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About the house fly.

When you see a house fly walking around on your kitchen window, do you ever wonder where it came from or where it has been?  You might just whip out the fly swat and be done with it because everyone knows that you don’t want flies in the house.  But are they just a nuisance, or is there something more to the deep-seated revulsion that we feel towards flies?

The house fly, Musca domestica is the most common and widespread species of fly in the world.  It is thought to have originated on the steppes of central Asia, but is now present in a wide variety of environments from rural to urban, and in all climates from tropical to temperate [1].

The house fly is considered a “filth fly” due to its association with manures and household waste.  It has developed a particularly close association with humans, and is able to complete its entire lifecycle within housing belonging to people or their domestic animals House flies are also abundant in other areas of human activity such as food markets, restaurants and cafes, and hospitals, as well as slaughterhouses and livestock farms where they can become a particular nuisance and pose problems of disease transmission.

Where did that house fly come from?

That adult fly you see buzzing around your house has been through three earlier life stages: egg, larva and pupa.

Female house flies lay eggs in moist microbe-rich decaying organic matter such as compost, rubbish, soiled bedding, rotting food or manure.  They typically lay about 100 – 150 eggs at a time and can produce several batches of eggs in their lifetime. 

The eggs hatch within a day, and the larvae feed for between 3 to 14 days depending on temperature and the richness of the food source. When the larvae stop eating, they migrate to a dry, dark place to pupate. 

The pupae develop a tough outer envelope around them called the puparium.  Inside this, the pupa transforms into an immature fly which breaks out of the puparium after about 5 days. 

The whole lifecycle can be completed in as few as 8 days under ideal conditions, but usually ranges from 10-21 days. Adults usually live 2 – 3 weeks in summer, but this lifespan can extend to several months during the cooler months of spring and autumn.

House fly larvae flourish in filth

The house fly’s relationship with bacteria starts right at the very beginning – when the eggs are laid. At egg laying, the females add a bit of bacteria for good measure, possibly to deter other females from laying their eggs in the same place, or as a “starter” food for the tiny hatchlings [2].  The eggs can be laid into any of a wide range of rotting organic materials (eg plant or animal material or manure or waste) and it’s the microbes and their by-products that become the food for the larvae [3], not the organic material itself.  That microbial community changes rapidly as the rotting takes place, causing changes to water content, pH and oxygen availability, and yet the house fly larvae thrive. For them, it’s a veritable feast!  Studies have found that while most of the microbes consumed by the larvae are digested, some manage to survive the pupation period, and are still alive on the fly surface or in their gut when the adult emerges [4].

House fly adults as microbial carriers

In the late 1800s and early 1900s, public health studies of conditions in the Boer War and around the docks of New York Harbor showed that outbreaks of typhoid fever could be correlated with fly population numbers, and bacteria could be cultured from flies [5]. Since then, public health campaigns around the world have worked to inform people about the need to control flies using advertising and posters like the one illustrated here [6].

Studies have shown that flies can carry and transmit diseases in several ways: on their mouth parts, through their vomitus, on their body and leg hairs, on the sticky pads of their feet and through the intestinal tract when they pass faeces. Over 100 pathogens, including bacteria, fungi and parasites, have been associated with house flies [7]. 

It’s their tendency to travel between areas of filth, such as garbage bins which may contain used nappies, dog waste or rotting animal and plant material, and then enter houses to feed on any exposed foods that makes flies the ideal carrier for microbes. Environmental sanitation has improved significantly over the years and we now have effective sewage treatment plants and regular rubbish collection. But flies are opportunists and can reproduce incredibly quickly when they find a suitable breeding source. And we humans still tend to provide those conditions in our back yards – an overflowing bin or open compost heap here, dog or chicken waste there…  Even if our own backyard is scrupulously clean and tidy, the flies can fly in from the neighbours place or the local tip miles away.

It wasn’t long ago that flies were considered by some to be “the most dangerous animal on earth” due to their ability to transmit diseases. Perhaps that’s why we are so quick to reach for the swat.  Even though generally, environmental hygiene is so much better now, it still makes sense to do whatever we can to reduce their numbers and keep them out from our homes.

Follow these tips to help prevent a house fly infestation:

  • Ensure kitchen bins have lids and are kept as clean as possible
  • Keep kitchen benches and other surfaces clean
  • Eliminate leaks and other sources of excess moisture
  • Dispose of waste regularly and store outdoor bins away from your home.
  • If you have dogs, horses or other domestic animals, remove and dispose of faeces regularly.
  • Set outdoor fly traps near your bins or other areas such as compost bins or manure heaps where flies may accumulate.
  • Keep doors, windows and vents closed whenever possible, or ensure mesh screens are fitted and in good condition.
Bibliography
[1]H. Sanchez-Arroyo and J. L. Capinera, “House fly, Musca domestica Linnaeus (Insecta: Diptera: Muscidae),” Institute of Food and Agricultural Sciences (IFAS) Extension, University of Florida, pp. 124-230, June 2014.
[2]K. Lam, C. Geisreiter and G. Gries, “Ovipositing female house flies provision offspring larvae with bacterial food.,” Entomologia Experimentalis et Applicata, pp. 292-295, 2009.
[3]D. Nayduch and R. G. Burrus, “Flourishing in Filth: House Fly – Microbe interactions across life history,” Annals of the Entomological Society of America, vol. 110, no. 1, pp. 3-18, 2017.
[4]K. Zurek and D. Nayduch, “Bacterial Associations Across House Fly Life History: Evidence for Transstadial Carriage From Managed Manure,” Journal of Insect Science, vol. 16, no. 1, p. 2, 2016.
[5]E. Hatch Jr., “The house fly as a carrier of disease,” The Annals of the American Academy of Political and Social Science, vol. 37, no. 2, pp. 168-179, 1911.
[6]Museum of Applied Arts and Sciences, “Flies have dirty feet”, Sydney: VCN Blight, Public Printer, NSW Department of Public Health, c. 1955.
[7]F. Khamesipour, K. B. Lankarani, B. Honarvar and T. E. Kwenti, “A systematic review of human pathogens carried by the housefly ( Musca domestica L.),” BMC Public Health, vol. 18, no. 1, p. 1049, 2018.

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