Timely Management Steps to Combat External Parasites

External parasites during the summer months can be a heavy burden on cattle and producers alike. Painful bites and risk of disease transmission are  common issues with  these nuisance pests. In cattle, culprits can include several fly species as well as ticks. Controlling these pests takes properly timed management.  This article will discuss the insects, their managementand control options.

The Culprits

It is estimated that horn flies alone may account for up to a billion dollars of damage and loss to the cattle industry each year. The horn fly is a blood feeding insect that inflicts a painful bite. Economic impact can be seen with as little as 200 flies per animal. This can be visualized by horn flies covering the withers and going about half way down the side of the animal. Infested animals are often seen switching their flanks, moving constantly, flicking tails, and even standing in water in attempts to escape the painful bites. All of these actions decrease grazing behavior and expend vital energy, increasing the stress on cattle. The economic impact hits producers in decreased average daily gain, and reduced weaning weight of calves. Horn flies spend the vast majority of their lives on the cattle themselves (usually on the backs and withers of cattle). The flies only leave the animal to lay eggs on fresh manure patties.

Face flies feed on the protein rich secretions from the eye and nose. Face flies are the primary vector for spreading the bacteria that causes the disease known as Pinkeye. Pinkeye in cattle is also a major economic concern to producers. Pinkeye accounts for 150 million dollars in estimated losses per year through decreased production and treatment costs. Although multiple factors play a role in pinkeye, face flies are generally a part of the spread of the disease. As with horn flies, face flies lay eggs on fresh manure patties. One main difference between the behavior of these two pests, is the face fly can travel several miles between animals, and spend less time on the animals themselves.

Other blood feeding insects such as the stable fly, horse fly and multiple species of ticks also commonly effect grazing cattle during the summer months. On top of painful bites and blood loss from heavy infestations, these pests can either be a mechanical or, in the case of ticks, a biologic vector of anaplasmosis. This disease infects cattle’s red blood cells and can cause death in mature cattle that have not been exposed to this pathogen before.


It is important to understand that combating these external parasites during the summer months takes a multimodal approach. There is a vast array of products on the market to help control these pests. Each product is designed to work in a specific way, against certain targets, for a specified amount of time. Expectations of a product to last from turn out to grass until the first frost, or to eliminate 100% of the pests is unrealistic and close to impossible. Producers should develop an integrated management plan to combat these pests.

Pasture management is vital to control external parasites such as flies and ticks. Brush and weed control in a pasture helps eliminate resting areas for ticks. Strategies such as burning and herbicide use can be vital to manage the risk. After a prescribed fire, cattle spend most of their time in the fresh lush growth where tick numbers would be very low. However, areas untouched by the fire may still harbor large numbers of ticks such as woody draws and waterways where brush, shrubs and ground litter are prevalent.

Stable flies tend to be more of a confinement or barnyard issue, but have increased occurrence in pasture settings. Typically, this is due to winter feeding sites and build-up of hay residue and manure. This mixture provides an ideal location for stable flies to flourish. Rolling out hay during the winter feeding months greatly reduces the number of stable fly larvae that survive the winter. Hay feeders tend to leave a large amount of residue on the ground into the spring. Cleaning feeding areas prior to late spring will reduce the habitat for stable fly larvae to develop.

Cattle Insecticides

There are a multitude of animal health products to help specifically control targeted external parasites in grazing cattle. They can include insecticide impregnated ear tags, pour-ons, sprays, oilers or dusters, injectables, and feed through products. Usually a combination of these products are utilized to provide coverage during the summer months. As with any animal health product, it is extremely important to read and follow all label directions foruse. It is also important to note that most of these products have a slaughter withdrawal time, so documentation of treatment dates is crucial. Common classes of insecticides include pyrethroids and organophosphates. Continued use of one class of product will promote resistance in the area fly population. So using one chemical class each season and rotating classes on a seasonal basis is critical to maintain the usefulness of these products. Timing is a critical component of insecticide use. Each of these products have an expected duration of efficacy. Producers often utilize these products early in the spring but the products start to lose potency and efficacy in the late summer months when we need them the most. Hold off treatment until fly levels on cattle hit the critical point (100-200 flies per animal). If additional treatment is indicated later in the season after a product has already been used, alternate the insecticide class when changing control methods. Work with your local veterinarian who understands your individual management strategies to tailor fit a plan.


Fly tags are excellent tools. To get the most benefit from them, wait until the middle of May or even June to put them in to ensure the tags still have effect later into the season when they are most needed. Many available tags may have an effective duration of 12-15 weeks. Use fly tags only  once per season. Remove these tags at the end of the season to avoid  continued exposure of these parasites to sub-therapeutic levels of the active chemical to reduce development of resistance .


These ready to use formulations are administered to the topline of cattle. Dose according to body weight. Common products have label claims against flies, lice and even ticks. These products contain  chemicals of similar classes to those in fly tags. Reapplication of these products may be necessary. These products last for varying amounts of time. Use the economic threshold of ~200 flies per animal as an indication for retreatment.

Dust bags/cattle rubs

We often rely on cattle to self-apply the products used on dusters or rubs in the pasture setting. To increase the effectiveness, fencing off and controlling entry points of commonly used areas can ensure more animals receive an application. These areas can be watering or mineral source areas. Read the label instructions carefully, these products may require specific carriers to work properly. These sites will need to be recharged with the proper product mixture at the recommended intervals during fly season.


Some products available come as a concentrate to be mixed with water before applying to cattle. These products can be useful for individual animals or groups of animals. Options for administration can range from high pressure, high volume spraying in a handling facility to low pressure, low volume hand held sprayers in the field.

Larvicide or Insect Growth Regulators (IGR)

These products are fed to cattle and are commonly included in certain mineral products. The products pass through the animal and have efficacy in the manure. They work by either destroying developing larvae or disrupting the normal development process. The process reduces the amount of new fly activity in a given area. However, flies do have the ability to travel over distances from neighboring operations, so overall elimination with these products is not practical.


Endectocides are the class of products whose main use is control of internal parasites (dewormers). The most common class are the Macrocyclic Lactones. Common ingredients in this class include the ivermectins. These products come in injectable and pour-on formulations. Even though their main action is against internal parasitism, their residual activity does provide coverage against external parasites for a period of time. Reliance and reapplication of these products during the grazing season is not recommended for fly control due to resistance issues with internal parasites. These products are often administered at the beginning of the grazing season and the activity against external parasites can be seen early in the season.

External parasites are annoying to livestock and costly to animal performance.  Timely management and appropriate use of insecticides will help minimize their impact.


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