In North America alone, there are over 700 species of flies that belong to the Muscidae (filth fly) group. Probably the single-most common of these is the house fly. No geographic locations or socioeconomic communities are spared the constant presence of the house fly. While they don’t bite, this fly’s habit of constantly crawling on nearly every surface – particularly manure – makes them very undesirable to have around.

Their feeding habits include constant regurgitation followed by sponging up the liquid. So the fly that just landed on your potato salad may have just come from the bacteria-laden manure.

Fly numbers are higher in the Southern states, especially more humid areas, since they have a year-round presence. During the warmer months, they can be found practically “everywhere.”

In a previous article, the biting stable fly was discussed as the most economically important pest fly to the dairy industry. The second-most important one is the common house fly. Even though they don’t bite humans or dairy cows and have little impact on the economics of production, they are “nuisance flies” and cause human beings much more concern than the previously discussed stable fly ever did.

House flies can transmit many pathogens (more than 60), ranging from bacterial and protozoan diseases, viruses and helminthes, causing such diseases as polio, hepatitis, cholera, anthrax, diphtheria, typhoid fever and yaws in humans and similar problems in dairy cattle, including pink eye.

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They may pick up disease-causing organisms from garbage, sewage, animal excrement, decaying organisms and any other source of filth. They can transfer these organisms from their mouthparts and other body parts to other animals including humans. Even though they don’t bite, house flies have been the basis of many lawsuits and have required the spending of thousands of dollars to get rampant disease issues under control, including E. coli and the previously mentioned diseases.

The principles of fly control are universal and, if properly understood, can be adapted to individual circumstances. Successful fly control requires a management approach involving an integration of control methods in a manner compatible with current animal practices.

House flies are 3/16-inch to 5/16-inch in length and black with four dark stripes on the thorax. The abdomen is grayish or yellowish at the sides and has a dark stripe down the middle. They have fleshy, sponging mouth parts and cannot bite. The eyes are reddish and have a golden stripe around the outer edge. The legs are hairy with adhesive pads and sharp claws. The wings are level and held straight back.

The female lays four to six batches of approximately 150 eggs on any wet, decaying organic matter, which usually hatch within 24 hours. Like the stable fly, the third and final larval instar (maggot stage) migrates away from the food source in search of a suitable site for pupation (cocoon stage). Adult flies may live up to three to four weeks but can go from egg to egg-laying adults in as short as eight days under optimum conditions, with five days in the larval stage and three days in the pupal stage.

Adult flies feed on many food sources and a dairy to a house fly is like a grand buffet. House flies are attracted to odor; large, bright-colored objects; and are always “exploring.” In a dairy, animal saliva, mucus and lachrymal secretions are a common source of sugars for the newly emerged adults but they feed equally well on manure and other attractive material. They are strong fliers; however, they most often develop at the same facility where the adults are found or within 1/4-mile range.

House fly breeding sites on a dairy are many and varied. Unlike the stable fly – where straight cow manure is not a major concern – with the house fly, any wet or decaying material is attractive for egg laying and larval development. Major areas of concern include, but are not limited to: lagoon, manure pile, silage pile, spilled feed, wet brewers grain, feed commodities, calf pens, animal feces, rotting fruit, dumpsters, etc. Like the stable fly, generally 90 percent of the house flies will develop in only 10 percent of the farm area. Finding and eliminating those areas provide the most effective fly reduction.

In a study conducted by the University of California several years ago, the preferred sites for fly development were identified with 99 percent of the fly larvae collected coming from those few sites. In those studies, plant material used as bedding in the calf hutches, silage and haylage piles – as well as the effluent that seeped from them – cow manure both alone and mixed with straw or hay, lagoons and sedimentation pits – were responsible for most of the flies that developed.

With this in mind, it makes an even stronger case for proper sanitation and manure management as the most important aspect of good fly control. Several species of flies may be developing in the same medium so cleaning it up will help with all. By finding the areas that are constantly wet (40 to 60 percent moisture) and digging to find the maggots, you can rapidly identify the source. If you find pupae, it will only be a few days before another hatch of flies emerges.

There are a number of tools that can be used for house fly control but no one single technique will provide total fly suppression. Pesticides are probably the single-most-used method of controlling house flies. However pesticide resistance can and has developed in many geographic regions, reducing the effectiveness of both sprays and baits. The cost of spraying varies with each facility and different geographic region. The cost per head will be less on larger dairies, with current costs from $0.50 to $3.50 per head per month. Sprays are often applied weekly early in the season and sometimes daily during peak fly activity. If only spot treatments on fly resting areas – not breeding areas – are used in conjunction with other tools such as the parasitic wasp, those pesticide costs will be less. The cost of the parasitic wasp can be comparable and often less than the cost of regular insecticide treatments. With sprays the number of pest flies is reduced immediately following treatment but will increase rapidly as additional generations continue to emerge. By using parasitic wasp biological control the number of pest flies emerging is typically reduced. Any of these costs – chemical or biological – will be decreased if a proper sanitation management program is put in place.

Traps, baits and sticky tapes are all effective in helping reduce the number of house fly adults present and will complement the use of biological controls and spraying. In a natural situation, only 2 to 4 percent of the house fly eggs that are laid ever reach the adult egg-laying stage. This high mortality is attributed to many naturally occurring parasites and predators, climatic changes, etc. If the first generations are suppressed when flies first start being active, they won’t build up as rapidly. The release of the parasitic wasp should be started early in the season to coincide with fly activity to limit the number of reproductive females present. An increase in numbers may be needed as the speed of reproduction of the house fly increases and releases should be continued for the entire fly reproduction period. PD

For additional information in both English and Spanish go to www.spalding-labs.com

Bill Clymer