How to Identify and Manage Fungus Gnats

For plant maintenance people, the worst pest problems are the ones you don’t notice until it’s too late. A fungus gnat infestation is a good example and can be very problematic.

Fungus gnats are common pests in the United States, found everywhere from greenhouses to interiorscapes. They’re attracted to damp places where fungi are most likely to grow. They are the most common interior landscape pest due to the fact that their numbers can rise so quickly. Because of their flying habit, most customers are not tolerant of a single one and often demand immediate eradication. This is one pest that it is a good idea to have a preventative treatment program for.

Why Are Fungus Gnats So Destructive?

While the adults do not feed directly on plant material, they can transmit plant pathogens as they fly. The larva live in the soil and feed on the roots of plants and organic matter. Most larger plants can handle the gnat’s larva feeding but this feeding can leave open wounds where pathogens can move in.

How to Identify Fungus Gnats

Fungus GnatFungus gnat adults are small but relatively easy to identify. They have long legs and resemble tiny mosquitos in body type and size (2.5 mm). Their wings are clear with a Y-shaped pattern. Although fungus gnats fly, they tend to hop from place to place when on soil. Adults are gray or black in color, Larvae are tiny, translucent, legless maggots with distinctive shiny black heads that can grow up to 8 mm long.

Do not get fungus gnats confused with shore flies, another problematic gnat. Adult shore flies are similar in size (2.5 mm) to fungus gnats, but are strong fliers that are stoutly built with short legs and antennae. Shore fly adults look like little fruit flies, having black bodies, reddish eyes, and dark wings with five distinctive light-colored spots on each wing. The spots on the wings are a key for identification and can easily be seen while the flies are at rest. They tend to thrive in moister conditions then fungus gnats, like in drains, but they can be found in plant material as well. Unlike fungus gnat larvae, shore flies do not cause direct plant damage. However, under extremely high populations, frass deposits on plant foliage can accumulate, decreasing the visual quality of the plant.

The Life Cycle of a Fungus Gnat

The adults live just seven to ten days, and in this time the female fungus gnats can lay up to 300 eggs. The eggs are laid in the soil where there is adequate moisture. Once they hatch, they will start to feed on plant roots and fungi near the surface of the soil. They will grow to about 8 mm long. They will then pupate and the adult gnat will emerge to take flight. The total length of their life will depend on temperature. At 80˚F they can complete their life cycle in as little as 12 days.

Do You Have a Fungus Gnat Infestation?

CLEAR INDICATORS:

  • Adult gnats on yellow sticky cards
  • Cream colored larva in the soil
  • Potato wedges can be pressed into the soil to check for the presence of fungus gnat larva. If the larva are present, they can be easily seen feeding on the potato. Only leave the potato in the soil for a few days.

It’s Time to Take Back Control

Because fungus gnats are most fond of moist soil conditions, it is important not to over water plants. Try to let the top of your soil dry out between waterings. Removing the top layer of soil will not help with management. A good control option is to use beneficial nematodes. These are microscopic worms that are applied with water to the soil. Steinernema feltiae is the species of nematode that works extremely well for this pest. They target the larva stage, preventing the gnats from completing their life cycle. Once nematodes are applied, you will not see a decline in the adult population for about a week unless you are trapping for the adults. For adults, using yellow sticky cards will trap them and prevent them from laying more eggs.

If you have shore flies you can use beneficial nematodes for control; however, S. feltiae is not the best nematode due to its cruising foraging strategy. Insect parasitic nematodes that use a cruising foraging strategy typically are found throughout the growing media profile and are looking for sedentary insects like fungus gnat larvae. Steinernema carpocapsae, on the other hand, uses an ambush strategy where it remains on the surface of the growing media where shore fly larvae predominantly are found.

Repeat applications of beneficial nematodes may be needed to control these fly pests. If fungus gnats are a reoccurring problem, a preventative program might be needed.

Preventing Fungus Gnats Infestations

The best way to prevent this common pest is with smart watering practices and a preventative drench program of beneficial nematodes. They can stop an outbreak from ever happening. Talk to your nematode supplier about frequency of application for a preventative program. Fungus gnats can be tough to get rid of once an infestation has taken over your account, so early detection and prevention is important. By monitoring your plants and keeping a strict watering and preventative nematode application, you can keep your plants free of fungus gnats.

Battling other common indoor plant pests? Check out my Professional’s Field Guide to Plant Pest Control for information on Spider Mites, Whiteflies, Thrips, Mealybugs, Aphids, and Scale insects.

 

Fungus Gnat photo courtesy Arthur Chapman via https://www.flickr.com/photos/arthur_chapman/3450753821/

Suzanne Wainwright-Evans is an Ornamental Entomologist specializing in integrated pest management. Suzanne has been involved in the Green Industry for more than 18 years with a primary focus on biological control and using pesticides properly. She is a graduate of the University of Florida with degrees in both Entomology and Environmental Horticulture. She has worked throughout the United States and internationally consulting to greenhouses, nurseries, landscapers and interiorscape companies.

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