Controlling Flies in Beef Feedlots

By Mike Catangui, Ph. D


Stable flies (both males and females) subsist mainly on the blood of cattle and many other warm-blooded animals, including humans; they are very aggressive biters of newborn calves, growing heifers, and milking cows.

Stable flies are known to reduce average daily weight gains of growing calves and heifers by up to 17 percent (Catangui et al., 1997) and reduce milk production of milking cows by up to 20 percent (Gerry et al., 2007). Stable flies appear to injure dairy cattle through direct blood loss, the wasting of energy by trying to ward off and avoid painful and irritating bites, and reduced or irregular feed consumption. Milking cows in free stall barns being attacked by stable flies tend to bunch together to defend themselves from the biting pests; this bunching behavior can exacerbate heat stress during hot summer days.

In contrast to stable flies, house flies cannot bite dairy cattle by virtue of their soft sponge-like mouthparts; their main role on dairy farms appears to be as vectors or carriers of microorganisms (bacteria, viruses, protozoans, worm eggs) that can cause diseases on dairy cattle and in human farm workers. The causative bacterium of cryptosporidiosis (Cryptosporidium parvum) had been conclusively shown to be vectored by house flies from contaminated farm substrates to humans and animals (Graczyk et al., 1999). Numerous house flies can alight and feed on the bodily secretions and excretions of farm animals, then on the water and feed provided to calves, heifers and milking cows. House flies can migrate in both directions: from the farm to nearby neighborhoods or from outside breeding sites into the dairy farm. Farm vehicle traffic may also inadvertently facilitate house fly migration in both directions.

Breeding sites

Commonly called maggots, the larval stages of stable flies can proliferate in plant materials that are provided to calves as bedding. Examples of bedding materials are wheat straws, oat straws, almond shells, rice hulls, oat hulls, sunflower hulls, corn stalks, wood shavings and others; they are highly attractive breeding sites for flies once they start fermenting and decaying over time. Numerous stable fly larvae and pupae in fermenting wheat straw used as bedding in a calf hutch. House flies appear to prefer to breed in fermenting feed; house fly maggots in this medium. Both fly species can complete their life cycles within 3–4 weeks on dairy farms during the summer months.

Controlling adult flies

House fly baits can be used in exterior areas close to calf hutches and dairy barns to lure and kill house flies. Liquid or sprayable baits are available for applications on vertical surfaces, ceilings, or areas where solid baits are not practical. [Note: Solid baits are designed for use against house flies and not for stable flies; stable flies feed on the blood of dairy cattle and other animals.

To quickly reduce adult house fly and stable fly numbers, on-animal sprays can be used to directly spray the flies that are feeding upon, or resting on or adjacent to the animals. On-animal sprays are quick knock-down and low-residue insecticides that can either be water-based [Table 2], oil-based or aerosol. Spray applications can be made using ordinary sprayers or recommended hand-held or tractor-mounted equipment. A few insecticides are labeled for use through automatic misting systems that are hardwired in the barn.

During cleanout or when the calf hutches or barns are empty of animals for a period of time, longer-lasting empty-barn residual insecticides [Table 8] can be applied on the interior and exterior surfaces of the empty structures. These residual insecticides cannot be applied directly on the animals; they are to be applied on empty or vacated hutches and barns. The animals can be brought back immediately in the treated barns or hutches after the spray droplets have completely dried up.

Parasitic wasps and fly traps are important components of a complete integrated pest management program (IPM) for dairy cattle. Releasing parasitic wasps augment natural parasitism rates already occurring in nature; fly traps provide long-lasting background control of active insects.

Controlling fly larvae in breeding sites

A successful fly control program needs to target both the larval (maggot) and adult stages of flies. The straw beddings, for example, can be treated with larvicides before the calves are placed inside the hutches. Larvicides are a group of insecticides specifically labeled to control the larval stages of insects; adulticides are aimed at the adult insects. Larvicides will effectively nullify the straw beddings as breeding sites for house flies and stable flies. Other potential fly breeding sites in and around the hutches and barns can also be treated with larvicides.

Elector® PSP is a contact larvicide that impairs the nervous system of the fly larva; mortalities are observed within 48 hours after application. The active ingredient in Elector® PSP (spinosad) is of natural origin; it is derived from the fermentation of a soil actinomycete (a group of soil bacteria) called Saccharopolyspora spinosa.

Tekko™ 10 (novaluron), Neporex® 2 SG (cyromazine), and NyGuard® IGR Concentrate (pyriproxyfen) contain so-called insect growth regulators (IGRs) that disrupt the molting process of insects. The effects of insect growth regulators can be better visualized in the pupal stage of the treated flies. Deformed house fly pupae caused by an insect growth regulator. These deformed pupae were collected from manure that was treated with novaluron (Tekko™ 10). Deformed or abnormal pupae collected from manure treated with cyromazine (Neporex® 2 SG). Deformed or affected pupae cannot transform into normal flies, thereby causing mortalities in the treated fly populations. 

Certain larvicides containing the insect growth regulator (IGR) diflubenzuron (ClariFly® Add-Pack Fly Control for Calves, Sav-A-Caf elim-A-fly™ Add-Pack with ClariFly® Larvicide, and JustiFly® Feedthrough) can be fed to the calves (feedthrough larvicides) through the milk, milk replacer or feed ration. The active ingredient is not digested but excreted as a larvicidal treatment on potential fly breeding sites such as the straw beddings and organic matter in and around the hutches. Diflubenzuron is a chitin synthesis inhibitor; treated larvae are not able to form new exoskeleton normally after molting.

About the Author

Mike Catangui, Ph. D

Entomologist, Parasitologist
MWI Animal Health
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