Flies and backyard chickens are never a good combination.  Not only are they a nuisance to you and your flock but they can also lead to some serious problems. There are two types of flies that can affect chickens and they are categorized as biting and non-biting.  The non-biting flies are called filth flies.  Your typical housefly falls into this category.  The biting flies are typically found near water sources.  Biting flies that mostly affect chickens are black flies and biting gnats.  Black flies are also called “buffalo gnats” or “turkey gnats” while the biting gnats go by  “no-see-ums”, “sand flies”, and “midges”.

Filth flies can cause the following trouble in chickens:
They are a nuisance.
They can lead to tapeworm in your flock as your flock ingests them.
They can spread pathogens that cause Exotic New Castle Disease and Caronavirus.
They can spread bacteria including Salmonella, Campylobacter, E. Coli, and Listeria.
They can lead to disputes between neighbors due to excessive fly populations.
They can leave spots on your eggs.

Black Flies/Biting Gnats can cause the following issues:
They can cause anemia/death- if a large swarm is present.
They transmit leucocytozoonosis.
They cause flocks to become restless.
They lead to decrease in your flock’s appetite.

Prevention:

Filth Flies:
1.  Keep the coop dry.  Flies love moisture.  Repair any known leaks in and around the coop where rain sneaks inside.
2.  Keep rain water from puddling in the run. Clear drainage areas from being choked by weeds.
2.  Promote good ventilation.  Fly eggs need a moist environment to thrive.  Drying out the coop air, manure and bedding helps to cut down on the number of hatching eggs.
3.  Be sure to clean the coop well when you change the coop’s shavings out.  By leaving just a bit of old damp bedding and manure, you are leaving behind hundreds of fly eggs.

Black Flies/Biting Gnats:
1.  Apply fine mesh screens to your chicken coop windows.
2.  Attempt to control larvae in the Spring through the use of pyrethrum.  Pyrethrum is a natural plant based insecticide made from the Chrysanthemum family.  Pyrethrum should not be used after the Springtime.  Flies can become resistant to it when it is used on a long term basis.
3.  Eliminate any stagnant water sources on your property.

Treatment:
1.  Keep coop and run dry.
2.  Try using fly sticky tape ribbons hung in the rafters.  Replace periodically.
3.  Add food grade diateomaceous earth (DE) into the chicken coop bedding/shavings and in the dust bathing areas.  Please wear a mask to prevent breathing in the DE dust. It can cause a chronic lung condition called Silicosis.
4.  Provide fresh clean drinking water daily.
5.  Keep soiled shaving and manure removed from the coop away from where the chickens can access it. This removes the fly eggs and larvae away from the coop.
6.  Clean up food and water spills.  Be sure the litter underneath is dry.
7. Harvest eggs promptly.
8.  Try using a fan to create a gentle breeze.  Flies will avoid wind.
9. Close the coop door at night to prevent flies from coming inside.


To learn more about why flies in and around your coop are bad for chickens, click here to read about flystrike.


References:
http://www.uaex.edu/Other_Areas/publications/PDF/FSA-7063.pdf

http://www.merckvetmanual.com/mvm/index.jsp?cfile=htm/bc/200106.htm

http://ohioline.osu.edu/b853/b853_2.html

Damerow, Gail. The chicken health handbook. Pownal, Vt.: Storey Communications, 1994. Print.