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Your Fungus Gnats Are Showing!

9/23/2008 5:02:00 PM

Tags: Taylor Miller, The Herb Companion, Gnome, Gardening, Planting, Tips, Secrets, Advice, Question and answer, Fruit Flies, Fungus Gnats, Indoor Gardens

 FungusGnatvsFruitflySome people scream “RATS!” when something goes wrong.  Sometimes, though, I think the saying should be “GNATS!”  If you’ve got a bug problem, don’t fret, hope is on the horizon, (or at least in the top layer of your soil).   

Question:  I have noticed more fruit flies in the house now that it's getting cooler. I know that they're attracted to my plants, but they're kind of embarrassing when I have people over. Do you know of a way I can get rid of them, short of walking around and trying to catch each one?? Thanks! - Susan, Kansas

Susan, great question!  First, it’s important to understand the problem:  It’s likely that these buzzers actually aren’t Fruit Flies (Drosophila melanogaster), but a relative insect known as the Fungus Gnat (Sciaridae).  I spoke with an entomologist who said it is a common misnomer.  Even a few exterminators I spoke with had no idea what a Fungus Gnat is (now that is scary, and a good litmus test for a potential exterminator!)   

Fungus Gnats are the most frequent houseplant annoyance and are distinguished from common Fruit Flies because of their darker color.  While Fruit Flies hang out primarily in exposed fruit, rotten food and in leaky fridges, you’ll find Fungus Gnats in wet plant soil, in sewer situations and in household drains.  They’re also attracted to CO2 (Carbon Dioxide) which explains why they’re always right up in your face. 

Adult Fungus Gnats live a week and a half, and in that span can lay up to 200 eggs each in moist soil.  The eggs hatch in 4 to 6 days and feed on plant roots in their larval stage for about 2 weeks.  They love moist, compost-rich potting soils, so it’s more likely you’ll find them in the brand names, like Miracle-Gro®.

Because Fungus Gnats need moist soil in order to complete this four-week life cycle, most experts think you can eradicate the problem by letting your plants completely dry out and get into the practice of watering "...just enough."  But if your watering methods aren't cryptically precise, there are other more accessible solutions.

Questions about Fungus Gnats:

So, do I have Fungus Gnats?  - To see if you have larvae in your plants, and can’t tell just by looking, cut a potato into ½ inch slices and set on top of the soil. FG larvae are attracted to the potato and will move to the surface to feed on it. In a couple days, check the slices, and discard if engulfed with Gna-ggots.  My advice, skip this step unless you’re morbid and have a strong stomach.

Ack, I have them!  Make them leave!  - Pour a generous layer of sand on top of the soil and cover with cedar mulch.  FGs are attracted to the top layer of wet soil. Because sand drains quickly, adults are confused by the newly dry top layer of soil, even though your plant is perfectly watered.  The cedar mulch is ornamental and most insects hate the smell.

OK, so the larvae are dying.  But how do I kill the adults? – Smear Vaseline® on a yellow sticky note and hang vertically.  FGs are specifically attracted to the color yellow and will bang up against it if it is hung vertically.  Another trick is to uncap and cut the top 1/3 off a soda (pop) bottle, turning it upside down inside the bottle (so it looks like a funnel).  Then fill the base with a mixture of vegetable oil and apple cider vinegar.  Attracted to the fermented smell, the FGs will drown when the surface tension is broken by the vegetable oil.     

I think my case is serious! – Most greenhouses use a peat mixture infused with an FG predator called Hypoaspis miles.  The mite lives and feeds on insect larvae and is commercially available for about $30.  You can also buy Gnatrol, a bacterial insecticide that is human and pet friendly and retails for around $20.  Residential exterminators can also control the problem but for, at minimum, $45.

For more information on indoor gardening, visit my blog entry:  5 Tips for Indoor Gardens 



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