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Fungus Gnats


Nancy Robitaille

photo - Happy D Ranch

Of all the insects that infest our African violets, the most common is the fungus gnat. Bothersome, annoying, embarrassing when guests visit, these creatures may be obnoxious but they are not too destructive to our plants.

Some say they do not bother the plant, they feed on fungus in the soil. But through research it has been discovered that Yes, they do damage plants—food crops outdoors can be devastated by them. But, generally speaking, they are not too harmful to our African violets.

If there is no fungus matter available for them to eat, gnats will attack weaker and younger plants. Leaf cuttings, baby plants and immature plantlets are vulnerable.

These "insignificant" pests are only a nuisance if their numbers build and a hardy infestation takes over. If more than once you have seen a fungus gnat flying even with those yellow sticky cards set up among your plants, you have an infestation that should be taken care of.

These small flies include 600 species found in North America. Thank heaven most are not interested in our violets. Dr. Charles Cole, author of INSECT AND MITE PESTS OF AFRICAN VIOLETS, says: "Only a few species will actually feed on and damage live plant tissue."

Fungus gnats (orfelia spp.) lay eggs in the soil. With each female laying between 100-300 eggs and when those eggs hatch in four to six days, depending on the temperature of the room, it is easy to see how they could multiply into a serious infestation.

What Do These Tiny Annoying Insects Look Like and How Can You Tell Them Apart from Thrips?

Thrips are tiny insects shaped like a bomb. They are in the range of 0.5-5 mm long. Color may be yellow, tan, brown or black. Thrips have four narrow wings fringed with long hairs. They feed on decayed plant material or fungi or in the case of other species, they feed on other insects such as mites, plant-feeding thrips and other small insects. Some thrips species feed on pollen, flowers or plant foliage. Besides the color difference between Thrips and gnats, their feeding habits are remarkable.

Gnats may be one-eighth to one-half inches in length, mosquito-like with long legs and antennae. Two wings of a grayish color are delicate and a clear color with a Y- shaped vein in the wing pattern. Fungus gnats may be gray, black, some are marked with yellow or white.

As larvae, the insects are tiny, translucent-to-white in color with distinctive shiny black head capsule. They have no legs and are small and thin, thread-like. They may be up to one quarter inch (5.5mm). Larvae are most damaging to seedlings, cutting and young plants. When larvae feed on tender young roots this provides an entry for pathogens.

Where there is an abundance of moist organic matter adults will breed. They feed on plant exudations and nectar, while the baby larvae feed on fungi and decaying organic matter.

Many fungus gnats are found outdoors and in nurseries where it is warm and moist. They may attracted to light and may be seen near widows.

What damage can fungus gnats do?

Damage is not visible; that is, damage to the foliage and flowers is negligible but what about the tiny delicate, often weak root systems of cuttings and baby plants? Roots may become small and show brownish scars from larval feeding. Especially delectable are the young feeder roots or root hairs that will become demolished in time. Plants may show signs of sudden wilting, loss of vigor, poor growth, yellowing and foliage loss because of the stress of root damage.

Fungus gnats may find their way into your plant room through air currents or other means such as on pet fur or even on your own hair or clothing. Other means of entry might be newly acquired plants.

The life cycle egg to adult is 3- 4 weeks depending upon the temperature of the room. Larvae are laid in cracks and crevices of the soil and will mature in four to six days.

Mixes that use coconut coir were at first thought to discourage gnats but experiments have proven the contrary; it seems to attract gnats.

How to Control Fungus Gnats:

Poor drainage and over watering may contribute to making the conditions which fungus gnat love. Correction of this may reduce some of the problems.

When chemical control is necessary you may decide to try the following suggestions:

"Black flies that flit around the plants are fungus gnats that breed in the rich organic planter mix. If manure is used in the planter mix these pests are sure to be troublesome." says the book, AFRICAN VIOLETS , published by Countryside Books.

The book goes on to say that although these tiny insects don't really do much damage you may wish to control heavy infestations by drenching the soil mix with Sevin or Chlordane solution.

