Why Is Your Compost Bin Full of Fruit Flies?

Composting is breaking organic matter down through aerobic decomposition into a rich soil amender that is rich in organic matter. While the process can be messy, your compost bin should not be full of flies. 

Your compost bin is full of flies because:

  1. Poor compost balance and composition can attract pests.
  2. Improper moisture levels within compost can attract pests.
  3. Aeration is important for decomposition and pest control.

In this article, I’ll explain why your bin is full of flies in more detail. I’ll also examine how you can get rid of fruit flies (also known as vinegar flies) in your compost bin, so read on. 

Poor Compost Balance and Composition Can Attract Pests

Whether you’re composting with or without a bin, maintaining the right balance of greens, browns, and moisture is essential to keeping your compost pile healthy and free of pests like fruit flies, bugs, ants, and other grubs. 

You should ensure that your composting feedstock has a good balance of browns to greens, in a ratio of 3:1. When compiling your feedstock, you have to consider the size of your operation, the type of composting unit you’re using, and how much time you have. 

If you’re composting in-ground, you have the time to wait about a year or two for your organic matter to break down into compost directly into the soil. You can probably add a greater variety of greens and browns into your pile in such a situation. 

However, if you’re composting with a pile in your yard or bins, you need to watch your feedstock composition closely. The compost feedstock should be made up of greens and browns that can be decomposed safely and fully in your home composting unit to avoid attracting pests like fruit flies. 

A few issues with your compost feedstock balance and composition might cause your compost bin to be full of fruit flies. 

Your Greens Had Fruit Fly Eggs

The greens in your compost feedstock are typically made up of kitchen scraps like rotting fruit and vegetable peels and trimmings. It is not unusual for the produce you bring from the supermarket to already have fruit fly eggs. 

Bananas, tomatoes, melons, squash, grapes, taters, onions, and other unrefrigerated produce are likely to have fruit fly eggs in the skin. As these fruits ripen or ferment in the open air, the fruit flies are drawn to the smell. Female fruit flies can lay over 500 eggs at once, so they can go around from fruit to fruit, laying eggs as they please. 

Throwing food scraps with the eggs to the compost pile filled with decomposing food serves as the ideal space for the fruit fly eggs to hatch, resulting in larvae that appear as white worms in your compost

While the larvae can be helpful to the process of decomposition, they can also overtake your compost pile and treat you to the unsightly vision of grubs crawling all over your nice compost. 

The best way to avoid this is to wash your produce as soon as you bring it home to eliminate any fruit fly eggs. When you add the peels to your compost, they will be free of any fruit fly eggs. 

Adding Coffee Grounds to Compost: Fruit Fly Concerns

Coffee grounds can be added to your compost feedstock easily. 

Coffee grounds can attract flies just like other decaying organic matter. However, they are not any more appealing to fruit flies than any other kind of green matter, and they won’t work as well as bread to bait fruit fly larvae or apple cider vinegar to bait adult fruit flies. 

As such, coffee grounds should be treated like any other green in the compost feedstock, or it will be full of fruit flies. 

You Don’t Have Enough Browns

Browns supply the carbon required for the microbes, which eat the carbon as they break down and process the organic matter in your compost feedstock. Having insufficient browns is one of the main causes of most problems associated with composting

If you don’t have enough browns in your composting feedstock, the greens will not be aerated enough, causing the aerobic bacteria to die. The lack of aerobic bacteria will cause a build-up of gases, and anaerobic decomposition will result in strong smells like rotten eggs, and fruit flies love it. This sulfurous smell will then attract pests to the compost pile. 

Insufficient browns, which means less carbon, can also result in a strong ammonia smell, which will also cause your compost bin to be full of fruit flies as they come searching for food sources. 

You Added Dairy, Meat, or Fats to Your Feedstock

Adding inappropriate materials to your feedstock is one of the main reasons your compost bin is full of flies or other pests like rats, bugs, worms, etc. 

The composition of your feedstock influences how fast it breaks down into compost. 

Dairy, meat, meat trimmings, bone, fats, and fish scraps should be kept out of home composting units. While all these materials are biodegradable, they take a very long time to decompose and release odors when they do.

These odors attract pests like worms, rats, and flies to your compost bin. The odors released by meat and similar materials breaking down may attract scavengers like bears to your compost.  

