Horn Fly Control: Dealing with Insecticide Resistance

Fly management and control are challenging to many beef cattle producers nationwide. Horn flies (Haematobia irritans) are the most prevalent pest in terms of economic losses to cattle producers and are estimated to account for around $1 billion in monetary damages to the livestock industry annually in the United States due to weight and blood loss and increased stress for cattle.^1,2 This publication will help beef cattle producers prevent production loss resulting from the presence of horn flies on cattle.

Introduction

Horn flies develop from the egg to the adult stage within ten to twenty days, and the adult flies live for about one to two weeks, feeding twenty to thirty times a day.³ While insecticide-impregnated ear tags have certainly made fly control a more manageable goal, each producer still has decisions related to fly management. There are four main types of insecticide ear tag products based on the active ingredient. These include organophosphate insecticides (diazinon and fenthion), synthetic pyrethroids (lambda-cyhalothrin and cyfluthrin), abamectin (the newest type of tag), and the fourth type that uses combinations of the previous three insecticides. Examples of combinations of insecticides include combinations of organophosphate and synthetic pyrethroids or synthetic pyrethroids and abamectin.

The early ear tags contained only pyrethroid insecticides and were very effective. After only a few years, horn flies began to develop resistance to pyrethroid insecticides; a crucial contributing factor was the widespread use and often misuse of pyrethroid tags.^4,5 Resistance management should be included in any fly control program regardless of the product or application method. Many insecticides used for horn fly control have documented cases of resistance, especially pyrethroid and organophosphate insecticides. North Dakota State first released guidelines to help prevent the development of insecticide-resistant horn fly populations.⁶ Adaptations of these guidelines are presented below to help effectively manage horn flies while preventing the establishment of insecticide-resistant populations.

Treatment

Research has determined that 200 horn flies per animal (100 per side) is the economic treatment threshold for horn flies on beef cattle.⁷ The economic threshold is the number of flies that must be present before treatment is justified. Waiting until populations exceed the threshold ensures treatments are necessary so that the fly population is exposed to fewer insecticide treatments. Figure 1 illustrates the criteria for estimating the number of horn flies on cattle.

Figure 1. A method of estimating horn fly numbers in the field. Image credit: Phillip E. Kaufman and Emma N. I. Weeks, University of Florida EDIS, Horn Fly Management.⁸

Insecticide Ear Tags

Many fall-calving herds are processed in early spring before 200 flies per animal are present. It is best to delay insecticide ear tag application until an economic threshold is met to save money and prevent the development of insecticide-resistant populations of horn flies. Insect growth regulators (IGRs) are the exception to this rule. The use of IGR products should begin early in the horn fly season, usually before flies are present.

Insecticide ear tags contain a 4- or 5-month effective supply of insecticide. After which, tags will still contain some insecticide, but the amount is not a lethal dose. Exposure to sub-lethal doses of insecticides is another reason horn flies develop resistance. Removing fly tags is essential for preventing fly population exposure to sub-lethal levels of insecticide.

Periodic Applications of Sprays, Dusts, and Backrubbers

With ear tags, horn flies are constantly exposed to the insecticide. However, periodic temporary treatments such as sprays and dusts (table 1) and backrubbers can be used temporarily in conjunction with ear tags to reduce fly populations to acceptable levels.⁹ Backrubbers provide a rubbing surface treated with insecticide and can be purchased or made by wrapping burlap over chains or barbwire. These periodic techniques limit the length of time horn flies are exposed to a particular insecticide and provide an opportunity to rotate to another chemical classification. Chemical rotation is a critical part of resistance management, and more discussion on rotation is provided below.

Insect Growth Regulators and Oral Larvicides

IGRs are excellent tools for horn fly control. IGRs are usually included in cattle minerals and are among the best tools for resistance management because flies will not develop resistance to this particular control mechanism. Most larvicides inhibit fly eggs from properly developing using hormones or hormone-mimicking compounds. The IGR passes through the gas tract after it is eaten and is present in the manure pat. Because horn flies only lay eggs in manure pats, feed-through insecticides are a very effective control measure.¹⁰ Oral larvicides are effective when consumed in sufficient quantities throughout the fly season. Adult horn fly numbers may appear unaffected if cattle consuming feed additives are near an untreated herd. An untreated herd may provide enough flies to keep fly numbers above the economic injury level for both treated and untreated cattle.

Late-Season Treatments

Horn flies overwinter as a pupa in the soil and emerge the following spring. An appropriate late-season treatment can limit the number of adults laying eggs that will become over-wintering pupae. It is crucial to select a late-season treatment that will help eliminate resistant flies before they lay eggs and produce resistant offspring that will over-winter. Insecticide resistance is heritable and is passed to the next generation of horn flies.¹¹ Using products with little or no fly resistance in the late season can limit the number of insecticide-resistant adults that emerge the following spring, enhancing the efficacy of traditional horn fly control methods. Product selection should be based on the status of resistance and the chemical class and active ingredient that aren’t being used at the time.

