Dealing with the fungus gnat.

Written by Matthew on January 19th, 2010

Thanks to Jean for inspiring this post,

Say no to bugs

Image by Steven Brown

It is not my goal to eliminate all critters from the worm bin, nor would that be desirable. The goal here is to keep bins maintained in a fashion that prevents mass numbers of fruit flies and other such critters from forming. Most of the creatures is a bin are very beneficial, especially from a composting point of view.

Dealing with pest such as the pesky fungus gnat we must take their life cycle into account. Fungus gnats eggs incubate for about a week then they stay as larva for 2 weeks. During this time they remain in the bedding of the worm bin. After the two weeks they become an adult and live for a week, lay more eggs and die.

I call the following the “Shock-and-Awe” approach. What we must do is break the before mentioned cycle. To do this wage an all out war one day and kill as many of the adults as possible on that day. Then any more that hatch will not be laying till 7 days later. Whew! So we can now rest easy and relax knowing that those fungus gnats are laying potentially millions of eggs, no more nightmares of horror film sticky traps. But exactly 7 days later, no later, we must wage war again because those new hatchlings are going to start laying. So after doing this the second time in theory you should not have had any eggs laid for 14 days. Awesome, we are almost there since their life cycle is 4 weeks. Do this once more, not waiting for more than seven days, and you should have significantly reduced their numbers. After you get there numbers down you can provably keep them down and reasonable by keeping something on the surface to obstruct their access to the food supply. A thin layer of bedding works for this.

Now that leaves one unanswered question, how do we kill then without killing the worms. They are several things are effective that should not hurt the worms. A very mild soap solution sprayed lightly over the surface of the bin should be effective without hurting the worms. I suggest testing this out on a small sample from the bin first just to be sure. There is also some bacterias that will kill fungus gnats but I have no personal experience with it. I think it is sold under the trade name Gnatrol. What I use and personally know to be safe for me and the worms is diatomaceous earth. It is most effective when dry, but this is ok because we know we only need it to kill them on our one day.

Hope this helps out,

Matthew Wilson
Worms Etc


9 Comments so far ↓

  1. Jean Kruse says:

    Matthew -that was the best explanation of how to deal with these pests I’ve ever seen. I do have one question – when I look around down inside the bin I see many of these gnats running around deep inside the vc. Are they going to eventually come to the surface to get zapped by the DE or will they just stay down in the bin and lay their eggs there?

  2. Matthew says:

    It is true fungus flies are very weak fliers and often times they just run around instead of flying. I am not entirely sure of DE effectiveness on fungus flies and gnats. Fruit flies are the main culprits in my bins. Since feeding chickens DE will reduce the common fly population by making the manure uninhabitable, seems to me it should have a similar effect in the worm bin. DE is great on slugs, fruit flies, and common flies, but does nothing at all to ants and a few other insects, so it does have limits.

  3. Jean Kruse says:

    Thanks for the added info -I guess I’ll try DE just because I’m desperate to keep these pests out of my greenhouse. Nice post about worm names-I like the keep it simple approach.

  4. Wendy says:

    I’m wondering what you can tell me about the groups of tiny red bugs in my worm bins. These are smaller than a comma on this page and seem to show up in groups and say that way. I’m not sure, but they don’t seem to be causing any problems in my bins. Thanks for any help.

  5. Matthew says:

    They are provably mites. They will not hurt anything, but if you want to get rid of them you can try adding some powdered limestone to your worm bin.

  6. GARY says:

    I have tiny little white bugs. When I put them under the scope they have ugly feelers, claws and look crusty, but I do not see anything flying. Does anybody know what they could be?

  7. Matthew says:

    Could you email a picture? Maybe spring tails or mites of some kind.

  8. GARY says:

    They are pretty small to get a picture of. I had to use a microscope to see how ugly they are. But this will give me a start. i have been using schreded paper for my bedding and the scrapes for food. So far it seems to be working. Any comments on the schreded paper?

  9. Matthew says:

    Shredded paper makes excellent bedding. Look on google/images for worm bin critters and see the pictures that come up. You will provably find a picture that will help you identify your critter. Don’t worry though, most bugs are a sign that all is going well in your compost bin.

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