There are so many reasons why people call a pest control company, but I’m always surprised that flies are frequently an afterthought. To many of our customers, flies are often just part of life, something irritating but not necessarily a problem.
With more than 100,000 species, flies can be parasites, predators, pollinators and, of course, decomposers. Despite this variety, a relatively small number of species are residential or commercial pests, the most common being the house fly (Musca domestica) and the green bottle fly (Lucilia sericata). Possibly the most concerning thing about these pests are their ability to spread pathogens that cause disease; the house fly is known to spread at least 65 different pathogens. We group many of the larger fly pests together as “filth flies,” which describes their role as decomposers, but also providing a clue to control.
FILTH FLY SPECIES.
House flies are relatively large insects and are around 1/4 inch big. Females are slightly larger than males and have noticeably wider spacing between the eyes. Adults are grey to black with four stripes on the thorax and have one pair of membranous wings. Behind the wings are interesting organs called halteres. These organs are a reduced wing that helps stabilize the fly in flight. Adult house flies have large red, compound eyes and a trio of ocelli. The larvae of house flies (maggots) are cream colored, about 1/3-inch long and very active. House flies are so widely distributed around the world that most adults will recognize them and their maggots on sight.
Green bottle flies are slightly smaller than the common house fly and have a distinctive metallic sheen to their thorax. These flies are commonly found around carrion and feces. Many homeowners with pets will notice these flies out in the yard. The presence of these insects is also of interest to forensic entomologists who use the life stage to determine a time of death or post-mortem disruption.
Flies go through complete metamorphosis and have an egg, larvae, pupa and adult life cycle. The speed at which all flies go through these life stages is strongly influenced by ambient temperature, but most of their life is spent as an adult. House flies can complete a life cycle in as little as a week in warm locations or a month in cooler climates. Adults, which might live up to two months, can produce up to 900 eggs. At this reproductive rate the population can rapidly grow beyond the tolerance of most residences and businesses. If you do the math, the numbers quickly become mind boggling, but it is very safe to say that a pair of flies could become thousands of flies in a month.
Adult house flies feed with a sponging mouth that is used to suck up liquid foods. In order to turn solid food sources into liquids, flies regurgitate saliva from their gut, liquefying the solids, which can then be sucked up. Any pathogens ingested in this manner can survive in the fly’s gut for several days. Because of the fly’s mobility and feeding style, these pathogenic organisms can be introduced at each new feeding site.
As with all pest control efforts, the starting point is always proper identification. Due to their potential to be a health hazard, fly identification keys are widely available from universities and government agencies. The primary point of interest between species is the preferred breeding sites. Beyond locating breeding sites, control efforts are relatively similar. Flies locate breeding sites using odors and house flies prefer fresh manure, fermenting vegetable waste and waste areas such as dumpsters or trash cans.
Breeding sites are easily identifiable by the numerous crawling maggots. Around households, poorly maintained compost and overuse of natural fertilizers in garden areas are common sources, especially in suburban areas. In commercial settings, the source is frequently poorly cleaned dumpsters or trash areas. It is important to realize that while flies typically do not travel far, the breeding site may be located on a neighboring property and beyond the scope of work. If this is the case, working with the other property owner may be necessary to achieve acceptable levels of control.
While it may be tempting to simply apply a liquid insecticide to these breeding sites, the most effective control method will be to eliminate the breeding site via thorough sanitation. This sort of work may be beyond the scope of the relationship between the customer and pest management professional. Identifying who will perform the sanitation must be communicated effectively. It is also important to ensure your customers understand the habits and behaviors that allow fly breeding sites to develop in the future. Sanitation is the single most controllable aspect of fly management.
Once breeding sites are eliminated, the adult fly population must be reduced. There are a few different strategies for this, and all should be used in conjunction for rapid relief. The first is simple exclusion. During the initial inspection, damaged or poorly fitting screens should be identified and repaired or replaced. Customers should be advised to keep unscreened entryways closed when not in use. In a commercial setting, air curtains also can be placed at areas of high traffic to prevent fly entry. Properly placed, these devices will create a strong negative pressure wave that will push flies away from the entryway.
