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Worm Bin Benefits are HUGE; Problems are Tiny; Plus: A Word or Two About Drain Flies


You Bet Your Garden
Question of the Week © 2018 Mike McGrath

Note: Hundreds of Mike's informative articles are available (in alphabetical order!) right here at the Gardens Alive website. To find Mike's answers to your specific garden problem, Click here and find your topic (like Weeds, Worms, Rhododendrons...) in our complete alphabetical archive of Questions of the Week.

Worm Bin Benefits are HUGE; Problems are Tiny; Plus: A Word or Two About Drain Flies

Q. Frank in Cherry Hill, New Jersey writes: "A couple of weeks ago you helped me figure out what to do with the very wet worm castings left in my finished worm bin trays. Now a new issue has popped up: a sudden explosion of fruit flies and drain flies. My worm bin is in an unfinished area of my basement, near florescent light fixtures for seed starting. I also have a sink in that area. Seemingly overnight, there were fruit flies and similar flying creatures with slightly longer bodies everywhere. I first tried controlling them with fly paper; the strips were black with fruit flies in a week, but the problem persisted.

"I also put out a glass with some red wine in it; same result. The drain flies were clearly also in the worm bin; I would see them crawling out of the little holes on the top of the bin. After three weeks of changing the fly strips (completely covered in fruit flies each time after a week) and freshening up the wine, I moved the bin into my unheated garage and cleaned out the drain in the sink. All is now well. I think that people should be made aware of this potential pest issue. I am assuming that the fruit flies came from the fruit that I was putting into the worm bin. Not sure why the drain flies were in the worm bin."

A. I have had my "Worm Tower" (stackable trays that the worms are able to move between) for a solid decade now, and I've had this problem maybe three times. The first couple of times I agonized over it and wasted small amounts from some really good bottles of red wine. (Interestingly enough, the better the wine, the more the fruit flies flocked to their drunken death in it. But they, like me, completely ignored the really crappy red wines I had been given as gifts over the years. At least the little buggers have good taste.)

Anyway, one day I had an epiphany and shook some BTI granules into each tray of my worm bin and freshly wet everything down. Within a week there were zero fruit flies. Why? Because BTI doesn't just protect us from mosquitoes, it {quote} "prevents all members of the fly family from successfully breeding in standing water", and the continually moist interior of a worm bin--where the little buggers breed--is almost as wet as water.

Short Diversion: The original Bt, sold under brand names like Dipel, Thuracide and Green Step, kills pest caterpillars and {quote} "agricultural worms" that eat the sprayed leaves of plants. It does not harm bees, butterflies, frogs, toads, people, pets, etc.

BTI, a different strain of the basic Bt bacillus, prevents mosquitoes from breeding in standing water that can't be drained. Female mosquitoes lay their eggs in the water, but the larvae die before they can become biting adults. BTI also does not affect bees, butterflies, frogs, toads, people, pets, etc. But it does affect other members of the fly family that breed in moist conditions, like blackflies, outdoor gnats, houseplant-bred fungus gnats, and fruit flies.

Oh--and drain flies, which are different than fruit flies (they look a little more moth-like) but just as susceptible because they are true flies. In fact, they're more susceptible because they breed in the standing water of the 'trap' that is an essential part of any home drainage system. This U-shaped pipe is designed to prevent sewer gas from entering the home, but it has also 'trapped' and saved many a wedding ring that would have been lost with a straight pipe system.

Anyway, step one is to go under the sink, place a bucket to catch any water, take apart the trap (very easy to do) and clean it well--outside. Then run a bottle brush up and down the sides of the straight pipe to get rid of other gunk that the flies can use for breeding. Put the piping back together, check for leaks and then rub some petroleum jelly inside a glass jar and place that over the drain. Remove it when you do the dishes or whatever, but keep it in place at all other times. Once you go a week without trapping anything in the jar, you'll know you solved the problem.

Drain flies and fruit flies are ubiquitous--they're also everywhere. And I mean everywhere. Although fruit flies often come into the house on store-bought and home-grown fruits, they can also just fly in with you when you open and close the door. Same with drain flies; they're all around us--and once inside will head for the places they like best; drains for drain flies and fruits for fruit flies. (Although I can see drain flies being attracted to a super moist worm bin, so don't overwater it and make sure to keep draining the 'worm tea' from the bottom of the bin.)

Neither is a threat to human health--just an annoyance. So keep BTI granules around to use in your worm bin, keep your drains clean and there won't be any little buggers trying to share your finest wines.

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