By: Kristi Waterworth
Worm bins are one of the best gifts any gardener can give themselves, even though they require a fair amount of attention. When worms eat your garbage and turn it into incredibly rich, black castings, there’s lots to celebrate, but even the best worm system is prone to vermicomposting pests. Fruit flies in vermicompost are an annoying problem but, thankfully, they’re not among the more serious pests you’ll encounter during your adventures in worm farming. A few changes in your worm routine should send any accumulating flies packing.
Preventing fruit flies in worm bins is a difficult challenge; most vermicomposters find they simply have to learn to manage these insects. Because fruit flies and worms have very similar needs, it can be a delicate dance adjusting your worm bin to conditions that will completely eliminate or prevent fruit flies. Here are a few tricks that work well to keep fruit fly populations away from your vermicompost for longer:
Feed your worms non-rotten food that’s cut into small pieces. The smaller sized chunks are easier for worms to eat completely before the food begins to decompose and attract flies. Rotten food is a great host for fruit fly larvae, so avoid adding more pests to the pile by feeding only still-edible choices.
Don’t overfeed your worms. For the same reason that rotten food or food cut in too large of chunks is an attractant, overfeeding brings mature flies to the vermicompost bin. Feed a little bit at a time, waiting until your worms have eaten all the food before adding more.
Conceal food items. Make sure to bury your food items and cover the top of the material inside the worm bin with a loose sheet of newspaper. These extra precautions help to prevent fruit flies from ever getting a whiff of the food you’re offering your worms.
If fruit flies become a problem despite good worm feeding practices, you’ll need to get control of them sooner rather than later. Fruit flies multiply surprisingly fast in a worm bin and can soon outcompete your worms for food. Start by reducing the moisture level in the bin, keeping the bedding just moist. Hanging fly paper or installing homemade traps can quickly kill adults, breaking the fruit fly life cycle.
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3If your household composts its kitchen scraps, you may have had problems with fruit flies. These tiny flies are harmless, but they are definitely annoying. And they can invade your bowl of fresh fruit, spoiling expensive produce. Whether you keep a compost pail on your countertop or use worms to break down scraps in an indoor compost bin (vermicomposting), you need to give fruit flies the boot!
If you start fooling around with your kitchen scraps, you will disturb the fruit flies and they will disperse. The first thing you need to do is start trapping them.
The quickest way to do this is simply to vacuum the fruit flies up. Station a vacuum cleaner where the flies are congregating. Switch on the vacuum cleaner and wave the hose in their general direction. Be careful not to vacuum up worms, worm bedding, scraps or water. When the flies come within a few inches of the business end of a vacuum hose, they will get sucked in. This seems to kill them – we have examined a bagless vacuum cleaner, and all the flies were dead. Repeat the vacuum treatment several times a day until the population has dwindled.
Meanwhile, set up a trap to catch the faster ones that outsmart your vacuum. You can buy them at the store or online. Or make your own fruit fly trap (see photo above):
Pour an inch of apple cider vinegar into the bottom of a jar. Add one drop dishwashing liquid. Place a funnel into the jar or make one using a sheet of paper. Tape the funnel in place. Fruit flies will check in, but they won’t check out. They have difficulty flying straight up. Soon enough, they will fall into the vinegar. The soap breaks the surface tension, and they will drown. Leave this near the source of the flies until the problem goes away (at least 2 weeks). You can replace the liquid if it gets dirty. Tip: don’t place it directly in the worm bin. This actually works – see photos at the bottom of this article.
Now that the vacuum and traps are in place, get to the root of the problem by taking away their food. Fruit flies are attracted to a yeast that results from the initial decomposition of plant material. They eat fruits and vegetables and lay their eggs in them.
If you have any fresh or scrap produce on countertops, simply cover them or put them in the refrigerator. Make sure any fruit flies are brushed away or vacuumed first. In the refrigerator, you might need to store them in air-tight containers if they are already contaminated. Kitchen scraps for composting can be frozen.
In your indoor vermicomposting bin:
Fruit fly eggs take around 2 weeks to develop into adult fruit flies. You need to break the breeding cycle by following this program for at least 2 weeks. As soon as you see flies, repeat the vacuuming. Make sure the fruit fly traps are in place. Keep produce off the countertops and bury the kitchen scraps in the worm bin.