An article written by Michael J. Kartuz from HANDBOOK ON AFRICAN VIOLETS AND THEIR RELATIVES (Brooklyn Botanic Garden) gives only slight mention on the subject of Fungus gnats. He offers a cure with Chlordane or Cygon 2E (dimethoate ) or use Aldrin 5 percent granular.

Tony Clements in AFRICAN VIOLETS says to remove the top three-quarter inch of soil and replace it with fresh soil then allow the plant to dry out before watering again.

Note that larvae may also be found at the bottom of the container.

Helen Van Pelt Wilson says in her book, AFRICAN VIOLET BOOK p. 213 that the larvae stage attacks root hairs. She recommends Fumi-Soil Capsules or Science Granular Systemic. She mentions to spray the plant with Raid House and Garden Bug Killer or use Terraclor or P-40.

Pauline Bartholomew in her book GROWING TO SHOW, says to use No Pest Strips. She says to tap on the pot to call them out then zap them with a pesticide spray. "If this is done each day for ten days or so the population will be greatly reduced if not eliminated entirely".

Montague Free has this to say in his book ALL ABOUT AFRICAN VIOLETS. "For a home remedy, take plant, pot and saucer to kitchen sink, water thoroughly with a fine spray of lukewarm water, and when the springtails swarm in the saucer, wash them down the drain. …Another is to put a teaspoonful of naphthalene flakes in the center of the saucer, rest the pot on it, and then fill the saucer with water; but it seems to me that this might be disastrous to plants in some cases. Other prescriptions include these three: watering with one teaspoon Chlorox to one pint of warm water; with one teaspoon ammonia to one quart of water; or with one teaspoon Lysol to one quart of water…"

He continues, "Black flies are said to succumb to most of the above measures. In addition, any of the insecticides containing DDT for garden purposes should be effective is sprayed on pots and soil (Note: This information is taken from a book printed in 1949. DDT for use indoors or out is NOT recommended.)

Other Ways to Rid Ourselves of These Pests:

The most practical solution is recommended by Pauline Bartholomew. She recommends using No-Pest strips manufactured by several companies. These yellow (or blue) cards are manufactured with a sticky substance which makes flying insects stick. These are most effective when placed just above the soil level.

Some growers have used yellow cards or tennis balls coated with Vaseline for the same purpose. One recommended using the product Tanglefoot on yellow cards since it lasts much longer than the Vaseline.

An internet article written by University of Connecticut staff has this to say about fungus gnats. The article is entitled INTEGRATED PEST MANAGEMENT.

"A bacteria called Bacilleis thuringensis var. israelensis, sold under the name of Gnatrol, is most effective against the young larvae when they are actively feeding. The bacteria must be ingested by the larva, after which a toxic protein crystal is released into the insects' gut. The larva stops feeding and dies. Gnatrol is only toxic to larvae for two days. Repeat applications…

"Steinermema filtiae is an insect killing nematode that can be also applied as a drench treatment"

Another predatory mite mentioned is Hypoasper miles, that feeds on fungus gnat larvae.

"Knox Out and DuraGuard ME are two microencapsulated products that may be used for both larvae and adults." Other recommendations are: Talstar, AtainTR, Decathlon 20 WP, Astro, Orthene TR, Pyrethrum and insecticidal soap which are labled for fungus gnat adults.

The University of Connecticut article continues by suggesting this: "Potato plugs, one inch diameter, one-half inch thick placed on soil surface to monitor for larvae"… may be used. "To use potato plug place it so there is contact with the soil so the plug does not dry out. First check soil under the plug then the surface of the potato. It may take one to three days before seeing larvae. Replace plugs weekly. Remember to remove the potato plugs."


Development Stage

  • Egg --- 4 to 6 days
  • Larva --- 10 to 14 days
  • Pupa --- 4 to 6 days
  • Adult --- 8 to 10 days
  • Egg Laying Period --- 7 days
  • Egg to Adult --- 30 days

Total number of eggs laid = 100 to 150