Biology The horn fly was introduced into the United States during the 1800s and it spread rapidly across North America. Females lay eggs in fresh cattle manure, which is where immature forms complete development. During warm weather the life cycle of a horn fly is 10-20 days; there are many generations of flies each year. The first flies appear in June or earlier; flies disappear with hard frosts in fall. Both sexes suck blood from cattle and other animals. If enough flies are present, this can lead to reduced weight gains or milk production of cattle. However, cattle can tolerate a low density of horn flies without measurable losses. The generally agreed upon economic threshold is 200 horn flies per animal.
- abamectin ear tag (XP 820)-Apply to all animals in herd when flies become a problem in spring. One or two tags/animal. Remove tags at end of fly season or before slaughter.
- beta-cyfluthrin + piperonyl butoxide ear tag (Cylence Ultra)-One tag to each ear of all animals older than 3 months when flies appear. Remove tags at end of fly season or before slaughter.
- diazinon ear tag (Patriot, OPtimizer)-One tag to each ear of all animals older than 3 months. Remove before slaughter.
- diazinon + chlorpyrifos ear tag (Warrior)-One tag to each ear of all animals older than 3 months when flies appear. Remove before slaughter.
- diazinon + coumaphos ear tag (Corathon)-One tag to each ear on all mature animals when flies appear. One ear tag per calf. Remove tags at end of fly season or before slaughter.
- lambda-cyhalothrin ear tag (Saber Extra)-One tag to each ear on all animals when flies appear in spring. Remove tags in fall or before slaughter.
- lambda-cyhalothrin (6.8%) + pirimiphos-methyl (14%) ear tag (Double Barrel VP)-One tag to each ear on all animals in herd when flies appear in spring. Remove tag in fall.
- permethrin ear tag (GardStar plus)-One tag per ear on all animals when flies first appear in spring. Remove before slaughter.
- pirimiphos-methyl ear tag (Dominator)-One tag per ear on all animals when flies first appear in spring. Remove tags in fall.
- zeta-cypermethrin + piperonyl butoxide ear tag (PYthon MagnuM)-One tag per head of all mature and weaned animals when flies appear in spring. Remove tag at the end of fly season or before slaughter.
- coumaphos spray (Co-Ral Emulsifiable) at 2.5 fl oz per 4 gal water. Apply to run-off. Check label for application restrictions.
- cyfluthrin pour-on (CyLence)-See label for dosing instructions. Pour along top of back and head of the animal. Repeat as needed but no more than once every 3 weeks.
- ddvp concentrated spray (Vapona)-Dilute 1 gal in 50 gal water (1% solution) and apply up to 2 fl oz of solution per animal as a fine mist spray. Do not wet hide. Do not treat Brahman animals or calves under 6 months of age. One day withdrawal.
- eprinomectin pour-on (Eprinex, drug) at 1 ml/10 kg (22 lb) body weight. Apply along midline of back in a narrow strip extending from the withers to the tailhead. No slaughter interval, no required milk discard time.
- gamma-cyhalothrin pour-on (Standguard) at 10 ml (0.3 fl oz) if < 600 lb or 15 ml (0.5 fl oz) if > 600 lb to backline. Do not apply to face. Repeat as needed but no more than once every 2 weeks and no more than 4 times in a 6 month period.
- ivermectin pour-on (Ivomec, drug) at 1 ml/10 kg (22 lb) body weight. Do not slaughter within 48 days of treatment.
- lambda-cyhalothrin + piperonyl butoxide pour-on (Exile Ultra) at 10 ml (0.3 fl oz) if <600 lb or 15 ml (0.5 fl oz) if >600 lb to backline. Do not apply to face. Repeat as needed but no more than once every 2 weeks and no more than 4 times in a 6 month period.
- permethrin ready-to-use pour on (1%; Atroban) at 0.5 fl oz (0.005 fl oz or 0.15 ml ai) per 100 lb of body weight to a maximum of 5 fl oz (0.05 fl oz or 1.5 ml ai) per animal. Pour along backline and down face. Treat once every 2 weeks as needed.
- permethrin + piperonyl butoxide pour on (Ultra Boss) at 3 ml per 100 lb body weight. Maximum of 30 ml (1 fl oz) per animal. Treat once every 2 weeks as needed.
- permethrin spray (Gardstar 40% EC) at 30 to 118 ml (1 to 4 fl oz) per 25 gal water (0.013-0.05% ai). 1 to 2 quarts high pressure spray over body of animal. For low-pressure spray, apply at 118 ml (4 fl oz) to 2.5 gal water, spray till wet but not to runoff.
- phosmet spray (Prolate/Lintox-HD) at 1 gal per 200 gal water. Apply as high pressure spray wetting skin and hair but not to point of runoff. Do not slaughter within 3 days after treatment. Repeat as needed every 7 to 10 days. See label for other restrictions.
- pyrethrins (EverGreen EC 60-6) at 1 to 2 fl oz/gallon of water. Wet hair thoroughly. Repeat as needed. Also available as aerosol.
- tetrachlorvinphos + ddvp spray (Ravap)-Mix 1 gal in 75 gal water. Apply as coarse spray, 0.5 to 1 gal spray per animal. Do not treat Brahman cattle or calves under 6 months of age. Do not treat more often than once every 10 days.