Harmless but irritating, fruit flies can’t stop you from having a composting program. With very little effort, drosophila melanogaster can be eliminated from your home.
The worms are most active when the temperature is between 55F and 75F. The most common location for bins is outside in an area protected from direct heat and cold. In cold weather, insulate the bin by covering it with an old sleeping bag, old carpet or even straw bales around the sides and top. In summer, keep it in a shaded area. Smaller bins can be kept in the garage or other spaces that have moderate temperatures. Inside bins need a tray or additional bin underneath to capture leachate.
Most worm farms raise two main types of earthworm: Eisenia foetida and Lumbricus rubellis. These worms are commonly used to produce vermicompost, as well as for fish bait. Both are referred to by a variety of common names, including red worms, red wigglers, tiger worms, brandling worms, and manure worms. These two species are often raised together and are difficult to tell apart, though they are not believed to interbreed. While several other species have been successfully bred in recent years, this fact sheet focuses primarily on the use of these species.
The night crawler (Lumbricus terrestrius) is also harvested and sold for fishing bait. This species does not breed well in captivity and is generally harvested from wild stock.
You’ll need a couple of 5-gallon buckets to make this project. Check the local bakery for inexpensive buckets — they often sell the ones that their ingredients come in for just a couple of bucks.
Worms like it dark. If you opt to use white or opaque buckets, you should cover the worm bin with an old towel or tarp to prevent light from permeating. Black or solid colored buckets will prevent sunlight from bothering the worms.
Not all worms are created equal. For this project, seek out red wigglers. They thrive in worm bins and gobble up the kitchens scraps that you generate.
You can order composting worms online. If you’d like to stick closer to home, though, talk with your local garden club or farmers market people. If you can find someone with an established worm bin, it’s easy for them to pull a handful out to share.
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Once you’ve acquired worms, you’ll start with a handful or two of fresh kitchen scraps and shredded newspaper.
It takes less than half an hour, start to finish, to put this vermicompost bin together.
While your wormery is getting established, don’t overdo it on adding waste. Add 2-3 cups or so every couple of weeks. Once a brown base develops under the paper (this is the worm castings, aka worm poop), you should be able to add a bit more. The worms will multiply based on how much food is made available to them.
This vermicompost bin takes up about a square foot of space and fits under most kitchen sinks. And I promise. If you’re doing it right, it does not stink.
When the top bucket begins to get full, you can add a second perforated bucket. Just add moist newspaper and scraps like you did during set up, and set it onto the stack. Make sure the top bucket rests on the waste in the bucket below. The worms will slowly migrate up through the holes.
When the contents of the first bucket are no longer recognizable, simply pull out the middle bucket and use the rich worm castings on your potted plants or in your garden. The worms will have migrated up and you can use the castings directly from the worm bin.
A worm bin of this size is best suited to a single person or couple. Larger households will generate more waste than this size bin can handle. Simply use the same method with larger Rubbermaid containers. (I’ve done both over the years.)
To harvest the castings without adding a second bucket, dump the contents into a large piece of cardboard. The worms will move down into the castings and unfinished kitchen waste, away from the light. Lift away large pieces of scraps. Skim off the top layer of worm castings using a garden trowel. Wait a few minutes for the worms to move deeper, and skim some more. Continue until you have harvested most of the castings, then put the worms back in the bin with fresh scraps and top it off with damp newspaper.
Q. I've been using worm bins for several years and currently have a fruit fly issue. I have a glass of red wine sitting next to the stack and use a vacuum cleaner to capture as many as possible. I just listened to several of your old shows and loved hearing about Bt. I use mosquito dunks in my rain barrels outside can I also use them to kill fruit flies in the worm composters inside?
---Donna in upstate (Hobart) New York
P. S.: I'm a long time listener and love to disseminate your info while I work (I'm a dental hygienist captive audience!)
A. And they can't talk back I love it!
OK—now she wrote "Bt" but clearly means BTI so let's discuss the difference.
