Cows, Horses and Flies

Written by Justin Reynolds

Flies on the eye of a horse. Photo by and (c)2007 Jina Lee. Wikimedia Commons.

Flies thrive where cows and horses reside. This page focuses on the various conditions of cow and horse habitats that promote fly breeding and development. There have been several measures suggested and implemented to reduce fly infestation in barns, pastures, stables, and corrals to improve cleanliness and reduce the spread of diseases. However, flies will continue thrive where manure, dust, food, and other conditions remain accessible to reproductive flies.

Cow and horse manure serve as desirable breeding grounds for house flies. In the early summer, female flies begin laying hundreds of eggs on manure in multiple settings. These eggs quickly, in a matter of hours, begin their life cycle. This process is catalyzed by the presence of heat and moisture (Bermondsey 1914). During summer months, specifically July, August and September, there is a great increase in the number of flies. The warm weather during these months stimulates rapid development of flies. Full development, from egg to mature fly, can occur in less than two weeks during the summer months (Willesden 1920). Different species of flies tend to prefer fecal matter from different animal species, but house flies, the most common type of fly, show preference for horse manure. When compared to areas with horse and cow manure, house flies breed more successfully in areas where only horse manure is present (London County Council 1907). The moisture, temperature and organic content of cow and horse manure provide perfect conditions for fly eggs to rapidly develop into mature flies.

As previously mentioned, house flies tend to prefer to lay their eggs in the manure of horses. A different species of fly, the horn fly, prefers the manure of other animals. A study was performed to compare horn fly development in manure of various livestock animals. It was found that horn flies will fully develop in the manure of horses, sheep, bison, and cattle, but not pigs. The results further showed that, contrary to house flies, horn flies show preference for cow manure to a greater degree than horse manure. It was assumed that behavioral factors prevent female horn flies from laying eggs in horse manure (Greer and Butler). So different fly species show specific preference for the manure in which they lay their eggs, which could make prevention efforts difficult when stables or pastures host a variety of animals.

There have been various efforts by humans to remove manure due to concerns of flies. Before the 1900s, sparse attention and regulations were directed towards the sanitation of barns and stables hosting livestock. Over time, increasing numbers of flies, because of livestock, forced people to act and control the issue. Once understood that a positively correlated relationship exists between manure prevalence and fly breeding, a campaign was directed against barns that did not contain a manure receptacle and barns that were unsuitable for the community or of defective condition (Kingston upon Thames 1927). Because flies were thought to spread a variety of pathogens, significant attention was given to manure removal programs, especially in urban areas (Poplar 1927). In the summer months, there were notices recommending stable owners to have all manure removed from stables at least twice a week (East Barnet Valley 1919). Unfortunately, this was not always effective because the practice of manure removal is costly and tedious. Another prevention measure targets insecticide application to horses, cows, and barns. However, long term use of insecticide can lead to insecticide resistance. Moreover, insecticides have only shown to be moderately effective, and it is even less effective for pasture animals (Tangtrakulwanich).

Flies have shown to mingle in areas with a significant amount of dust (Bethnal Green 1905). Controlling dust in human habitats is a difficult task, but limiting dust in a cow and horse habitat is nearly impossible. Trash bins host a great amount of dust, and more so when it is not taken out consistently, which is often the case at cow and horse barns. Trash covers are frequently used at barns to inhibit flies from gathering in and around dusty trash bins. It is common for manure to end up in trash cans at barns, and, as previously mentioned, manure is a perfect breeding ground for flies. In this case, it is necessary to take out the trash every day (Hampstead 1914).

Closeup of Angx.Closeup of Angus ste
Closeup of Angus steer with flies on his head. USDA Photo by: Larry Rana. Wikimedia Commons.

Flies are also attracted to several different food products, like milk and sugar. Cows are the number one milk producers, and environments in which milk is exposed create an opportunity for flies to obtain food. Cow-keepers must ensure their work space is kept tidy and the milk is protected in order to prevent flies (Sutton 1910). Also, when cows or horses are nursing their young, they secrete milk. Some of the milk may not make it to their young, and this excess milk, which will often remain standing for long periods of time, can attract flies. Vegetable substances and decaying matter are also enticing to flies, as it can serve as another suitable ground for flies to lay their eggs (Hackney 1912). Hay and horse feed, which contain vegetable substance, are used constantly in stables and pastures for feeding. Feed rooms are often infested with flies. It is common for small rodents and other insects to scavenge throughout stables and pastures. Upon the death of these rodents and insects, flies take advantage of their decaying bodies to lay eggs.

The habitats of cows and horses provide suitable environments for flies to prosper, while variousĀ other environments also offer fitting conditions. Manure serves as an ideal breeding ground for flies to lay their eggs. The summer months bring warm weather which promotes development of fly larvae. As there are various species of flies, each species tends to show preference for manure of specific livestock animals. Human interaction with cows and horses in urban areas demands the need for manure removal programs because flies can spread disease. There have many efforts to control the prevalence of flies, but the efforts have been relatively unsuccessful.


Greer, N. I., and J. F. Butler. “Comparisons of Horn Fly Development in Manure of Five Animal Species.” The Florida Entomologist 56, no. 3 (1973): 197-99.

Tangtrakulwanich, Khanobporn. “Sensory Morphology and Chemical Ecology of the Stable Fly, Stomoxys Calcitrans: Host-Seeking and Ovipositional Selection.” Order No. 3504210, The University of Nebraska – Lincoln, 2012.