by Cassandra Olds, extension entomologist
With warming weather and cattle going out onto pasture soon, many of you may be considering your fly control options. Every year, stable and horn flies cause significant economic losses, but a good fly control program can minimize this impact. Although often grouped together, these are very different flies that need different control approaches.
The first step in developing a good fly control program is knowing who you have. Horn flies are smaller flies, found predominantly on the back and belly of the animal (Figure 1) while the larger stable fly is found on the legs, especially front pair (Figure 2).
Horn flies feed 20-30 times a day and stay associated with their chosen animal 24/7, with females only leaving briefly to lay eggs. Stable flies in contrast only feed once or twice a day, remaining on the host for a short period of time (3-5 minutes). When not feeding, flies are resting in shaded areas such as building sides and vegetation. This short feeding time means that producers often underestimate their stable fly burdens. While both flies impact pastured cattle, horn flies are not a problem in confined settings like dairies and feedlots. This is because horn flies need fresh, undisturbed manure as a breeding site while stable flies can develop in any decaying plant matter such as hay bales, feed bunk spill over and decaying grass.
With either fly species, using non-insecticidal control methods are essential for slowing insecticide resistance. For horn flies, pasture burning in spring kills any flies overwintering which can significantly reduce fly populations emerging as weather warms. A healthy dung beetle population will also significantly reduce your fly numbers, for free! Dung beetles are very susceptible to macrocyclic lactones so avoid using injectable and pour on avermectins (abamectin, eprinomectin, ivermectin etc.). Because horn flies die within hours of being removed from cattle, walk through traps such as the Bruce’s trap can be very effective if animals pass through it regularly (https://www.iowabeefcenter.org/bch/HornFlyTraps.pdf).
Round hay bales result in significant wastage, which when mixed into the manure contaminated mud around bales provides a prime breeding site for stable flies. Each round bale is able to produce 200,000 stable flies! Reducing hay waste and spreading/drying areas around finished bales is key to reducing stable fly numbers. In feedlots, minimizing feed spillage and waste is critical to remove breeding sites for stable and house flies. Parasitoid wasps are available from multiple sellers and should be released around fly breeding sites. These are very effective if released before fly populations emerge and released repeatedly though the fly season. Be careful using insecticides if using parasitoid wasps as they are very small and sensitive to these chemicals. Keeping vegetation surrounding pen areas short and exposed will remove sheltered resting areas, making life more difficult for the flies.
Chemical control options should be used as a supplement to the options above, and not the basis of your fly control program. For horn flies, insecticidal ear tags are an effective method of control if correct rotation is used. Rotate the chemical class of your tag annually, in year 1 using pyrethroid-based products, year 2 use organophosphate-based products and year three use macrocyclic lactone tags. Repeating this 3-year cycle will reduce the selection pressure on the fly populations, slowing down the spread of resistance. Some tag options are given below with their chemical class (Table 1).
Table 1: Insecticide ear tags options for a three-year rotation strategy.
| Organophosphate (Group 1B) | Pyrethroid (Group 3A) | Macrocyclic lactone (Group 6) |
| Corathon | CyLence Ultra | XP820 |
| Dominator | GuardStar Plus | TRIZAP |
| Max40 | PYthon II | |
| Optimizer | Python II MagnuM | |
| Patriot | Saber Extra |
For effective tagging, tag both ears and place the tag directly into the ear. For the tag to be effective, it must come into direct contact with the animal’s skin, which is greatly reduced when daisy chained. In Figure 3 you can see how little of the tag actually touches the body when attached to another tag.
Do not tag young calves. Mature bulls with thick necks may not benefit from tagging unless the tag can touch the skin. Although the box may label products as effective for 4-5 months, field trials have shown that tags only remain effective for 90-100 days. If possible, wait till fly populations are noticeable before tagging animals to get control over peak fly activity period. After 90 days, remove the tag to reduce the risk of insecticide resistance developing.
Pour-ons of the same chemical class as the ear tag may be used to increase coverage, however, be aware that a macrocyclic lactone pour-on will impact dung beetle populations. Make sure animals are dosed accurately according to weight and ensure head to tail coverage. Due to their low contact time with the host and preference for the legs, topical insecticidal treatments are generally not useful against stable flies. Spraying the legs may provide some relief although it should be used sparingly as most sprays are pyrethroid based, not allowing for effective annual rotation. Baits and premise sprays can be useful in controlling both house and stable flies, look for areas where flies are found resting such as building walls, fence posts and inside sheds and shelters.
Feed through Insect Growth Regulators (IGRs) can be used to control horn fly burdens if cattle consume the correct amount, which can be difficult under free choice conditions. Under dosing will result in resistance developing over time, reducing product efficacy. Although labeled for stable fly control also, when manure containing the IGR is diluted in the mud and hay, it is no longer effective. Often marketed as dung beetle safe, evaluations of these claims in most species have not been carried out and their true impact remains unknown. Insecticide resistance to IGRs can and does happen, to slow this, rotate annually between methoprene based (Group 7A) and diflubenzuron based products (group 15).
The Veterinary Entomology website (https://www.veterinaryentomology.org/vetpestx) provides a searchable database which can help you select the right products. You can select from type of animal, insect and application method. The Insecticide Resistance Action Committe (IRAC) codes are given for each product in addition to trade names. For on animal use, select the best product to allow an annual rotation between pyrethroid (Group 3A), organophosphate (Group 1B) and macrocyclic lactone (Group 6) groups.
Note: All references to commercial products or trade names are made with the understanding that no discrimination is intended and no endorsement by K-State Research & Extension is implied.
