5 pp., 1 photograph, 1 tableHorn flies are the most damaging insect to cattle in Texas. This publication explains biological, cultural and chemical methods of controlling horn flies. Various insecticides used to suppress horn flies are liste

Tomberlin, Jeffery K.

Texas A&M University

E-208
5/04
For cattle in Texas, the most damaging insect pest is
the horn fly (Fig. 1). Research has shown that a calf
infested with more than 200 horn flies will gain 15 to
50 pounds less than normal from birth to weaning and
sale (about 4 to 6 months). Horn flies can also reduce
milk production in dairy cows by up to 20 percent.
To control horn flies effectively and economically,
it is helpful to know how to distinguish them from
other flies, what control methods to use for different
stages of their life cycle, when to take action and how
to reduce the pest’s resistance to chemicals.
Identifying horn flies
Horn flies look like houseflies and stable flies, but
horn flies are slightly smaller. However, they do have
piercing mouthparts like that of the stable fly.
To distinguish horn flies from stable flies, observe
their feeding behavior. Horn flies rest on a cow
between feedings; stable flies remain on the cow 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.
Life cycle
Horn fly eggs are laid in cow manure pats, where
they hatch as maggots. Horn flies develop from the egg
to adult stage within 10 to 20 days, and the adults 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
September or October, depending on the weather.
Control methods
Three types of control methods are used to sup-
press horn flies: biological, cultural and chemical.
Producers who use all three methods (that is, they
employ integrated pest management practices) will
reduce horn fly populations the most while incurring
the least expense. 
Assistant Professor and Extension Entomology Specialist,
The Texas A&M University System
Figure 1. Horn flies resting on the back of a cow.
Protecting Cattle
from Horn Flies
Jeffery K. Tomberlin*
Biological control: Parasitic wasps and fire ants
suppress horn fly populations naturally. Producers
wanting to use parasitic wasps for controlling horn flies
can order fly pupae parasitized with the wasps from
insectaries in Texas or across the United States.
However, to date, no research results have been pub-
lished showing that releasing parasitic wasps suppress-
es horn flies.
The parasitized pupae can be spread around barns
near where the pest flies are developing. These pupae
should be placed in areas where they will not be
stepped on and where they will be out of direct sun-
light.
Cultural methods: Removing and properly dispos-
ing of fresh manure from barns and stalls interrupts
the horn fly’s life cycle and helps prevent new popula-
tions from developing.
Chemical control: Several chemical control meth-
ods can help reduce the number of horn flies on cattle:
ear tags, sprayers and dusters, feed additives and bolus-
es.
Ear tags
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 of tags are available. This
large selection can make it difficult to decide which tag
to use and when.
Following these guidelines will help you use ear
tags effectively:
➢ Avoid tagging cattle until there are more than 100
horn flies per cow. This will minimize the chance
for the flies to develop early-season resistance to
the insecticide in the tag. If you do not tag cattle
until March or later, the tags will still be effective
late in the year, when horn fly numbers are high.
➢ Read the ear tag labels carefully to determine when
to remove them from the animals. Do not use the
tags beyond their recommended useful life – if left
in longer,  the flies will be exposed to lower doses
of the insecticide and may increase the chances of
fly populations developing resistance.
➢ Rotate classes of insecticides (not brand names of
tags) every year. Most ear tags contain one of two
classes of insecticide – pyrethroid and organophos-
phate. 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 increase
this problem.
➢ After tagging cattle, add a supplemental treatment
– such as pour-on, back rub, etc. – at mid-season
(July or August).
Sprayers and dusters
Free-range cattle can be treated with small sprayers
(Table 1) and dusters powered electrically from the
back of a truck. Sprays can also be applied during sea-
sonal roundups. However, they usually do not suppress
horn flies for long.
Feed additives
Both free-range and confined cattle can be given
feed additives that suppress horn flies. These products
contain insecticides that pass through the digestive sys-
tem of the animal and exit in the manure where horn
fly maggots develop. Feed additives can kill many fly
larvae.
A disadvantage of using feed additives is the diffi-
culty of regulating each cow’s feed intake. Some ani-
mals might take in the proper amount; others might
receive too little. 
Boluses
Another method for controlling horn flies is bolus-
es, which are large pills. When given to a cow, boluses