- coumaphos dust (Co-Ral)-Suspend bags where cattle congregate. Place bags so they hang 4 to 6 inches below animal's backline. Direct applications of 2 oz per animal may be repeated no more frequent than 10 days apart and no more than 12 applications in 12 months.
- coumaphos (Co-Ral Fly & Tick Spray) at 9.75 fl oz per gal of No. 2 diesel or fuel oil. Place backrubber where animals congregate.
- permethrin backrubber or self-oiler (Gardstar 40% EC) at 118 ml/10 gal mineral oil or diesel oil. Keep rubbing device charged. Results improve with forced daily use.
- permethrin dust (several brands, 0.25%)-Suspend bags where cattle congregate. Place bags so they hang 4 to 6 inches below animal's backline. Direct applications of 2 oz per animal may be repeated as needed.
- phosmet backrubber (Prolate/Lintox HD)-Dilute 1 gal into 50 gal fuel oil or other suitable carrier. Charge backrubber device as needed. Do not slaughter within 3 days of treatment. See label for restrictions.
- tetrachlorvinphos + ddvp backrubber (Ravap)-Mix 1 gal in 25 gal water. Mix with approved oil and use in mechanical rubbing device or burlap or rope backrubbers. Charge as needed.
- zeta-cypermethrin + piperonyl butoxide dust (PYthon)-Apply in dust bag, shaker, dusting glove, or mechanical duster. Suspend bags where cattle congregate. Place bags so they hang 4 to 6 inches below animal's backline. Direct applications of 2 oz per animal may be repeated as needed but not more often than once every 3 days.
- diflubenzuron feed supplement (Clarifly 8%)-Prevents development of immature forms in manure of treated animals. Mix into grain according to label to provide 0.1 mg ai/kg (4.55 mg/100 lb) body weight/day. Start feeding early in spring before flies appear and discontinue when cold weather limits fly activity.
- diflubenzuron add-pack for milk (elim-A-fly Add-Pack with Clarifly)-Prevents development of immature forms in manure of treated calves. Mix daily into milk/milk replacer according to label to provide 0.1 mg ai/kg (4.55 mg/100 lb) body weight/day. Start feeding early in spring before flies appear and discontinue when cold weather limits fly activity.
- (S)-methoprene premix (Altosid IGR Custom 2%)-Use formulas on label to determine proper amounts to mix with feed or minerals. Begin use 30 days prior to fly season; continue feeding until cold weather restricts horn fly activity
- (S)-methoprene mineral tub (Mineralix fly tub with IGR)-Feed free choice. Place tubs out before flies appear and continue use until cold weather marks end of fly season.
- tetrachlorvinphos mineral block (Sweetlix Rabon)-Feed free choice. 1 block for 5 head of cattle. See label for details.
Management of Resistance to Pyrethroid Insecticides
Horn fly resistance to pyrethroid insecticides is in all cattle-raising areas of the United States. Strategies for combating the flies, while minimizing further buildup of resistance, need to be addressed. If you treat the herd with the proper dosage of a pyrethroid, but within days of using it see many more flies than before the treatment, then resistance is likely. This resistance may be to a spray, dust, or ear tag treatment. Remember that the following guidelines pertain to horn flies only. If resistance is suspected, do not use pyrethroids.
The following control strategies are recommended (may be used singly or in combination):
- Do not treat for horn flies. Cattle can tolerate moderate levels of horn flies (up to 200 per head), so no treatment at all is an option. This has the greatest effect on reducing or slowing insecticide resistance.
- Separate mature animals from calves. There is no evidence that horn flies affect mature cattle, other than reducing milk production in lactating cows. Calves should be treated to optimize weight gain. However, cows without calves and replacement heifers (animals for which efficient weight gain is not imperative) probably should not be treated. With cow-calf pairs, it is more effective to tag cows than calves. Calf weaning weights have been shown to be greater when cows are tagged because of higher milk production. Generally, horn flies aren't a problem on calves until the end of the season.
- Use a four-year rotation strategy to prolong the effectiveness of insecticide-impregnated ear tags. An example of a rotation follows: for the first year, use tags impregnated with abamectin; the second year, use endosulfan tags; the third year, use an organophosphate ear tag; and the fourth year, use a pyrethroid tag. With this plan, a different class of insecticide with a different mode of action is used each year.
- Delay control until flies exceed the treatment threshold. To avoid wasting insecticide and getting poor results, do not apply until horn flies build up in the spring or summer. However, show cattle or other special animals may require intensive treatment.
- Treat periodically with organophosphate sprays, dips, backrubbers, ear tags, oilers, or dusts to reduce early buildup of fly populations. Insecticide resistance can be delayed or reduced by periodic treatments that give high levels of immediate control, followed by a period of no control during which time the pest population again builds up. One to two treatments in early summer may delay the need for more sustained controls.
- Use feed-through fly control products that contain insect growth regulators instead of pyrethroid products. These products are fed to cattle during the months when flies are present. The risk of insecticide resistance development is very low for insect growth regulators such as s-methoprene and diflubenzuron.
- Treat late in the season. This should begin before the horn fly enters its overwintering phase. Any effort to reduce the number of flies that overwinter may hold down the initial density of flies the following spring. For late-season control, use an insecticide with a different mode of action than the one used during peak periods. If flies are not a problem late in the season, skip the late-season control.
- Remove ear tags in the fall; this eliminates the low insecticide pressure during the winter that could foster the development of resistance.