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Fungus Gnats: Tiny Flying Houseplant Pests

Q. A friend gave me a large tropical houseplant in December. With it came very small, dark, flying bugs. Is there any non-chemical solution I could spray that might eliminate these little pests? Thanks!

    ---Merrily in Severna Park, MD

How can I get rid of gnats on my houseplants and prevent them from coming back? I don't overwater my plants so I don't know where they're coming from. I've tried sticky traps, red pepper, water that cigarettes had soaked in (which I heard about on a TV show), insecticidal soap, and Raid House and Garden spray. These gnats are really getting on my nerves! Please help.

    ---Valerie in Bowie, MD

Mike: I've had a horrible fruit fly problem for six months now. I believe the pests came from an ornamental orange tree, which I removed months ago. I've bleached my cabinets and counters and poured vinegar down the drain, but nothing works. Please help!!!! I can't even enjoy a glass of red wine with dinner without fruit flies landing in the glass!

    ---Melissa from Fairfax, VA

A. Wow! It's toxic home-remedy week here at YBYG!

Melissa—were you hoping to achieve bleached white fruit flies? I'm as protective of my old vine Zinfandel as anybody, but please—everyone—don't use bleach! Although some addle-brained garden and food writers suggest it for everything short of mouthwash, bleach is dangerous stuff. Its risky to handle, the fumes were used as deadly chemical agents in World War I trench warfare, and it creates cancer-causing dioxins. If you need to disinfect a surface, use soap and hot water. If that's not enough for your peace of mind, plain old white vinegar kills germs with the best of them.

It also controls fruit flies. Fill little dishes with vinegar (or that red wine that turned out to be too gnarly to drink; [and yes, I do mean "Yellow Tail"]); the fruit flies will drown themselves in the liquid. And just keeping all the fruit and vegetable matter off of your counters for a few days is often enough to break the flies' very short life cycle.

But you mention that they first came from a potted plant, and that makes me think you are actually a member of our fungus gnat trio—especially if you have other plants in the house that the gnats could have migrated over to. As our good friend Bill Quarles, director of the Bio-Integral Resource Center ( in Berkeley, California notes in a feature article in the latest issues of the BIRC's fine journal, Common Sense Pest Control, "fungus gnat" is an all-inclusive name for a huge number of similar-looking small flying creatures that breed in houseplant soil.

Bill explains that, like whiteflies, the gnats are attracted to yellow sticky traps, which are available mail order and at most garden centers. Place the sticky yellow rectangles on their holders and push the stakes into the soil. As each generation hatches, the annoying-but-harmless adults will get stuck and be unable to mate. When all the luckless gnats have left the soil, your problems will be over—at least till you bring new plants in.

These pests are always being re-introduced because they are epidemic in greenhouse situations. Smart growers now control them with beneficial nematodes, which is also another home cure. Water some of these microscopic predators into your houseplant soil before you release the rest outside to control lawn grubs and flea larvae in your yard.

Or use BTI—the non-toxic, naturally occurring larvicide used in standing water or on wet patches of ground to prevent mosquito and black fly problems; homeowners can use the same BTI to kill baby gnats down in the soil (where they are probably chowing down on your poor plants' roots!). Almost everyone carries the doughnut shaped BTI dunks, but look for the granular form; it's best for this use. Or use one of the fungus gnat specific BTI products that are EPA approved for use as soil drenches for controlling the pests.

Or sprout any grain—cat grass from a pet shop, wheat grass, etc.—in a pie plate filled with soil near your plants. The gnats will fly into the lush young grass, which you then throw away. Repeat this until you've gone through all the life cycles in the soil.

And while overwatering doesn't CAUSE the gnats, they do love moist soil, so having a light hand with the water can help minimize the problem. As can a trick that's 90 years old! As Bill reports, it was discovered back in 1916 that a one-half inch layer of sand on top of the soil is an excellent deterrent to adult egg laying. Prevent the eggs, and you prevent the larvae, and thus the adults. Probably looks nice too.

By the way, if you want to see what those nasty larvae look like, Bill says there's a newly devised (1997) way to do so. Slice a potato into chunks at least one inch in diameter and one half inch thick, and imbed them in the soil of your houseplants. Remove 48 hours later and look for nasty little quarter-inch long wormy things with black heads.

And finally, remember "Kitchen Sink" Val in Bowie, who said she had tried EVERYTHING to get rid of the pests, including {quote}: "water that cigarettes had soaked in"? Now, you know that I'm always urging allayouse out there to garden without chemicals, but some of these ill-advised "home remedies" are more toxic than the most egregious commercial pesticide. Never soak tobacco products to make nicotine tea!

Cigarettes are bad enough when their smoke is inhaled! Soak them in water you create a very nasty toxic brew that could easily send you to the hospital. Nicotine has been illegal for garden and farm use under federal law for many decades—and for good reason; get just a little bit on your skin and you risk serious and immediate health problems.

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