are retained in the reticulum (second stomach) and dis-
solve slowly, releasing the insecticide into the digestive
tract. Unlike feed additives, boluses release the insecti-
cide continuously.
Summary
Regardless of the method chosen, follow these
guidelines:
➢ Do not treat infestations of less than 200 flies per
cow. Treating when horn fly populations are below
this level will not provide an economic return, and
the unnecessary use of insecticides can speed the
development of resistant fly populations.
➢ Read the label of the treatment to make sure it is
suitable for use on beef or lactating dairy cows.
➢ Be careful when applying insecticides, and wear
protective clothing.
Acknowledgments
The author would like to thank Mike Merchant and
Allen Knutson, Professors and Extension Entomo-
logists, the Texas A&M University System, and Kim
Lohmeyer, USDA-ARS, Kerrville, TX, for their review
of this manuscript.
Table 1. Various insecticides used to suppress horn flies on cattle. Please read the label to determine if the product is
approved for use on lactating animals.
Insecticide and dosage Method Comments
Pour-on sprays
GardStar 40% 4 fl oz/50 gal water 1 qt coarse spray per animal.
High pressure spray See label for other spray considerations.
GardStar 40% 4 fl oz/25 gal water Follow label directions.
(High-pressure spray) or
4 fl oz/2.5 gal water
(Low-pressure spray)
Rabon + Vapona (Ravap) Use 0.5 to 1 gal spray per Treatment interval no less than 10 days.
23% + 5.3% = animal. No waiting period.
2 qt/100 gal water
Permethrin: (Pyrethroids) Spray to thoroughly Can reapply in 14 days if needed.
Ectiban 5.7% EC wet animal. No waiting period.
1 qt/25-50 gal water
Insectaban 5.7% EC
1 qt/25-50 gal water
Atroban 11% EC
1 pt/50 gal water
Permectrin 10% EC Re-treat in 14-21 days.
1 qt/200 gal water
Permectrin 25% WP
1 lb/50 gal water
Synergized DeLice 1% Ready-to-use pour-on Pour along back. Do not treat more
+ 1% PBO Maximum 5 oz/animal often than every 14 days.
0.5 fl oz/100 lb body PBO = piperonyl butoxide, a material
weight that increases the effectiveness of some
pyrethrin and pyrethroid insecticides.
Follow label directions.
Brute 10% Ready-to-use pour-on Read the label carefully for proper rates
based on the weight of the animal.
Cyfluthrin 1% Ready-to-use pour-on Read the label for proper rate based on
(Cylence) weight of the animal.
Lambda-cyhalothrin Ready-to-use pour-on
Ear tags
Organophosphate tags: Read label. Follow label instructions.
Diazinon 18-21.4%
Terminator
Optimizer
Optimizer calf
Diazinon 18% + 2% PBO
Z-Diazinon
Boyagard
Diazinon 40%
Patriot
Cutter 1
Diazinon 30% + Chlorpyrifos 10%
Warrior
Insecticide and dosage Method Comments
Ear tags (continued)
Organophosphate tags:
Pirimiphos-methyl 20%
Dominator
Fenthion 20% + PBO 15% PBO = piperonyl butoxide, a material
Cutter Blue that increases the effectiveness of some
Ethion 36%
Commando
Pyrethroid tags:
Permethrin 10%
Ear Force
GardStar Plus
Atroban, Deckem, Expar,
GardStar,
Z-Permethrin
Fenvalerate 8.6%
Ectrin
Cyfluthrin 10%
Cutter Gold
Lambda-cyhalothrin 10%
+ PBO 13%
Saber Extra
Zetacypermethrin 10%
+ PBO 20%
Python Magnum
Zeta Gard
Python
Combination ear tags with
pyrethroids:
Diazinon 30% + chlorpyoifos 10%  
Warrior               
Cypermethrin 7% +
Chlorpyrifos 5% + PBO 3.5%
Max Con
Lambda-cyhalothrin 6.8%
+
Pirimiphos methyl 14.0%
Double Barrel  
Backrubber applications
Mix the insecticide formulations listed below as directed on the label for use in homemade or commercial backrubbers. 
Backrubbers are most effective when they are placed in a forced-use situation, such as gateways, doors or alleyways. Install
them so that all animals use them once a day. If backrubbers cannot be installed in a forced-use situation, place them in
areas where animals often loaf. To be effective, backrubbers must be maintained and filled often. Use only No. 2 diesel fuel,
No. 2 fuel oil or an approved backrubber oil to mix insecticides for backrubbers. Do not use fresh or used motor oils.
Co-Ral 11.6% ELI Restricted-use pesticide See footnote.
Co-Ral 5.8%
Livestock Insecticide Spray (LIS) Backrubbers only
4 qt/13 gal (9.75 oz/gal) of No. 2
fuel oil or No. 2 diesel fuel
pyrethrin and pyrethroid insecticides.
Insecticide and dosage Method Comments
Backrubber applications (continued)
Malathion 57% EC Follow label instructions for mixing.
2.25 pt/4 gal No. 2 Backrubbers No waiting period before slaughter for
diesel fuel or approved any of these materials
backrubber oil
Rabon + Vapona
(Ravap) 23% + 5.7%
1 gal/25 gal diesel or 
5 oz/1 gal diesel
Permethrin:
(Pyrethroid insecticides)
Ectiban 5.7% EC
Insectaban 5.7% EC
1 qt/10 gal No. 2 diesel fuel or
approved backrubber oil.
Permectrin 10% EC
1 qt/20 gal No. 2 diesel fuel or
approved backrubber oil.
Synergized DeLice 1% + 1%
PBO PBO = piperonyl butoxide, a material that
1 pt/gal No. 2 diesel fuel or increases the effectiveness of some 
approved backrubber oil. pyrethrin and pyrethroid insecticides


