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Horn Flies, Managing a Painful Pest
Flies are often associated with rot and disease, and for good reason, as many flies carry problematic bacteria for humans and other creatures. Many of them make their homes in rotting matter and dung, accentuating the image. Commonly a pest in nearly every situation they are found in, flies are certainly not our favorite insect. Amongst flies, the horn fly species is a major pest of cattle and livestock in the US, with some estimates hitting upwards of a billion to two billion dollars a year. So what exactly causes them to be such a problem, and how can we control their numbers in our fields?
What are Horn Flies Exactly?
The horn fly is scientifically known as Haematobia irritans irritants (Linnaeus). The average horn fly is 3 to 5 mm long. Five millimeters is often compared to a pencil eraser in size, but what they lack in size, they tend to make up for in numbers. Unlike its larger cousin the common house fly, horn flies’ have a mouth designed for piercing hides and sucking on the blood within. Both males and females feast on the blood of cattle, sometimes up to 30 times a day, with some scientists suggesting even higher numbers, like Dr. Norman in this article, who suggested an upper limit of around 40 blood meals a day! Each meal consists of around 10 µL, which is only .01 milliliters. This translates to .001 liters, or approximately .0042 cups, which is not that much. Even if the fly ingested 40 meals in one day, that would only equal out to about .4 milliliters in total for one fly. The problems begin when hundreds of flies begin attacking each cow.
Image from the USDA ARS Gallery taken by Scott Bauer
The horn flies will often live most of their life on their chosen host, only leaving to lay eggs in fresh manure, or to get out of the way of disturbances before settling back down again. Typically, they remain on the top, sides, and back of a cow, but will huddle on their belly when it gets exceptionally hot to avoid the heat. Horn fly females prefer to lay their eggs in fresh manure pats, typically within 10 minutes of it dropping. Once there, they will deposit their eggs and quickly return to their host to continue feeding. In under 24 hours, the eggs will hatch, and the new horn fly larvae will emerge. This larval stage is the first stage in horn fly development, which continues through three instars, or the developmental stage between a moult.
The horn flies will often live most of their life on their chosen host, only leaving to lay eggs in fresh manure, or to get out of the way of disturbances before settling back down again. Typically, they remain on the top, sides, and back of a cow, but will huddle on their belly when it gets exceptionally hot to avoid the heat. Horn fly females prefer to lay their eggs in fresh manure pats, typically within 10 minutes of it dropping. Once there, they will deposit their eggs and quickly return to their host to continue feeding. In under 24 hours, the eggs will hatch, and the new horn fly larvae will emerge. This larval stage is the first stage in horn fly development, which continues through three instars, or the developmental stage between a moult.
These instars only take 3-5 days before the larva is fully mature and begins its pupal stage. In another 3-5 days, new adult horn flies will emerge and seek a host. Females can begin laying new eggs, of which they can lay up to 200 in a batch, within 3-8 days of emerging from their pupae. This rapid life cycle allows them to reproduce extremely rapidly, which can make controlling their numbers difficult.
How do Horn Flies Affect my Cattle?
Horn flies have been shown to have several negative effects on cattle. To begin, the cattle expend a large amount of energy exhibiting defensive behaviours to try to rid themselves of their unwelcome riders. All the time and energy put into trying to deal with the flies can result in less feeding time and less weight gain. It can also result in a reduced milk production, lowering weaning weights.
Large numbers of horn fly bites can also lead to damaged hides, especially when lots of bites become infected. Flies are normally carriers of disease wherever they go, and for the cattle, it’s no exception. The flies have been associated with Mastitis or teat infection in cattle, due to their carrying and transmitting of Staphylococcus aureus as well other other problematic bacteria.
While a small infestation can affect grazing patterns and cause the cows to stop eating briefly, a massive infestation can cause severe disruptions. This article from the University of Minnesota mentions that cows may exhibit behaviours when under intense fly pressure, such as abandoning their food and bunching close together, trying to limit the number of places the fly can bite them. This bunching, though, can cause them to experience heat stress from being grouped so closely. The more stress and anxiety the cows face while dealing with the flies, the lower their production. Several hundred flies can cause problems, and there have been cases where the cows have been dealing with thousands of flies per cow.
Imagine going to get your blood drawn. The doctor pokes you with a needle once and takes a minuscule amount of blood. However, when you leave, they follow you home, and live in your house poking you with their needle taking more blood around 30 times each day, without warning. Now multiply that by two hundred doctors all poking you at different times through the day all day every day. I think I can understand why the cow might be getting stressed! Imagine 1000 different pokes thirty times a day, at random with no breaks in between! This is why it’s extremely important to try to keep horn fly numbers under control.
How Do I Control Horn Flies on Cattle?
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While not yet fully understood, this Journal of Integrated Pest Management article, in its section on pasture management, suggests that silvopasture systems may provide increased horn fly control. This could be due to the increased biodiversity in silvopastures, and the propensity to have more parasitic and predatory insects that work against horn flies. The addition of predatory birds that feast on the flies also helps to control their number, depending on how many birds make their home in the silvopasture. According to the research team, more investigation was needed, as this benefit was not noticed on all silvopastures studied.
While there are a large number of available control methods, most target one lifecycle stage or another and will not fully wipe out a horn fly population. Constructing a horn fly control strategy, and mixing several different methods is the most effective way of keeping horn fly numbers down. We have included additional reading and resources below to assist with your horn fly management strategy.