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How flies can affect the productivity of your dairy herd


by Dr Jennifer Lawrence, Technical Consultant, Dairy and Antibiotic Ambassador at Elanco Animal Health

Many people perceive flies as merely a nuisance and an irritation, but they can be a serious hazard to both humans and livestock. High fly numbers are worrisome to livestock and are carriers of important pathogens and diseases. This has negative consequences for production and profitability of dairy herds by impacting on the growth of calves and heifers and the milk production of lactating cows. Flies also pose a public health risk to the people working on the farm and to consumers.

Flies belong to the Diptera family of insects and the two most important species affecting livestock are the house fly (Musca domestica) and the stable fly or biting fly (Stomoxys calcitrans).

Flies are prolific pests and their lifecycle can be as short as one week in the summer months when the temperature and humidity are optimal for fly reproduction. Female flies have a life span of about two weeks to two months and can lay up to 120 eggs per week.

House flies can negatively impact herd productivity when their numbers increase in the hot summer months. As they are sucking flies, they cause significant irritation and discomfort to animals by feeding on their eye and nasal secretions and on small wounds. House flies also act as important mechanical carriers of many pathogens and diseases due to their feeding and reproductive habits and the structure of their legs and mouth parts. Some of the major diseases transmitted by flies include eye (pink eye) and enteric (Salmonella and Campylobacter) diseases as well as mastitis.

Stable flies are biting flies and feed on the blood of livestock. This feeding behaviour results in pain and irritation to the animals, especially when fly populations are high. Stable flies can also have a negative effect on the immune system of animals due to a salivary gland extract being released when the fly bites the animal, influencing T lymphocyte proliferation¹. An antibody response to the salivary gland proteins has also been noted in animals bitten by stable flies. The immunosuppressive effect of the salivary gland extract predisposes animals to an increased risk to vector borne (such as tick borne diseases), bacterial, viral and protozoal diseases.

Studies have shown that fly infestations can cause annual production losses of up to 139 kg of milk per cow, and 6 kg, 29 kg and 9 kg body weight loss in pre-weaned calves, grass fed young stock and feedlot cattle respectively². When fly populations are high, grazing time is reduced and increased defensive behaviour such as foot stomping, head shaking, body twitching and tail switching is seen.

This defensive behaviour leads to unnecessary expenditure of energy that would have been used for production whether it be growth or lactation. Lying down time is decreased due to irritation and animals tend to bunch together to try and get away from the flies. The bunching increases the risks of heat stress and injury, and the decreased lying down time negatively impacts on milk production.

It is very clear that flies can have a negative impact on the productivity and profitability of your farming enterprise, so it is extremely important to implement fly control measures that are effective, long lasting and safe for the animals, humans and the environment.

Note the flies feeding on the secretions from the eye and causing discomfort to the cow.


  1. Swist S.L., Wilkerson MJ., Wyatt CR., Broce AB., Kanost MR. Modulation of Bovine Lymphocyte Response by Salivary Gland Extracts of the Stable Fly, Stomoxys calcitrans (Diptera: Muscidae) Journal of Medical Entomo-logy 2002;39(6): 900-907
  2. Taylor DB., Moon RD., Mark DR. Economic impact of Stable Flies (Diptera: Muscidae) on dairy beef cattle production Journal of Medical Entomology 2012;49(1): 198-209

ELANCO ANIMAL HEALTH, a division of Eli Lilly (SA) (Pty) Ltd. (Co. Reg. No.: 1957/000371/07). Private Bag X119, BRYANSTON, 2021, Republic of South Africa. Tel.: (012) 657-6200 Fax: (012) 657-6216.ZADRYBIO00006

To find a safe solution please contact Dr Queen Sofe, Technical Consultant, Elanco Animal Health, at 071-4929-489.

This calf is showing obvious signs of discomfort and distress due to the large number of biting flies on its body. These flies also act as carriers of important pathogens and diseases.

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