English

Protecting Cattle from Horn Flies

For cattle in Texas, the most damaging insect pest is the horn fly. Research has shown that a calf infested with more than 200 horn flies will gain 15 to 50 pounds less than normal from birth to weaning and sale (about 4 to 6 months). Horn flies can also reduce milk production in dairy cows by up to 20 percent

Texas A&amp;M Repository

E-2085/04For cattle in Texas, the most damaging insect pest isthe horn fly (Fig. 1). Research has shown that a calfinfested with more than 200 horn flies will gain 15 to50 pounds less than normal from birth to weaning andsale (about 4 to 6 months). Horn flies can also reducemilk production in dairy cows by up to 20 percent.To control horn flies effectively and economically,it is helpful to know how to distinguish them fromother flies, what control methods to use for differentstages of their life cycle, when to take action and howto reduce the pest’s resistance to chemicals.Identifying horn fliesHorn flies look like houseflies and stable flies, buthorn flies are slightly smaller. However, they do havepiercing mouthparts like that of the stable fly.To distinguish horn flies from stable flies, observetheir feeding behavior. Horn flies rest on a cowbetween feedings; stable flies remain on the cow onlywhile feeding. Also, horn flies feed most often on ananimal’s back, shoulders and sides, whereas stable fliesfeed principally on the legs.Life cycleHorn fly eggs are laid in cow manure pats, wherethey hatch as maggots. Horn flies develop from the eggto adult stage within 10 to 20 days, and the adults livefor about 3 weeks, feeding 20 to 30 times a day. In Central Texas, horn flies are usually firstobserved in early spring. Populations tend to peak inearly summer, then decline when the weather becomeshot and dry. In the fall, horn fly populations usuallysurge again when the temperatures drop and rainfallincreases. Generally, they are no longer a problem afterSeptember or October, depending on the weather.Control methodsThree types of control methods are used to sup-press horn flies: biological, cultural and chemical.Producers who use all three methods (that is, theyemploy integrated pest management practices) willreduce horn fly populations the most while incurringthe least expense. Assistant Professor and Extension Entomology Specialist,The Texas A&M University SystemFigure 1. Horn flies resting on the back of a cow.Protecting Cattlefrom Horn FliesJeffery K. Tomberlin*Biological control: Parasitic wasps and fire antssuppress horn fly populations naturally. Producerswanting to use parasitic wasps for controlling horn fliescan order fly pupae parasitized with the wasps frominsectaries in Texas or across the United States.However, to date, no research results have been pub-lished showing that releasing parasitic wasps suppress-es horn flies.The parasitized pupae can be spread around barnsnear where the pest flies are developing. These pupaeshould be placed in areas where they will not bestepped on and where they will be out of direct sun-light.Cultural methods: Removing and properly dispos-ing of fresh manure from barns and stalls interruptsthe horn fly’s life cycle and helps prevent new popula-tions from developing.Chemical control: Several chemical control meth-ods can help reduce the number of horn flies on cattle:ear tags, sprayers and dusters, feed additives and bolus-es.Ear tagsEar tags (Table 1) are 2- to 3-inch plastic tagsimpregnated with an insecticide and attached to acow’s ear. Several insecticides are formulated for use inear tags, and many brands of tags are available. Thislarge selection can make it difficult to decide which tagto use and when.Following these guidelines will help you use eartags effectively:➢ Avoid tagging cattle until there are more than 100horn flies per cow. This will minimize the chancefor the flies to develop early-season resistance tothe insecticide in the tag. If you do not tag cattleuntil March or later, the tags will still be effectivelate in the year, when horn fly numbers are high.