Chemical Classes Rotation

Insecticide ear tags are still very useful fly control tools. Rotating between organophosphate and pyrethroid products is recommended. The suggested rotation schedule is organophosphate tags for two consecutive years, rotate to a pyrethroid tag for one year, and then back to organophosphate tags. Now that a macrocyclic lactone fly tag is available (XP 820), we can implement a three-way rotation of fly tags. Chemical rotation is a good idea with any method of fly control.

Remember that rotation between chemical classes is necessary (more than changing brand names). The Insecticide Resistance Action Committee (IRAC) groups insecticides based on the mode of action. IRAC groups are numbered 1–32. If you use an insecticide from one group, you must rotate to an insecticide with a different group number to manage resistance. Table 1 lists common fly insecticides by chemical class and IRAC Group that can be utilized for horn flies. Check the active ingredient of the products you are considering and determine their chemical class.

Some classes of insecticides are more prone to developing resistance than others. Horn flies will easily and quickly build resistance to pyrethroid insecticides (Group 3). Organophosphate insecticides (Group 1B) have documented cases of resistance but aren’t as prone to resistance as pyrethroids. Macrocyclic lactone (Group 6) and IGRs (Group 15) have very few or no documented cases of resistance. These characteristics should be considered when developing a resistance management strategy.

Table 1. Common insecticides for fly control.

Source: Adapted from L. Townsend, 2016.¹² Updated from the 2020 South Carolina Pest Management Handbook.
* = insecticide ear tag

Control throughout the horn fly season will likely require two or more methods from different chemical classes. Keep insecticide resistance management in your fly control plans, and always read and follow directions on the product label to ensure products are used safely and according to manufacturer guidelines. Please contact the Livestock and Forages Agent at your local Clemson Cooperative Extension County office for additional information. Contact information for county offices is available on the Clemson Cooperative Extension website.

References Cited

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2. Cutkomp LK, Harvey AL. The weight responses of beef cattle in relation to control of horn and stable flies. Journal of Economic Entomology. 1958 Feb 1;51(1):72–75. doi:10.1093/jee/51.1.72.
3. Foil LD, Hogsette JA. Biology and control of tabanids, stable flies and horn flies. Revue scientifique et technique-Office international des épizooties. 1994 Dec 1;13:1125–1158. doi:10.20506/rst.13.4.821.
4. Williams RE, Westby EJ. Evaluation of pyrethroids impregnated in cattle ear tags for control of face flies and horn flies. Journal of Economic Entomology. 1980 Dec 15;73(6):791–792. doi:10.1093/jee/73.6.791.
5. Denholm I, Rowland MW. Tactics for managing pesticide resistance in arthropods: theory and practice. Annual Review of Entomology. 1992 Jan;37(1):91–112.
6. Myer R, McBride D, Christie R. Management strategies for pyrethroid insecticide resistant horn flies. Fargo (ND): North Dakota State University Extension Service; 1991. E-1012. https://library.ndsu.edu/ir/bitstream/handle/10365/17474/E-1012-1991.pdf?sequence=2&isAllowed=y.
7. Gordon DV, Haufe WD, Klein KK. Determination of economic thresholds for horn fly control in Western Canada: a farm level simulation approach. Canadian Journal of Agricultural Economics. 1984;32(2):399–421. doi:10.1111/j.1744-7976.1984.tb02134.x.
8. Kaufman PE, Weeks ENI. 2012. Horn fly management. Gainesville (FL): University of Florida, Institute of Food and Agricultural Sciences; 2012 Sep. ENY288/IN952 EDIS 2012. doi:10.32473/edis-in952-2012.
9. Harvey TL, Brethour JR. Dust bag and backrubber applications of insecticides for control of pyrethroid-resistant horn flies, Haematobia irritans (L.). Preventive Veterinary Medicine. 1986 Mar 1;3(6):537–40. doi:10.1016/0167-5877(86)90031-0.
10. Lysyk TJ, Schaalje GB. Binomial sampling for pest management of stable flies (Diptera: Muscidae) that attack dairy cattle. Journal of Economic Entomology. 1992 Feb 1;85(1):130–136. doi:10.1093/jee/85.1.130.
11. Dobzhansky T. Genetics and the Origin of Species. New York (NY): Columbia University Press; 1937.
12. Townsend L. ENT 11 – Insect Control on Beef Cattle. Lexington (KY): Cooperative Extension Service, University of Kentucky; 2016. https://entomology.ca.uky.edu/files/ent11.pdf.