On the exterior of the structure, traps can be used to intercept flies. This strategy is especially important when breeding sites cannot be eliminated. There are a variety of traps on the market. They generally feature some sort of one-way entry combined with an attractant. Placement of these is key for two reasons: First, flies typically fly around 5 feet above the surface level so traps should sit at this height. Second, many of these traps use a lure that tends to smell objectionable, thus the traps should not be close to household windows or areas where customers frequent at a business. While traps are effective in reducing the numbers of adults, the reproduction rate of house flies means they will rapidly become full and need replacement.
On the interior of commercial structures, insect light traps (ILTs) are also an effective tool for managing adults. These devices emit UV light waves that flies find attractive. Inside the device there is typically a sticky board or electrifying element. Placement of these devices should be at a similar height to exterior traps with the added need for some distance from food and food-prep surfaces to avoid contamination of these areas.
Insecticides are also a tool that can be used to reduce adult populations. Most general-use residual insecticides will eliminate adult flies, however, many of these insecticides take up to a few hours from contact to kill the adult. Fly-specific products are much more effective due to the addition of an attractant. These insecticides are actively sought out and ingested by the flies and typically work within a few minutes.
The downside to rapid elimination of adult flies at a customer’s location can be the false perception that the problem is resolved, and that the sanitation tasks are now unnecessary. Nothing could be further from the truth. The reprieve in adult activity should be used to eliminate as many breeding sites as possible. Again, because of the speed at which flies reproduce, failing to do so will result in a failure of control and a rapid rebound of the population.
As always, carefully review all pesticide labels and ensure that the application complies with all local, state and federal laws. All pesticides should be applied in a manner that will not contact people or pets. Several fly-specific products on the market contain noenicitinoids, which may have serious impacts on pollinators if used incorrectly.
THE NEXT STEP.
Once breeding sites are eliminated and adult control methods implemented, customers should expect relatively immediate relief, within a day. Before leaving the site, ensure your customers understand what types of breeding sites were located and what types of sanitation tasks are required in the future. If problems still exist when following up a few days later, it is likely that breeding sites were not located and eliminated.
During follow-up services, the PMP should conduct a thorough inspection of previously identified breeding sites. If the PMP finds that the customer has not addressed these locations, it is important to determine why and reinforce the need for this change. If necessary, provide photos or walk customers over to active breeding sites. The sight of numerous maggots often inspires customers to change their ways. The ideal time to do a follow-up inspection is mid-day through the early afternoon, when the flies are most active. Life isn’t always ideal though, so asking how often and where flies are seen can also be effective. It is important to understand everyone has a different definition of a problem, so try to find out how many flies are seen at a time. Look closely for home remedies like customer-placed sticky traps.
While this article focused on the house fly, there are other flies that are quite common structural pests such as blow flies, flesh flies, cluster flies, as well as smaller flies like fruit flies, fungus gnats, drain flies and phorid flies. The strategies to control these flies remain the same: identify the breeding site, conduct through sanitation, control entry points, eliminate the adults and monitor the situation.
References: Khamesipour, Faham, et al. “A Systematic Review of Human Pathogens Carried by the Housefly (Musca Domestica L.).” BMC Public Health, BioMed Central, 22 Aug. 2018, www.ncbi.nlm.nih.gov/pmc/articles/PMC6104014/. “COVID-19 Mythbusters.” World Health Organization, World Health Organization, www.who.int/emergencies/diseases/novel-coronavirus-2019/advice-for-public/myth-busters#houseflies. World Health Organization. Division of Control of Tropical Diseases. (1991). The Housefly: training and information guide. World Health Organization. “Get Rid of House Flies: House Fly Control Information.” PestWorld.org Your Partner in Pest Prevention, www.pestworld.org/pest-guide/flies/house-flies/.