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Fungus gnats are a common greenhouse pest, especially in the moist environment common in propagation houses. They are tiny mosquito-like insects, about 1/8 inch in length. You will generally first notice them darting about new seedlings. Being so small, they can enter greenhouses through the tiniest openings. Most often they arrive as eggs, either in plant soil or in damp potting soil bags.
Adult fungus gnats are mostly an annoyance, but the larva can damage young plants and seedlings by devouring new, tender roots. This stresses the plants and provides an entry for pathogens, such as Pythium, Botrytis, Fusarium and Thielaviopsis. The first symptom of damage is usually wilting, followed by general decline of the plant.
The life cycle of the fungus gnat may be as short as 3 to 4 weeks, depending on temperature. Eggs in cracks and crevices will mature in 4 to 6 days. The larvae eat roots or stems at the base of a plant; at 70F they will develop for two weeks. The larvae pupate in the soil, with adult gnats emerging after 4 to 5 days. Gnats are hard to control because the generations often overlap. Special challenges arise with plant species the fungus gnats like, such as poinsettias, or if the soil contains hiding places in bark or peanut hulls.
Biological controls are best used preventively, when populations are low. A regular monitoring program is needed for early detection of this pest and to insure the success of a biological control program. Yellow sticky cards can either be placed horizontally at the media surface or laid flat on the rims of pots to capture resting adults. Potato chunks (peeled potatoes cut in quarters for plugs) can be placed on the media surface to attract larvae. Check yellow sticky cards weekly and inspect the potatoes after 2 days. Regular inspection of developing root systems for signs of fungus gnat feeding (blunt root tips) is also helpful. Cultural controls include; avoiding overwatering, avoiding puddling on the floors, rigorous weed controls, and controlling algae. These cultural controls are critical before starting a biological control program for fungus gnats.
Pyrethrins or a pyrethroid are labeled to provide temporary, fast-acting control. Pyrethrins have low toxicity to people and pets and are the active ingredients in the botanical pyrethrum, from flowers of certain chrysanthemums. Pyrethroids (e.g., bifenthrin, permethrin) are synthesized from petroleum to be chemically similar to pyrethrins, but often are more effective and persistent, as well as being more toxic to beneficial insects. When using these on houseplants or interiorscape containers, it may be best to move plants outdoors for treatment and wait about a day after application before bringing them back inside. Peaceful Valley insecticide formulas labeled for fungus gnats and based on pyrethrins are Evergreen Crop Protection, Pyganic, and Safer Yard & Garden Insect Killer
The PVFS product Ecotrol which contains clove, thyme, cinnamon, and wintergreen oils, states on it’s label that it can be used as a soil drench for fungus gnats.
Thank goodness for beneficial insects. If you are in a greenhouse growing business you may want to consider introducing the “good bugs” into your houses to eat the “bad bugs”. Both Fungus Gnat Predators and Parasitic Nematode / Steinermema Nematodes are benefical insects that will eat fungus gnats, but not harm your plants or other “good guy insects”. Predatory Mites and Nematodes are compatible with a number of different pesticides. However, they are generally not compatible with organophosphates, carbamates, and nematicides, and none of the products mentioned above contain these ingredients.
If you can catch this problem quickly, you will be likely to eradicate it easily. Good luck in all your growing.
Adam Kufeld Says:
Jun 11th, 2012 at 7:37 pm
“The PVFS product Ecotrol states on it’s label that it can be used as a soil drench for fungus gnats.”
So states this page. However the label as far as I could see didn’t say anything about fungus gnats, nor did it work, at all. They seem to like the stuff. So others should examine this more carefully.
We love Peaceful Valley, but I really think this info is wrong, and was a waste of time and money, at least as it relates to fungus gnats.
Charlotte from Peaceful Valley Says:
Jun 18th, 2012 at 1:32 pm
Adam, Thank you for your loyalty and for your feedback. This post was written in 2010 when we still carried Ecotrol (it’s now unavailable from the manufacturer). Back then the Ecotrol label said “effective as a soil drench for controlling soil-borne insect larvae (e.g. fungus gnats)”
Since we cannot carry Ecotrol we now carry Ecotec G http://www.groworganic.com/ecotec-g-20-lb.html which, is similar, but is not labelled for use against fungus gnats.
I’m very sorry you are plagued by the fungus gnats. Which product did you use? We’d like to help you solve the problem.
Aug 6th, 2013 at 7:50 pm
look for a rotten potato or bag of them, like under the sink? that’s what gets them started.
Charlotte from Peaceful Valley Says:
Aug 7th, 2013 at 12:18 pm
jp, Thanks for the tip!