Beef producers frequently reduce fly control efforts in late summer — a mistake that can lead to disease outbreaks and entrenched chemical resistance.
“As late summer turns into early fall, it is easy to overlook the importance of pasture fly control through the end of the fly season,” says Dr. Michelle Arnold, DVM, ruminant Extension veterinarian at the University of Kentucky Veterinary Diagnostic Laboratory (UKVDL). “Many fly control products utilized in the spring and early summer are at the end of their useful life, meaning fly populations can increase quickly.”
Arnold emphasizes in a recent Kentucky Beef Cattle Newsletter fall control directly determines pest pressure for the following spring.
“Good fly control in the fall is essential to reduce insecticide resistance through control of the overwintering fly population,” she explains.
Horn Flies vs. Face Flies: Biology, Damage and Treatment Thresholds
Effective control begins with recognizing that horn flies and face flies behave, feed and respond to insecticides differently.
| Characteristic | Horn Fly (Haematobia irritans) | Face Fly (Musca autumnalis) |
|---|---|---|
| Feeding Mechanism | Piercers (20–30 blood meals daily) | Blotters (feeds on tears, saliva, nasal mucus) |
| Habitation on Cattle | Continuous (shoulders, backs, sides, belly) | Brief (spends only minutes at a time on the head) |
| Flight Range | Stays with the herd | Strong flier (2+ miles) |
| Primary Disease/Impact | Blood loss, grazing disruption, weaning weight drop | Vector for pinkeye (Moraxella bovis) and eye trauma |
| Economic Treatment Threshold | 100–200 flies per animal (100/side) | Fewer than 10 flies per head |
“Horn flies stay on cattle almost continuously, leaving only when disturbed or to lay their eggs in fresh manure,” Arnold explains. “The fly’s constant presence on the animal means they are almost certain to come in contact with applied insecticides.”
Face flies present a different challenge. “Face flies are ‘blotters’ that feed on tears instead of ‘piercers’ that feed on blood as horn flies do,” Arnold notes. “Female face flies irritate eyes when feeding, with abrasive, blotting mouthparts that rasp, scrape and penetrate the conjunctival tissues, triggering tear and mucus production that feed the insects. Bacteria in the secretions of pinkeye-infected cattle can survive on or inside face flies for two to three days and infect other animals when the flies feed again.”
Because face flies spend minimal time on the animal, they often do not receive a lethal dose of insecticide from standard whole-body treatments.
How Do Fly Tags Cause Insecticide Resistance If Left in Place?
Most commercial insecticide ear tags offer 12 to 15 weeks of labeled suppression, but deliver their strongest active release during the initial 45 to 60 days.
“Fly tags applied around the first of May will be exhausted by early September, but flies will usually continue into October and sometimes longer,” Arnold warns. “Insecticide resistance in fly populations tends to develop in late summer when insecticide levels drop to sub-lethal doses that allows a resistant fly population to survive and reproduce.”
To halt this cycle:
- Remove Old Tags in the Fall: “All insecticide ear tags should be removed at the end of the season to avoid exposing pests to low, residual chemical doses that accelerate resistance,” Arnold advises.
- Apply Short-Duration Knockdowns: Utilize pour-ons or animal sprays providing two to three weeks of protection to bridge the gap until cold weather arrives.
How Should Cattle Producers Rotate Insecticide Modes of Action (MoA)?
The Insecticide Resistance Action Committee (IRAC) groups active chemistries by their physiological Mode of Action (MoA). Repeatedly using the same group selects for resistant genetics.
“The recommended practice to manage resistance is to alternate insecticide IRAC Mode of Action (MoA) groups,” Arnold stresses. “Rotate between MoA groups every fly season and if possible, during the fly season. Products within the same group, but in different subgroups (for example, carbamates 1A and organophosphates 1B), should not be used in the same year unless there are no other chemistries available that are in totally different groups.”
Primary Cattle Insecticide MoA Groups:
- IRAC Group 1A (Carbamates) & Group 1B (Organophosphates): Acetylcholinesterase inhibitors (e.g., diazinon, stirofos/tetrachlorvinphos).
- IRAC Group 3A (Pyrethroids / Synthetic Pyrethrins): Sodium channel modulators (e.g., cyfluthrin, zeta-cypermethrin, permethrin).
- IRAC Group 6 (Macrocyclic Lactones / Abamectins): Glutamate-gated chloride channel allosteric modulators.
- Synergists (PBO): Piperonyl butoxide (PBO) is a chemical synergist added to some cattle fly tags to boost tag potency.
Producers can look up commercial product chemistries and their exact IRAC numbers using the Veterinary PestX database.
What Application Methods Work Best for Late-Season Fly Control?
Arnold suggests these four application methods for late-season fly control:
- Forced-Use Backrubbers and Oilers: Hanging backrubbers with fly bullets or flyps placed at 4- to 6-inch intervals at water or mineral entrances forces cattle to treat their own faces and polls consistently.
- Insecticide Strips: Add-on insecticide strips can be snapped over existing identification tags to provide supplemental face protection without punching a second tag hole.
- Pour-Ons and Sprays: Practical for late summer when ear tags have expired and cattle gather for fall working or preconditioning.
- Non-Chemical Pasture Traps: Sticky traps positioned around mineral feeders and loafing areas capture substantial numbers of biting flies without adding chemical selection pressure.
How Should Feed-Through Larvicides and IGRs Be Used?
Both horn flies and face flies lay eggs exclusively in fresh cattle manure — less than 24 hours old. Feed-through products pass through the digestive tract to kill developing maggots.
“Feed-through products can reduce the number of fly maggots developing to adults, provided they are started early enough in the spring and cattle consume a sufficient daily dose,” Arnold says. “Feed-through products should be started in mid-spring, 30 days prior to fly season, and be consistently available until 30 days after fly season is over.”
She suggests these common feed-through options:
- Methoprene (Altosid): Insect Growth Regulator (IGR) for horn fly control.
- Diflubenzuron (Justifly): Chitin synthesis inhibitor for horn fly and face fly control.
- Tetrachlorvinphos / Stirofos (Rabon): Organophosphate oral larvicide premix for horn fly and face fly suppression.
Arnold notes larvicides do not kill adult flies flying in from neighboring herds. Always verify meat withdrawal times, adhere strictly to label application rates and dispose of chemical packaging in accordance with local regulations.
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