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A few weeks ago I put up a compost bin in my garden, similar to this one (bottomless and placed directly onto the soil). Soon after I began to use it, I noticed swarms of fruit flies inside.

The first thing I added to my compost was shrub and other plant clippings. Other than that, it gets mostly bad apples from my apple tree, as well as fruit and vegetable scraps (including leftovers from the apples I process), some egg shells (crushed), spent coffee grinds and occasionally a piece of stale bread. I have someone to mow my lawn and take out the clippings, so they don’t go on the compost. There may be leaves in fall, but that will take another month or two. Other than these things, I do not add anything that has been cooked, marinated, fermented or treated with pesticides, and make sure all fruit/vegetable scraps are domestic

Obviously, rotten apples will attract fruit flies but I have to dispose of the apples somehow, and would rather return the nutrients they contain to the soil. So I am looking for ways to contain the fruit flies.

Since I noticed the flies, I have added some compost starter (microorganisms to speed up the decomposition). I also started mixing in finely shredded cardboard and small wood chips in order to balance out the carbon-nitrogen ratio as well as moisture. I also tried sprinkling some lime on top, which one source recommended specifically against fruit flies.

There are still plenty of flies, but I only started the countermeasures recently, so results may not yet be visible.

As for climate conditions, I live in the northeastern EU, and we are experiencing a warm summer with daytime temperatures frequently around 30°C.

Can I reasonably expect the countermeasures I tried to help me get rid of the fruit flies in my compost? If not – short of disposing of the entire compost and starting over from scratch, what gives?

3 Answers 3

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You've taken the right measures to help as much as possible, but in future, chop the apples into very small pieces so they biodegrade more quickly. They ferment before they rot down completely, and that attracts not just fruit flies but wasps; obviously whole apples will take a lot longer to disappear. Any fruit scraps, when it's hot particularly, will attract fruit flies, but they will certainly disappear once the weather cools down during autumn. One other thing to check - as it's been hot for so long I assume, like everywhere else in this part of the world, you've had little to no rain, so if the pile is very dry, add some water to help with the decomposition process and turn the pile twice a week if you can.

UPDATE: In answer to your comment, yes, turning absolutely makes a difference to composting materials if they are in a covered bin. You don't have to do it, but turning compost means it heats up more and decomposes faster - this is what's known as aerobic composting. If you just leave it sitting there, that's anaerobic composting, so it doesn't heat up much and takes a lot longer. The additional heat generated in a turned heap will kill off most pathogens, whereas a cold, anaerobic heap of compost retains active pathogens. This isn't a concern if you want to use the resulting compost on open soil, but it won't be suitable for adding to pots for plants. If the compost gets hot enough, it will also kill off most seeds so you don't get unwanted growth when you do finally use it - the opposite is true of anaerobic cold compost bins. As for making sure the compost isn't really dry, that's essential too - moisture is part of the composting process, and worms, bacteria, fungi and other organisms need it to break down the contents of the pile. Really dry compost bins or heaps often get occupied by ants who make their nests in it, so just check it isn't too dry.

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  • The weather has indeed been mostly dry, but since the compost is in a covered container (see linked image), there should be little evaporation. (And wouldn’t dry compost inhibit insect growth rather than facilitate it?) Also, does the advice on turning the pile also apply to a covered compost bin?
    – user149408
    Aug 30, 2022 at 13:29
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    See updated answer...
    – Bamboo
    Aug 30, 2022 at 16:36
  • Fast forward a year. I’ve been careful to chop up most things I added into the pile, add in sources of carbon (wood chips, chopped up cardboard and wood ash), and to turn (and loosen) everything regularly using a garden fork. This summer I have had hardly any flies, the pile is noticeably getting warm and the odor is close to the “earthy” note it is supposes to have. So, the combination of providing a source of carbon along with turning/loosening (for which the garden fork is indispensable) seems to have done the trick.
    – user149408
    Aug 25, 2023 at 18:06
  • Good news, thanks for the update!
    – Bamboo
    Aug 25, 2023 at 22:13
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In addition to all the great advice I would like to insist one of the best "emergency carbon sources" is wood ash.

It assimilates immediately, unlike wood chips or leaves.

A tiny BBQ fire for 2 people generates enough ash+embers that when thrown into the bin they cover everything in a thin film thus isolating the fruit flies form their dinner. Just be sure it's cooled enough (the largest ember should NOT be warm when grasped in hand). Try to do it before it rains as water removes all the Potassium(K) from the ash. Obviously make sure no plastics like cigarette buts go in the fire.

Because wood ash has a high char content, it can be used as an odor control agent, especially in composting operations.[11]

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First, try to ignore the fruit flies - they're harmless and all part of nature's rich tapestry :-) Second, ask the person who cuts your grass to leave a bag of clippings behind every now and then. Spread the clippings over the contents of your compost bin (ie try to bury the rotting apples under grass clippings).

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