By: Sonja L. Swiger and Jeffery K. Tomberlin
The most damaging insect pest for beef cattle in Texas is the horn fly (Fig. 1). Research shows that a calf infested with more than 200 horn flies will weigh 15 to 50 pounds less at weaning. Horn fly feeding on dairy cows can also reduce milk production up to 20 percent. To suppress horn flies effectively and economically:
- Identify them properly
- Understand the insect’s life cycle, and
- Use a combination of control strategies.
Identifying horn flies
Horn flies look like house flies and stable flies but are slightly smaller (Fig. 2). Like the stable fly, horn flies have piercing mouthparts.
To distinguish horn flies from stable flies, observe their feeding behavior. Horn flies rest on an animal between feedings; stable flies remain on the animal only while feeding. Also, horn flies feed most often on an animal’s back, shoulders, and sides, whereas stable flies feed principally on the legs.
Horn flies lay eggs in fresh manure pats, where they hatch as maggots. They develop from the egg to the adult stage within 10 to 20 days and live for about 3 weeks, feeding 20 to 30 times a day.
In Central Texas, horn flies are usually first observed in early spring. Populations tend to peak in early summer, then decline when the weather becomes hot and dry. In the fall, horn fly populations usually surge again when the temperatures drop and rainfall increases. Generally, they are no longer a problem after October or November, depending on when temperatures start to drop.
To suppress horn fly populations efficiently, use an integrated pest management (IPM) approach. IPM relies on multiple tactics including cultural, biological, and chemical methods to suppress insect pests.
Biological control: Parasitic wasps suppress horn fly populations naturally. Producers who want to use parasitic wasps to control horn flies can order fly pupae parasitized with the wasps from insectaries in Texas or across the United States.
The parasitized pupae are best used around barns where manure accumulations allow for the development of fly pests. However, research has not proven that releasing parasitic wasps suppresses horn flies or that the use of parasitized pupae reduces them in pasture situations.
Dung beetles and fire ants also suppress horn fly populations. Dung beetles compete for manure use and shrink the manure pats where horn fly larvae grow. Be careful when applying pesticides— moxidectin and, to a greater extent, avermectin kill dung beetles.
Fire ants feed on horn fly larvae and pupae, thus reducing the number of horn fly adults. Fire ant control is effective when horn fly populations are small, but as they increase in the summer, the impact is not as apparent.
A walk-through trap designed to collect horn fly adults from cattle can also lower populations.
Cultural methods: Remove and properly dispose of fresh manure from barns and stalls to interrupt the horn fly’s life cycle and help prevent new populations from developing.
Chemical control: Several chemical control methods can help reduce the number of horn flies on cattle: ear tags, sprayers and dusters, feed additives, and boluses. To prevent resistance, rotate chemical classes of insecticide products each year— and even within a year—if a mid-to-late season horn fly increase warrants further insecticide applications.
Ear tags (Table 1) are 2- to 3-inch plastic tags impregnated with an insecticide and attached to a cow’s ear. Several insecticides are formulated for use in ear tags and many brands are available. This large selection can make it difficult to decide which tag to use.
These guidelines will help you use ear tags effectively:
- Avoid tagging cattle until there are more than 200 horn flies per cow. This delay minimizes the chance for the flies to develop early-season resistance to the insecticide in the tag. If you do not tag cattle until the horn flies appear, the tags will remain effective late in the year when horn fly populations rise.
- Read the ear tag labels carefully to determine when to remove them from the animals, and do not use the tags beyond their recommended useful life. If left in longer, the flies are exposed to lower insecticide doses, which may increase chances for fly populations to develop resistance.
- Rotate classes of insecticides (not brand names of tags) every year. Most ear tags contain one of two classes of insecticide— pyrethroid and organophosphate. If you use the same class of insecticide 2 years in a row, horn flies can quickly become resistant.
- Do not use ear tags that contain both pyrethroids and organophosphates. These combination tags do not slow resistance development and may actually increase it.
Sprayers and dusters
Free-range cattle can be treated with small sprayers and dusters powered electrically from the back of a truck. Although sprays (Table 1) can also be applied during seasonal roundups, they usually do not suppress horn flies for long.
Confined and free-range cattle can be given feed additives that suppress horn flies. These products contain insecticides that pass through the animal’s digestive system and remain in the manure where horn fly maggots develop. But, the herd will still be susceptible to biting flies moving from neighboring properties.
One disadvantage of feed additives is that it is difficult to regulate each cow’s feed intake. Some animals might take in the proper amount; others might receive too little.
Another method for controlling horn flies is boluses, which look like large pills. Boluses are retained in the cow’s reticulum (second stomach) and dissolve slowly, releasing the insecticide into the digestive tract. Unlike feed additives, boluses release the insecticide continuously in the manure and kill immature horn flies. However, the herd will still be susceptible to biting flies from neighboring properties.
Regardless of the method you choose, follow these guidelines:
- Do not treat infestations of less than 200 flies per cow. Treating when horn fly populations are below this level is not cost-effective, and the unnecessary use of insecticides can speed the development of resistant fly populations.
- Read the treatment label to ensure that it is suitable for use on beef or lactating dairy cows.
- Be careful when applying insecticides and wear protective clothing as recommended on the product label.
Ed Bynum, Allen Knutson, and Chris Sansone, Extension entomologists with the Texas AgriLife Extension Service, and Ron Swiger reviewed this manuscript.
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