Pesky Worm Bin Varmints – Fruit Flies

Fancy Schmancy Fruit Fly Trap

There is one thing I need to get off my chest right away – I’m far from perfect when it comes to vermicomposting! Despite my best efforts (or due to laziness) I often run into some of the same issues as most people new to the hobby.

There I said it!

In a lot of ways I just don’t “sweat it” when it comes to a slightly out of balance bin. Worm composting is not meant to be “perfect” according to human standards – it is already a demonstration of the perfection of nature itself. If conditions in the bin change, the ecosystem inside typically changes in response. Sometimes this results in new populations of different critters.

A perfect example of a ‘critter’ that can make an appearance in your bin from time to time is the fruit fly (Drosophila melanogaster). Not surprisingly, fruit flies and other flying ‘varmints’ tend to annoy worm bin owners more than most bin creatures because they easily escape when the bin is opened. Add to that the fact that fruit flies can easily survive the conditions outside the bin (an exception to one of the rules in this post), and it’s no wonder people are frustrated by their presence.

Well have no fear, there are some solutions!

One simple practice that tends to help a lot is making sure to maintain a thick layer of bedding material over top of the main composting zone. This alone will go a long way towards eliminating fruit fly infestations. Also, it will definitely help if you bury your food scraps rather than placing them on the surface.

If you seem to still have recurring fruit fly problems in your bin or around your house in general I would definitely recommend using fruit fly traps. These are very simple in principle – basically, they are made up of an enclosed reservoir of some tempting liquid (apple cider vinegar is ideal), with a downward funnel and small hole where the fruit fly can crawl through. Once inside, it is virtually impossible for the flies to escape.

Out of curiosity a number of years ago I purchased fruit fly traps (see above picture) and have been very happy with them. The vinegar inside will eventually evaporate away, but you can simply refresh the supply by pouring some in through the hole.

Ugly Homemade Fruit Fly Trap

You don’t have to buy traps though. This morning I attempted to make one myself (pictured to the right) – while it certainly doesn’t look pretty, it is very easy to make your own using a cup and a piece of tin foil (perhaps an elastic as well). The advantage of the homemade design is that you can take off the funnel and periodically clean out the contents.

So there you have it! Fruit flies in your worm bin can certainly be annoying at times, but there are definitely ways to fight back!

Stay tuned for more posts in my “Pesky Worm Bin Varmints” series!

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    • Linda
    • June 28, 2008

    I started my bin a week ago in anticipation of my worms an entire swarm of fruit flies set up camp. The apple cider vinegar smells exactly like what they love–a little sour a little sweet and it’s pretty clear that it works because after less than 24 hours I can’t find a fruit fly flying around–they’re all trapped in the cup. A ziplock bag also works really well as a funnel

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    • Mike
    • June 4, 2010

    I’ve now got a huge fruit-fly infestation after accidentally leaving my bin open for half a day. I’ll try the traps, but more important to me is this question: Are fruit flies harmful to the worms and composting? Or just an annoyance?

    — Mike in San Jose, Calif.

    • Will
    • February 8, 2011

    FYI a rock solid fly catcher can easily be made from your favorite (plastic) bottled beverage. Cut said plastic bottle about 2/3 up from the bottom, puncture the cap with a small hole, invert capped top 1/3 into bottom. Voila!

    I hope a layer of paper will solve our problem, but how do i drill more holes when the thing is full of food, compost and flies? Yuck.

    W. in Sunnyvale.

    • Kristina
    • March 24, 2011

    Actually you don’t need anything fancy to catch fruit flies at all. A shot glass, some apple cider vinegar, and a drop of dish soap (not dishwasher soap). The soap breaks the surface tension on the liquid so the flies can’t just stand on top of the vinegar, therefore they get wet and can’t fly away. I use an old egg dipper from one of those dye sets you get around easter, to suspend the glass right into the bin, takes care of my issues with no problems.

    • M@
    • December 7, 2011

    I have used the 1/2 balsamic vinegar 1/2 water with a few drops of dish soap in reused plastic food container. But I wanted to find a less lethal course. And I found this on the MA Dept. of Environmental Protection webpage.

    Fortunately, fruit flies have an excellent sense of smell and are strongly attracted by bananas. A simple, nontoxic, inexpensive, humane way to trap them is to place a banana peel inside a clear plastic container and make three or four holes in the cover with a standard round toothpick. Be sure to pull the toothpick all the way through the plastic and wiggle it around to make a hole large enough for a fruit fly to crawl through. Place the plastic container in or near the fruit bowl, not inside the worm bin. (If the worm bin is not in your kitchen, place the fruit fly trap on a surface above the worm bin; if it is inside or too close to the worm bin, the odor of the banana peel will not be distinct enough to attract the fruit flies as effectively). Within 24 hours, about 99 percent of the fruit flies will be inside the plastic container, having entered the holes and not found their way out. Each day, take the container outside and release the fruit flies, unless you are a biology teacher or entomologist and want them for genetics experiments. After three or four days, the fruit flies will be gone (if no additional banana peels or other potential source of larvae have been added to the worm bin).

    Some species of fruit flies are larger than others. If you see fruit flies crawling around on the surface of your plastic container but not going inside, make the holes larger.

    • ShereeLynn
    • May 10, 2013

    Fruit flies have been my main concern as we have an infestation in the house every summer to begin with. We already have the store bought traps around the house; this post was an extreme stress reducer. Thank you again for all your great information!

    • Dan
    • January 16, 2015

    Bentley, have you discovered any new ways to kill off fruit flies? I’ve never them take over worm bins before, and I have some big ones, but this fall and winter they just won’t quit. We’ve killed thousands with traps and vacuuming. (Found out they like beer, soy sauce, vinegars, wine,…) I’ve moved all the compost and worms to one giant Rubbermaid storage container (it has vents) and covered it with a fairly thick painter’s dropcloth with the ends tucked under the bin. We’ll see how this goes now.

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