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Small Flies/Gnats Infesting My Worm Bin


March 14, 2024

I’m hoping you can help me solve a problem with my worm bin. This is my first year composting, and it started out great. The worms seem to be very happy. However, once my worm bin was full and I stopped adding to it, a large population of small flies/gnats (not sure exactly what they are. But smaller than fruit flies) moved in. I’ve put an apple cider vinegar/dish soap bowl in there to trap them. But I feel like I need a stronger approach. Any advice? [Video was attached for reference. Not included here.]


Answer by YW:   Thank you for reaching out. Glad to hear your wormies are happy! I have several composting bins in my backyard, and I have noticed a large population of flies appeared in/around the bins since the beginning of last month. My question to you is: Do they bother you? For example, do they follow you into the house? Do they bite? Are they causing any issues outside of the compost bin? The reason I’m asking is that flies are great decomposers. When the compost pile conditions meet their needs, these hungry creatures can always find their way to food. With the temperature being in the mid 50s and 60s during the day recently, they are thriving. That said, acceptance is one approach.  Alternatively, you can try a few things to reduce the flies population: 1) aerate your pile - something could be rotting inside which is attracting the flies. Airing the pile could speed up the decomposition process. (I recently turned all my piles, and it seems that the the flies population has reduced a lot already.) 2) reduce feeding - looks like there are still plenty of food for the worms to consume in your video. Wait it out. 3) add blankets on top of the piles. Cardboard or trash bags would work great. (Looks like you might already have one) 4) when you are ready to add scraps in again, cut them into smaller pieces and mix in some browns (leaves, paper, etc).

Hope it’s helpful, I’m curious if other MCs have any suggestions. BTW, what’s the brand of your bin? Is it Subpod?

Happy composting!

Response from Original Questioner: Thank you!

They aren’t biting or following me inside - mostly just bothering me by swarming up into my face when I open the bin. But I’m glad to know they aren’t hurting anything and are potentially helping!

I started composting in the fall, so I haven’t yet done a full year yet. I mostly wanted to make sure it wasn’t something I needed to get under control before it got (more) out of hand. But since it sounds like it shouldn’t be an issue worth worrying about, I will wait it out.

On the subject of waiting it out, do I need to do anything specific with my bin once it is full? Do I need to continue adding browns occasionally? Or just aerate it every now and then until it is finished?

And yes, it is indeed subpod! I’m glad I was able to snag one before they went out of business.

Thank you for your insight!

Answer by MG: The small flies sound like fungus gnats, but I have always had a hard time identifying those flying critters.

YW is correct -- it looks like you have a whole lot of food in your bin, so you will, of course, attract lots of "visitors" to your bin.  I have gotten fungus gnats even in my indoors bins and have, pretty much, beaten them by using two approaches.  The vinegar traps with a drop of soap in them like you mentioned, and a vacuum hose, always plugged in, always standing by.  That would probably not be practical for an outdoor bin like yours, which, by the way, looks wonderfully productive!

What should help, over time, is to always have a thick layer of DRY shredded cardboard (or some other dry mulch not attractive to bugs) on top of your worm habitat.  It's amazing how much a 6 or 8" layer of dry material will discourage visitors.  They are always going to be attracted to anything moist, especially in our climate.  Having to burrow thru a lot of dry stuff will cut way back on your visitors.  But keeping the material dry means a heavy lid will always be required, or thick sheets of plastic always held down with bricks or boards.

The sad, but natural, bottom line is that a worm bin outdoors is ALWAYS going to have uninvited guests, which is part of the reason I stopped using my outdoor bins.  Like YW said, many of them are doing some composting on their own, but it's not much fun to deal with them as you built your accommodations for worms.  And this disappointing fact is true even for indoor bins, because there are always teeny eggs laid on the skins of fruits and vegetables that you feed to your worms.  You usually just have to decide what you can live with and what you can't.  (I couldn't deal with the zillions of rolly bugs and earwigs outdoors!)  One thing to look forward to is that when you next harvest your worm castings, that will be a good opportunity to start over with fresh, uninhabited bedding material for your worms.  The only way to MAYBE keep your bin somewhat critter-free is to use something like an old freezer as a bin, with its hinged door kept closed, to raise your worms in.  This will keep the visitor population way down.  And normally, there is enough oxygen for your wormies, provided you never allow the worm habitat to go anaerobic.

Your worms look healthy & productive, so don't get discouraged!  Just choose your battles.  If visitors to the bin aren't hurting your worms (or you), you might just have to live and let live.

Worm regards ...

Response by Original Questioner: Thank you, MG! That is also very helpful information. I’ll definitely try adding more dry material! Your suggestion of the cooler also made me think to check on the settling of my subpod - and sure enough, the side worm holes had become exposed over the soil. I think that’s likely how more guests were able to move in, since the subpod has a pretty firm closing lid like a cooler.

I appreciate all your guys’ help! Very beneficial. Also I just saw the Composting with Worms class coming up this month, so I went ahead and registered. Thanks again!

Response by YW: I’ve been following Subpod for years, and I really enjoyed their educational content and the composting advocacy. Their products are definitely not designed for NM climate, since they are based in Australia. Yes, way too many holes! 

I forgot to mention the class in my last email, but glad you signed up! See you then. :)

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