From a Correspondent:
I recently attended a composting class you taught at the library, and started composting about 6 weeks ago. It’s going well, but there are a ton of fruit flies. I stopped putting in banana peels and I started adding more brown yard waste, but they are still a pain. I would appreciate any advice. Thank you so much.
Good morning Dianne,
Aren't those little fruit flies annoying? First, do know that fruit flies are completely harmless; that doesn't make it any more pleasurable to have them zip around and fly up your nose, but at least that's the worst of it. The other good news is that fruit flies thrive in exactly the same conditions as a healthy compost pile so, um, congratulations!
Now, on to business.
With any pest, our best course of control starts with understanding their interaction with the environment in which we find them. That give us the information we need to then alter the environment in such a way as to diminish or eliminate the pest. It might sound touchy-feely to say you have to understand things from the fruit fly's point of view, but it's really rather hard-nosed and practical.
You are correct in fingering the banana peels as being complicit in the fruit-fly issue. Fruit flies love banana peels, and are most commonly introduced into our homes and into our compost piles as larvae that arrive on bananas. From there, they hatch and begin hovering around the ripening bananas and other fruit that may be sitting in the fruit bowl on the kitchen counter, or around the fruit peels that we put in the compost pile. Ripening and fermenting fruit is their source of food, and even though they most frequently arrive on bananas, any fruit will give them an ongoing source of nourishment. Even wine (fermented grape juice, of course) is a great fruit fly attractor, and although the flies are harmless and carry no diseases, there is nonetheless something faintly disturbing about finding fruit flies riding the bubbles up and down in your glass of champagne.
Eliminate the attractors and food sources, and we eliminate the fruit flies.
Increasing the "brown yard waste" or carbon-rich materials in your compost pile is a great first step. This should speed up the decomposition of the fruit wastes in your pile, and also dilute the percentage of fruit-fly attractors in the pile. You have two choices for the next step; you might need to try one and then the other to see what works for you.
First, try withholding all fruit waste, not just banana peels, from your compost pile, and turning the pile daily, or every other day, for the next little while. That should eliminate existing food sources and dry up the fruit-fly population. Plus, it's good practice: any time we have a pest in the compost pile, our first step is to turn the pile more frequently.
The other approach is to continue to add fruit waste, including banana peels, but bury it deeply in the pile out of reach of the fruit flies. Turn your pile before you add new fruit waste, then open a hole at least a foot deep in the center. Place the fruit wastes in the bottom of the hole, and cover it deeply with other material from the bin; this will both exclude it from fruit-fly access and speed up its decomposition, as it will be in the warm and active center of the pile. I tend to favor this approach, as it allows you to keep using your kitchen wastes.
Just so you know, you can also try to trap the little critters. Poke some small holes in a plastic container, place a banana peel in the container, then set the container near the fruit fly swarm. In a day or so, most of the fruit flies will be in the container, and you can dispose of them. This seems like a lot of trouble to me, but if neither of the first options work, give it a try.
For the record, the one thing we can never do for pest control in a compost pile is use any kind of insecticide. There are a number of good reasons why we don't, but, above all, using pesticides indicates a certain lack of creativity.
Do let me know which method you try and how it works for you.
I hope this helps,