BATAVIA — Fungus gnats may be one of the most annoying pests that your houseplant can come down with.
They flit around the room and their small size makes you wonder if you are seeing things. It is most annoying when they end up floating in your coffee cup or whatever you may be drinking.
If your houseplants spent the summer outside or even if you did some re-potting recently, you may be experiencing these scenarios.
Fungus gnats are part of the fly family Diptera. Adult fungus gnats are tiny, most are only 1/16 to 1/8 inch long.
They have a black body, two clear wings, long legs and antennae. Flying around you may mistake them for a mosquito.
They aren’t strong fliers so they don’t usually travel far from home base, which is usually a house plant. The worm-like larvae are tiny, legless and have a clear or white body, with a black head.
The females lay eggs in moist potting soil or other organic debris. Indoors they can lay eggs year-round.
The smooth, oval eggs are barely visible as they are semi-transparent and a shiny-white color.
Eggs can be laid individually or in groups. A female fungus gnat can lay up to 1,000 eggs in its brief life.
Once they hatch, the larvae will feed for about two weeks. They then spin a thin silky cocoon to pupate in near the surface of the soil.
After three to seven days the adult emerges and can fly in just a few hours.
Adults live for around eight days. Females can start mating and laying eggs almost immediately.
The whole life cycle takes three to four weeks depending on the temperature.
Adult fungus gnats don’t bite so they are primarily a nuisance. They actually eat very little, mainly flower nectar and water.
It’s the larvae that are the problem. Once they infest the soil of a houseplant, they mainly feed on organic material such as dead vegetation, algae and fungi.
Once that is gone they will feed on plant roots which can result in stunted plants, yellowing foliage and leaves dropping off.
In the outside garden fungus gnats aren’t usually a major concern since predators and even the weather keeps them under control, but they can be a serious problem in greenhouses and nurseries.
Fungus gnats can cause damage to plants being grown commercially for sale, especially African violets, carnations, chrysanthemums, cyclamen, geraniums, lilies, poinsettias and foliage plants. They may also be a problem when growing indoor herbs or vegetable seedlings.
If you have multiple potted plants and are trying to determine which are infested, you can use slices of potato to do some scouting.
Take quarter-inch slices or wedges of raw potato and insert them into the potting soil. The fungus gnat larvae will start feeding on the potato in a few days.
You can check the slices for the larvae and start treatment if needed.
Fungus gnats can take some time and persistence to get rid of. Since the adults fly around and don’t sit still, it is best to direct your efforts at the larvae.
First of all, don’t bring infested plant material home with you. Check house plants over before you bring them home.
Quarantine them from your other plants for a week or two before adding them to your collection.
Good sanitation helps. Remove dead leaves and other debris from the soil periodically as the larvae will feed on this.
If you notice any fungus or algae growing on your medium or containers, clean it up as these are also food for the larvae. Potting soils that are high in organic components, like peat moss or compost, are more attractive to fungus gnats.
Regularly check your houseplants for infestations. When you water is an ideal time.
If you disturb the foliage and see small critters flying about, take a closer look. If they are black it is likely to be fungus gnats.
If it is white, then you are dealing with white flies.
You can try letting the top inch or two of the potting soil dry out between watering if your houseplant can tolerate it. Fungus gnats need moisture for the eggs and larvae to survive.
The females are also less likely to want to lay their eggs on a dry surface. You can try this if you are growing seedlings indoors, but you have to be careful as seedlings are less tolerant of drying out.
Another trick to try would be to cover newly seeded trays with plastic to keep female gnats out.
There is a biological control that you can try. Bacillus thuringiensis israelensis (Bti) has been shown to be effective against the larvae in potted plants. It is the active ingredient of Mosquito Bits.
It does not have a long residual effect so it has to be reapplied and it does not kill the adults.
Follow the label directions. It may be best used as a preventative as you try to eliminate them from other plants.
The best overall control is to treat the soil with an appropriate insecticide to kill the larvae. Imidacloprid will kill fungus gnat larvae when applied to the potting soil.
You can find this in a number of houseplant insecticide products.
Follow the directions on the product label. Repeat if needed. It may take more than one treatment to eliminate them.
Once you have them under control, you can use yellow sticky traps to monitor for the flying adults. This is an easy way to give you a heads up that a problem may be brewing. With persistence you can win the battle over fungus gnats.
Have a gardening question?
The Master Gardener office is open. Please wear a mask when visiting the CCE office and check in at the reception window.
Master Gardener volunteers are normally in the office 10 a.m. to noon weekdays at 420 East Main St. in Batavia. Call (585) 343-3040, ext. 127, or e-mail them at: email@example.com. Visit genesee.cce.cornell.edu or like us on Facebook www.facebook.com/CCEofGenesee.
The Master Gardeners will be hosting a number of programs via Zoom this fall.
What would Halloween be without witches? Find out what might be in a Witch’s Garden on Wednesday at Noon with Master Gardener Connie.
If you have questions about orchids, join our Nov. 5 Garden Talk as Master Gardener Jane gives us her tips for happy orchids.
These classes are free but you do need to register at our CCE website events page at http://genesee.cce.cornell.edu/.