They can bring not only disease to the chicken coop, but also one of the deadliest conditions to affect chickens. A condition that happens quickly, often without warning, and kills if not treated.
In this article we'll examine what it is, why it happens and how it should be managed.
And to understand that, it's critically important to look at some information about flies in general.
So we'll start there.
How long do flies live, where do they come from and how do they reproduce?
The average house fly (musca domestica) can live for up to 28 days - not long, you might think.
Except that during that time, they reproduce by laying at least five or six batches of eggs, each containing between 75 and 150 eggs(1).
So, potentially almost a thousand eggs from just one fly. And the eggs hatch within between just eight and twelve hours.
Which is why they need to be stopped quickly. One fly can very quickly become an infestation.
There are four stages in a fly's life cycle: egg, larva, pupa and adult(2).
In order for the eggs to hatch, they must remain warm and moist. So the fly will lay on any surface which is damp to the touch. They're particularly fond of decaying matter, so they'll use garbage, manure, compost, and any kind of animal waste.
They also love moist food.
So a fly which has been on animal poop a few minutes ago will have no problem about depositing bacteria and laying its eggs on your unrefrigerated food.
Or on your chickens.
Contrary to what you'll see on some websites, house flies do not bite. Horse flies and deer flies do, but not house flies.
A house fly transmits disease in a different way.
While it's there laying its eggs, pathogens (diseased matter) from the original source - let's say your fly had been on some dog poop - will be regurgitated or excreted from the fly onto your food. Or your chickens.
Which is why, even though it doesn't bite, the common fly is no good either for you or for your flock.
We've seen that flies like warm, humid conditions to lay eggs and the eggs need those conditions to turn first into a larva (the maggot), then become a pupa, which finally hatches as another fly.
What better place than a chicken coop in the summer. Warm bedding, particularly if it's kept moist by spilled water, rain or droppings, is an ideal place to lay their eggs.
But that's by no means the worst of it.
Here's what happens...
It can happen in rural, tropical regions and it's not unknown in war when wounds go untreated. Other than that, flystrike in humans would be very unusual these days.
It is, though, a common affliction in other mammals, particularly horses, rabbits and sheep.
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As with any health related problem, prevention is far better than cure - and fly strike cannot always be cured.
Given that it's caused by flies, the obvious way of preventing it is to get rid of flies from your coop.
This article discusses the pros and cons of fourteen different and complementary ways of preventing fly strike in chickens.
Don't wait until your flock is struck. Act now.
A lot of "facts" you'll find on the internet are often people's individual views, based on inaccurate information repeated from poor quality sources.
The information I provide in this article and others is based not just on my own experience, but on evidenced facts from scientific, peer-reviewed research and books from highly respected and experienced poultry keepers such as Gail Damerow.
Some of the trusted sources I have used in this article are these.
1. Rentokil: Flies. Updated annually.
2. Sanchez-Arroyo, H., and Capinera, J. L.: The House Fly. Pub. University of Florida, Department of Entomology and Nematology.
3. British Hen Welfare Trust: Fly Strike.
4. Loftin, K. L, et al: Biology and Control of Flies in Poultry Facilities. Pub. University of Arkansas, Department of Agriculture.