Your Greens Are Clumping

Cutting up your greens and browns into smaller pieces helps the feedstock break down more evenly. It is also important to layer your greens and browns correctly in the ‘lasagna layering’ method to ensure that the greens and browns are distributed more or less evenly throughout the composting unit. 

When you have large chunks of greens or don’t layer your compost well, the greens can stick to each other as they provide moisture to your compost. This results in clumping, and the larger the clump, the harder it is for microbes to break the clump down. 

The clump then becomes cold and slimy as the process of decomposition slows down and releases bad odors, which in turn attracts pests, bugs, and fruit flies to your compost bin. 

Your Greens Are Uncovered

It is important to ensure that all food is completely buried when it comes to layering your compost feedstock. If your greens are left uncovered, they will attract pests, and your compost bin will be full of fruit flies. 

Covering your compost refers to two things.

The first is covering your greens with your browns thoroughly. The layer of browns should be three times the layer of the greens so that the browns can absorb all the moisture and odors that will be released as the greens decompose. This will prevent flies and other pests from approaching the compost pile as there will be no smell to attract them. 

The second is to cover the compost bin with a tarp or a lid. This coverage will increase the overall temperature of the compost pile, which is necessary for the smaller piles of compost inside a composting bin as opposed to one built outside with yard waste. The increased temperature will deter pests like flies and white worms from inhabiting your compost bin. 

Improper Moisture Levels Within Compost Can Attract Pests

The moisture of your compost pile is essential to deliver good compost and ensure that the decomposition process is active and healthy. While the greens in your feedstock provide the majority of the moisture that your compost needs, you might also need to water your compost as necessary. 

The ideal moisture percentage of finished compost is between 40-50%. Compost with sufficient moisture is usually damp and squishes when you compress it in your hands. Too dry compost is crumbly and dry and usually attracts pests like ants, increasing the compost’s acidity. 

On the other extreme, compost that is too wet is usually cold, soggy, drips when picked up, and may even leak liquids out of the bin and around the composting unit. Overly wet compost that leaks will attract pests, and you’ll find your compost bin full of fruit flies that are attracted to a food source.

Your Compost Is Too Cold and Stagnant 

The right moisture levels in your compost support the microbes that break down organic matter, allowing them to process the compost faster. This process raises the compost temperatures, which accelerates the decomposition of the greens and browns in the feedstock. 

Cold and stagnant compost is caused when there’s too little moisture, which slows the entire decomposition process down. This causes clumpy bits of greens that ferment inside your composting bin and attracts flies. 

Too much moisture has the same effect, with the additional problems of leakage and terrible odors similar to rotten eggs or rancid butter. When there’s too much moisture, the water drowns the microbes involved in the decomposition process, which in turn slows the process down. 

Too little moisture can result in cold, slimy compost that attracts flies to your bin. 

The Compost Is Breaking Down Unevenly

The compost feedstock decomposes unevenly when the moisture in your compost pile isn’t distributed evenly. 

Uneven decomposition is usually caused by the incorrect layering of greens to browns, as there is likely a clump of greens somewhere in your composting feedstock. This clump is likely too big to break down at the same rate as the rest of the feedstock. 

The moisture in the clump of greens further affects the microbes, as the amount of oxygen and airflow around the clump is likely limited. 

So even if the rest of your feedstock is progressing nicely along to decompose into compost, this clump of greens will break down very slowly, releasing odors that are either sulfur or ammonia, which attracts pests like fruit flies to the compost bin. 

Aeration Is Important for Decomposition and Pest Control

Aeration is the key to quick composting. Without air, the microbes that break organic matter down cannot survive, and the feedstock will remain cold, stagnant, or start breaking down under the effect of anaerobic bacteria. 

Stagnation and anaerobic decomposition release strong odors, which attract scavengers like bears and pests like fruit flies to your composting bin. This also leaves you with no compost after all the hard work you’ve put into gathering your feedstock and putting it all together. 

For aeration, you have to make sure that your compost bins have holes for airflow or that your compost is a pile outdoors in your yard. Apart from these obvious checks, here are a few reasons and scenarios how a lack of aeration attracts fruit flies to your bin. 