➢ Read the ear tag labels carefully to determine whento remove them from the animals. Do not use thetags beyond their recommended useful life – if leftin longer,  the flies will be exposed to lower dosesof the insecticide and may increase the chances offly populations developing resistance.➢ Rotate classes of insecticides (not brand names oftags) every year. Most ear tags contain one of twoclasses of insecticide – pyrethroid and organophos-phate. If you use the same class of insecticide 2years in a row, horn flies can quickly becomeresistant. ➢ Do not use ear tags that contain both pyrethroidsand organophosphates. These combination tags donot slow resistance development and may increasethis problem.➢ After tagging cattle, add a supplemental treatment– such as pour-on, back rub, etc. – at mid-season(July or August).Sprayers and dustersFree-range cattle can be treated with small sprayers(Table 1) and dusters powered electrically from theback of a truck. Sprays can also be applied during sea-sonal roundups. However, they usually do not suppresshorn flies for long.Feed additivesBoth free-range and confined cattle can be givenfeed additives that suppress horn flies. These productscontain insecticides that pass through the digestive sys-tem of the animal and exit in the manure where hornfly maggots develop. Feed additives can kill many flylarvae.A disadvantage of using feed additives is the diffi-culty of regulating each cow’s feed intake. Some ani-mals might take in the proper amount; others mightreceive too little. BolusesAnother method for controlling horn flies is bolus-es, which are large pills. When given to a cow, bolusesare retained in the reticulum (second stomach) and dis-solve slowly, releasing the insecticide into the digestivetract. Unlike feed additives, boluses release the insecti-cide continuously.SummaryRegardless of the method chosen, follow theseguidelines:➢ Do not treat infestations of less than 200 flies percow. Treating when horn fly populations are belowthis level will not provide an economic return, andthe unnecessary use of insecticides can speed thedevelopment of resistant fly populations.➢ Read the label of the treatment to make sure it issuitable for use on beef or lactating dairy cows.➢ Be careful when applying insecticides, and wearprotective clothing.AcknowledgmentsThe author would like to thank Mike Merchant andAllen Knutson, Professors and Extension Entomo-logists, the Texas A&M University System, and KimLohmeyer, USDA-ARS, Kerrville, TX, for their reviewof this manuscript.Table 1. Various insecticides used to suppress horn flies on cattle. Please read the label to determine if the product isapproved for use on lactating animals.Insecticide and dosage Method CommentsPour-on spraysGardStar 40% 4 fl oz/50 gal water 1 qt coarse spray per animal.High pressure spray See label for other spray considerations.GardStar 40% 4 fl oz/25 gal water Follow label directions.