Bt stands for Bacillus thuringiensis, which nobody wants to try and remember, spell or pronounce so we use the shorthand Bt. Thousands of different strains of this soil dwelling organism have been identified and studied over the years, but researchers have only found uses for a handful, which are individually identified by the use of a third letter in their abbreviated name. (BTI, BTK, BTG…)
The original strain, officially known as BTK but often just called "Bt", has been used since the 1920s to control pest caterpillars. You spray it on crops you want to protect and any caterpillars that eat the sprayed plants immediately stop feeding and soon die. But only caterpillars it has no effect on any other living creature.
Then there's BTI. The "I" stands for 'israelensis'. This strain, discovered by a researcher in Israel, has been used since the 1990s, mostly for mosquito prevention. This strain of Bt has no effect on caterpillars, but prevents members of the fly family that breed in wet environments from reaching adulthood.
The most popular use of BTI is in standing water for mosquito prevention. Female mosquitoes will lay their eggs in the water, but the larvae will die before they can become biting adults. It doesn't hurt anything else in the water, like fish or frogs. And yes, mosquitoes are technically in the fly family, a fact I discovered when I was researching the use of BTI against fungus gnats those annoying little creatures that breed in the soil of overwatered house plants…
…and learned that BTI targets many members of the "Diptera" family—what science calls the 'true flies'. It's a HUGE family, with over a million different members, including mosquitoes, biting midges and gnats.
Plus house flies and even horse flies. But thankfully not dragonflies, because these extremely beneficial mosquito eaters also breed in water. But they're in a completely different insect family that BTI doesn't affect. (They aren't 'true' flies it's just a common name in their case.) So you can treat standing water to prevent successful mosquito breeding without harming any dragonflies that may also be breeding in that water.
OK—now back to worm bins and fruit flies! Number one: Don't blame the worms! To misquote Shakespeare, "the fault, Dear Brutus, lies not in our worms, but in our fruit". There is a constant chance of these creatures entering the house on any fruit we bring inside from the store or garden. And they often just fly in with us when we open a door they're everywhere in the outside environment.
I've personally had to fight them twice: Fruit Fly I—the war (I thought) to end all wars and then Fruit Fly II—the Empire Strikes Back. Persistence with fruit fly traps—little dishes of vinegar or red wine set out in the kitchen and near my worm bin—ended the first conflict. The problem returned a few years later, but that time, it started in the Spring—so I just decided to do a worm bin Spring cleaning.
I dumped everything from my worm bins—finished castings and fresh layers of kitchen waste and shredded newspaper—into my outdoor bins (the sealed ones with locking lids to keep out vermin—and in this case, to also maintain the moist environment the worms require). That material finished composting fast, and my shredded leaves really seemed to benefit from the extra worm help.
But I didn't want to have to find a fresh supply of worms later on so as I dumped, I also plucked, and saved a couple dozen worms that I hoped would survive for awhile in a plastic container of moist finished compost with air holes punched in the lid.
Then I washed the stackable trays that make up my worm bin 'tower' over top of my raised beds, so that all of the washed-off worm castings and brown liquid from the washing would feed my plants. Then we just followed fruit fly rule #1 and made sure not to leave any fruit or scraps out in our kitchen for awhile. With no fruit out in the open to support them (we moved our compost crock outside), we were quickly fly free.
After a couple of weeks without a sighting, I decided to start the bin up again—and was delighted to see that my worms in storage were happy and alive.
And this is when I started using BTI. Fruit flies are a lot like fungus gnats and so I figured it had the potential to stop their breeding in a moist environment as well. Now I add a few fresh sprinkles of BTI granules to the bin every couple of weeks. (Our listener could bust up her donut shaped dunks into smaller pieces, but I find the granules to be just the right size.)
And it has worked perfectly. Whenever I prepare a new tray for my Worm Tower, I put in a layer of fruit scraps and coffee grounds and cover it with some shredded newspaper, as always. Then I sprinkle some BTI granules on top and add the water you need to make everything nice and damp in there for the wormies.
There's a spigot at the bottom of the tower that makes it easy to drain off the 'worm tea' that accumulates inside. You can either use this tea as a liquid fertilizer or pour it back into the top tray to keep the insides of the bin moist. I used to use it both ways, but now I always pour that liquid back onto the top tray, because I figure I'm also recirculating any BTI that's still active every time.
I repeat: No more flies. Which is great, because now I know I can recycle my kitchen scraps indoors in the winter again. No more fighting through deep snow to get to my outdoor bins because of fear of fruit flies!