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Fungus gnats - Funky Problem
Jun 24, 2010 - Kalita from Peaceful Valley
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 greenhouse through the slightest of openings. More often they come in as eggs, either in the soil of plants that have been outside for the summer or in damp bags of potting soil.
Adult fungus gnats are mostly an annoyance, but the larva can do damage to young plants and seedlings by feeding on the new, tender roots. It is also thought that they feed on the developing callus of cuttings, delaying the development of new roots. Their feeding stresses the plants and provides an entry way for disease pathogens, such as; Pythium, Botrytis, Fusarium and Thielaviopsis. The first symptom of damage is usually wilting, followed by general decline of the plant.
The fungus gnat’s life cycle from egg to adult may be completed in as little as 3 to 4 weeks depending on temperature. Eggs are laid in cracks and crevices in the media surface and mature in four to six days. Fungus gnat larvae feed on roots or stems at the base of the plant and develop for about two weeks at 7oF. Pupation occurs in the soil. After four to five days, adults emerge. Overlapping and continuous generations make control difficult. Control may be especially challenging with favored plant species, such as poinsettias, or if the growing media contains bark or peanut hulls, where they like to hide.
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 can 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 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 green house 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, which none of the above mentioned products are.
If you can catch this problem quickly, you will be likely to eradicate it easily. Good luck in all your growing.