Sunday, June 27, 2010

What are those bugs in my compost?

"A nice afternoon, perfect for floating in the pool," I muse. "But first, let's jam out a quick rant about those grubs in the compost pile that gardeners are always fretting about, specifically, the white segmented grubs and those big, buzzing flies, like the ones I have in my Can O' Worms composting bin."

The easy, fast answer about those critters, which are probably soldier flies: they're harmless. Let 'em be.

Ha! Easy, my ass. The sun is setting, and the more that I read about soldier flies, the more I realize that I have a lot to learn. Depending on the composting system, soldier flies and their larvae may be either good guys... or a major detriment. The pool will have to wait for another afternoon. 

Soldier flies do such a good job digesting your garbage, there are websites that will be glad to show you how to build a system to raise soldier flies.

 Soldier flies can be found throughout the U.S., wherever there is moist, rotting food.
The adults look like inch-long black wasps and do a fair share of loud buzzing when disturbed. But the soldier fly is you. They do not bite, they only live a couple of days, usually die soon after having sex and laying eggs on the tops of partially composted kitchen scraps. Some critters have a nice life.
The soldier fly larvae - the maggots - are also about an inch long, gray-white, and segmented. When exposed to light, they will scurry deeper into the compost bin, where they are voracious feeders on decaying, nitrogen-rich foods.

Those maggots are very good decomposers or organic material, helping along the process in your compost bin. According to the University of California, the soldier fly also inoculates the compost with beneficial bacteria from other sources.

It is their appetite that is a double edged sword for home composters.

In a regular composting bin, the soldier fly larvae are a welcome addition, speeding along the decomposition. 

Can O' Worms
But in a vermicomposting system - a worm bin such as the Can O' Worms - soldier fly larvae are competing with the worms for food. Plus, they consume so much material, they generate a lot of heat. And in the summer, that may be more heat than your composting worms would be willing to tolerate. Temperatures over 90 degrees will bring worm work to a near halt in a worm bin, or worse, unless you work to keep them cooler.

For worm aficianados, the University of California offers these tips if you want to remove the soldier flies and their larvae from your vermicomposting system:

• Make sure you have enough leaves, dry grass, shredded paper and other organic "brown" material in the pile to cover the nitrogen food sources by at least two to four inches. 

• Be sure to bury food scraps deeply in the pile and cover them well.  

• Put window screen over any holes in the bin and glue it down with a waterproof caulking (like an exterior household caulk) on the inside of the bin to help exclude the flies in their egg laying stage.

• Birds love soldier fly larvae so you can remove them and feed them to chickens or just toss them on the ground and other birds may find them.

The Tumbleweed
And now that my pool time today has been usurped by studying the soldier fly, I'll take out my revenge in the evening: remove the maggots and put them into another of my composting systems, my favorite, the Tumbleweed, which produces usable compost quicker than the Can O' Worms...but the Can O' Worms is more fun! Even with the maggots.

More information about small scale vermicomposting from the University of Hawaii 

A good book about vermicomposting: "Worms Eat My Garbage" by Mary Appellhof.


  1. I'm noticing that the soldier fly larvae is only an issue in my worm bins if I put in a lot of fruit scraps- especially melon rinds. I was considering just putting the fruit in my compost bin and leaving the vegetables scraps for the worms (mostly because I am too lazy to bury the scraps). Do you think that would help the problem?

  2. Burying those scraps a couple of inches deep can help keep soldier flies away. If you don't want to bury the scraps, then shred some newspaper to put on top of the fresh scraps, maybe keep the shredded newspapers handy next to the bins to remind you to cover new scraps with the newspapers.

  3. Thanks for the info - having a hard time with soldier fly lavae in RSA

  4. Thank you so much for your post! I just started my new tumbler compost system a month ago and I was wondering what the heck I was seeing in there!

  5. Thanks for that!

    Didn't know how to feel about those grubby fellas when I first saw them!!

  6. Mystery solved. Great article. Thanks!

  7. It's been so many years since you posted this, but thanks for the post and for keeping this up! I was wondering what was going on in my compost and you helped me solve what I thought were wasps and maggots! Appreciate your work!