population level more than the climate of the area.

The micro-climate of the breeding medium is also of major importance in determining the rate of fly development and fly abundance. Accumulated manure and other fly-breeding media (silage, hay, feed) generate considerable heat as they ferment.

The temperature factor

The temperature in the breeding medium is usually higher than the surrounding air (whether indoors or outdoors). The greater the accumulation, the more the heating, providing fly larvae with greater opportunities to find a suitable habitat for development.

During the winter, slow development of fly larvae and long-time survival of pupae routinely occur indoors in confined-animal facilities. The same occurs outdoors in piles of manure, silage and hay.

In the summer, the rate of fly development in accumulated manure or other breeding media is often faster than expected from weather data. Confined-animal housing systems and a concentration of manure and feeds make for an artificial environment, protected from the extremes of weather and conducive to fly breeding.

Since the confined-animal housing system is an artificial creation, it can be manipulated to either encourage or discourage fly breeding. Covering and protecting feeds (silage, hay bales, grain) from moisture, and minimising fermentation provides a less desirable habitat for fly breeding. Management practices in this regard are major factors in determining fly abundance.

It is acceptable to do spot treatment in stables, provided that the entire stable is cleaned out once a week. In such a system, a small amount of parasitic wasps should be released near the stables so that the manure to be removed contains its wasps. Only surface applications of contact insecticide on walls can then be used.

The stable contents must be removed entirely to a distant location (>2 km), ploughed in (>30 cm) or treated with the correct number of parasitic wasps. Placement of traps en route from the potential breeding areas to the stables, helps identify the breeding sites and alleviates the problems before they reach the horses.

Many yard owners swear by their chickens. They are the ultimate in biological control, they maintain. The chickens patrol breeding areas and keep these free of fly larvae. Guinea fowl fulfil the same function, and according to those with experience, even better.