You Aren’t Turning Your Compost Enough

Turning the compost is a process by which you mix the pile. This could be as simple as turning your bin that is on a convenient, rotatable stand or turning and mixing the pile with a pitchfork. 

If you don’t turn and aerate your compost enough, you’ll limit the amount of oxygen available to the microbes, which will slow the process of decomposition down and result in cold, soggy composting feedstock that attracts flies. 

Turning your compost piles is also important for controlling odors. The breakdown of the greens in your compost will inevitably result in some odor. 

Turning the compost allows these to be mixed in with the browns and introduces fresh air into the pile, absorbing and dissipating the smell. If the odors aren’t controlled, they’ll announce to pests and scavengers that there’s a handy food source, and your compost bin will be full of fruit flies. 

If you find that your bin is overrun with flies and fly larvae, you might have to turn your compost pile daily till you’ve controlled the population. 

Anaerobic Decomposition Has Set In

Not aerating your compost pile enough causes anaerobic decomposition to set in, essentially fermentation. This will attract fruit flies, as they are attracted to the smell of fermented foods. 

When you don’t aerate your compost pile, the moisture level increases, as the excess moisture can’t evaporate. This drowns the microbes that have survived the dwindling oxygen in your compost pile, leaving only the anaerobic bacteria.

Decomposition by anaerobic bacteria is a slow, cold process that releases malodorous gases like hydrogen sulfide. The organic matter is broken down into organic acids and ammonia, adding to the odors emanating from your compost pile. 

While the matter will decompose, what remains will be unusable as it won’t be sterilized like compost is sterilized through high temperatures. The cold and damp of your compost feedstock, which is undergoing anaerobic decomposition, will serve as a good place for flies to lay their eggs, causing your compost bin to become even more full of fruit flies. 

How Do I Get Rid of Fruit Flies in My Compost Bin?

Fruit flies aren’t necessarily bad for your compost. But if your compost bin is full of fruit flies, something is wrong with your compost. 

To avoid fruit fly invasion or get rid of fruit fly infestation, you should check your feedstock and ensure the ratio is 3:1 browns to greens and the feedstock is well mixed and covered. Ensure that your compost is damp but not wet and turned regularly. Finally, cover the ventilation of your bin with mesh and set a fruit fly trap.

We’ve already discussed how you can ensure that you have the right ratio of brown materials to greens and why you need to mix your feedstock well, keep the moisture balanced, and turn it.

However, even if you’ve corrected the issues in your bin, the larvae that have already hatched will grow and continue to feed and propagate in your compost bucket or bin. 

To prevent the flies from laying any more eggs, you can add mesh over the air holes on your composting bin, as these holes are the likeliest entry point for these flies. A mesh will ensure there is airflow in your compost. However, the holes in a mesh are too small for flies to enter, so they won’t lay any more eggs. 

Another way to get rid of or avoid fruit flies is to set fruit fly traps by mixing apple cider vinegar with dishwashing liquid in a jar. You can mix about a quarter cup (60 ml) of apple cider vinegar with a few drops of dishwashing liquid. The flies will be attracted to the smell of the ACV and come to your trap. Then they’ll get sucked in by the weight of the dishwashing liquid. 

Does White Vinegar Work?

Apple cider vinegar is ideal for trapping fruit flies as it smells distinctly of fermented organic matter, which is the smell that attracts fruit flies. 

You can use white vinegar to set up a trap for fruit flies in a pinch, but it won’t be very effective. A better alternative for apple cider vinegar for fruit flies would be any other kind of cider vinegar like red wine or balsamic vinegar. 

Final Thoughts

If your compost heap or bin is full of fruit flies, your greens are likely decomposing unevenly, clumping together, in a higher proportion than your browns, or uncovered. You may have also added non-compostable materials to your feedstock. 

Other problems to check are if there’s sufficient moisture in your pile and ensure that you’ve turned your pile regularly. You may even need to turn the pile daily till you’ve addressed the fly problem. Meanwhile, you should add mesh over your bin’s air holes and set a trap for the flies.

Alexander Picot

Alexander Picot is the founder of TheGrowingLeaf.com and its lead content writer. He created the website in 2022 as a resource for horticulture lovers and beginners alike, compiling all the gardening tips he discovered over the years. Alex has a passion for caring for plants, turning backyards into feel-good places, and sharing his knowledge with the rest of the world.

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