(High-pressure spray) or4 fl oz/2.5 gal water(Low-pressure spray)Rabon + Vapona (Ravap) Use 0.5 to 1 gal spray per Treatment interval no less than 10 days.23% + 5.3% = animal. No waiting period.2 qt/100 gal waterPermethrin: (Pyrethroids) Spray to thoroughly Can reapply in 14 days if needed.Ectiban 5.7% EC wet animal. No waiting period.1 qt/25-50 gal waterInsectaban 5.7% EC1 qt/25-50 gal waterAtroban 11% EC1 pt/50 gal waterPermectrin 10% EC Re-treat in 14-21 days.1 qt/200 gal waterPermectrin 25% WP1 lb/50 gal waterSynergized DeLice 1% Ready-to-use pour-on Pour along back. Do not treat more+ 1% PBO Maximum 5 oz/animal often than every 14 days.0.5 fl oz/100 lb body PBO = piperonyl butoxide, a materialweight that increases the effectiveness of somepyrethrin and pyrethroid insecticides.Follow label directions.Brute 10% Ready-to-use pour-on Read the label carefully for proper ratesbased on the weight of the animal.Cyfluthrin 1% Ready-to-use pour-on Read the label for proper rate based on(Cylence) weight of the animal.Lambda-cyhalothrin Ready-to-use pour-onEar tagsOrganophosphate tags: Read label. Follow label instructions.Diazinon 18-21.4%TerminatorOptimizerOptimizer calfDiazinon 18% + 2% PBOZ-DiazinonBoyagardDiazinon 40%PatriotCutter 1Diazinon 30% + Chlorpyrifos 10%WarriorInsecticide and dosage Method CommentsEar tags (continued)Organophosphate tags:Pirimiphos-methyl 20%DominatorFenthion 20% + PBO 15% PBO = piperonyl butoxide, a materialCutter Blue that increases the effectiveness of someEthion 36%CommandoPyrethroid tags:Permethrin 10%Ear ForceGardStar PlusAtroban, Deckem, Expar,GardStar,Z-PermethrinFenvalerate 8.6%EctrinCyfluthrin 10%Cutter GoldLambda-cyhalothrin 10%+ PBO 13%Saber ExtraZetacypermethrin 10%+ PBO 20%Python MagnumZeta GardPythonCombination ear tags withpyrethroids:Diazinon 30% + chlorpyoifos 10%  Warrior               Cypermethrin 7% +Chlorpyrifos 5% + PBO 3.5%Max ConLambda-cyhalothrin 6.8%+Pirimiphos methyl 14.0%Double Barrel  Backrubber applicationsMix the insecticide formulations listed below as directed on the label for use in homemade or commercial backrubbers. Backrubbers are most effective when they are placed in a forced-use situation, such as gateways, doors or alleyways. Installthem so that all animals use them once a day. If backrubbers cannot be installed in a forced-use situation, place them inareas where animals often loaf. To be effective, backrubbers must be maintained and filled often. Use only No. 2 diesel fuel,No. 2 fuel oil or an approved backrubber oil to mix insecticides for backrubbers. Do not use fresh or used motor oils.Co-Ral 11.6% ELI Restricted-use pesticide See footnote.Co-Ral 5.8%Livestock Insecticide Spray (LIS) Backrubbers only4 qt/13 gal (9.75 oz/gal) of No. 2fuel oil or No. 2 diesel fuelpyrethrin and pyrethroid insecticides.Insecticide and dosage Method CommentsBackrubber applications (continued)Malathion 57% EC Follow label instructions for mixing.2.25 pt/4 gal No. 2 Backrubbers No waiting period before slaughter fordiesel fuel or approved any of these materialsbackrubber oilRabon + Vapona(Ravap) 23% + 5.7%1 gal/25 gal diesel or 5 oz/1 gal dieselPermethrin:(Pyrethroid insecticides)Ectiban 5.7% ECInsectaban 5.7% EC1 qt/10 gal No. 2 diesel fuel orapproved backrubber oil.Permectrin 10% EC1 qt/20 gal No. 2 diesel fuel orapproved backrubber oil.Synergized DeLice 1% + 1%PBO PBO = piperonyl butoxide, a material that1 pt/gal No. 2 diesel fuel or increases the effectiveness of some approved backrubber oil. pyrethrin and pyrethroid insecticides

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Protecting Cattle from Horn Flies

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Protecting Cattle from Horn Flies

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