Fungus gnats on Plants

Symptoms

One sign of fungus gnats is a plant that wilts even though you are watering it enough. The wilting comes from direct damage by larvae that feed on the roots of the plant. That same root feeding can damage the roots and stunt the growth of the plant.

A second sign is on the soil itself, where larvae leave shiny slime trails across the surface. The adult gnats are a nuisance, but they do not damage the plant themselves. All of the harm described here comes from the larvae.

When larvae are numerous, they may bore into larger roots or stems. In a severe infestation the root damage can be significant, and the plant can even die. Root feeding also gives plant pathogens a way to enter the plant. So a plant that looks thirsty may actually be struggling with damaged roots rather than a lack of water. Stunted growth and wilting are therefore signs to take seriously, since severe cases can end in the death of the plant.

Causes

Fungus gnats are linked to wet conditions, and several causes come down to moisture. Excess humidity, overwatering, poor drainage, prolonged wet surfaces and standing water all appear among the causes. Leaking plumbing or irrigation systems are also on the list.

Moist, decomposing grass clippings, compost, organic fertilizers and mulches are listed as causes too. Incompletely composted or unpasteurized organic matter in potting or growing media is a cause as well. In production areas, algal scum and weeds are listed as causes.

Some of the treatments match these causes directly. One is to let the surface of container soil dry between waterings and to improve the drainage of the potting mix. Another is to buy and use only pasteurized container mix or potting mix. A third is to clean up standing water and fix plumbing or irrigation leaks. The treatments also advise against fertilizing with excessive amounts of manure, blood meal or similar organic materials.

Diagnosis

Wilting in a plant that is being watered enough can point to root damage from larvae. Wilting alone is not enough to decide, so look for the other signs on the list. Shiny slime trails on the soil surface are left by the larvae. Stunted growth is another sign of larvae feeding on the roots.

Adult gnats are a nuisance that do not damage the plant themselves, so any damage you see comes from the larvae. Larvae may also bore into larger roots or stems when their numbers are high, which adds to the damage. The conditions behind an infestation include overwatering, poor drainage, prolonged wet surfaces and excess humidity.

If the soil surface stays wet and the plant is wilting anyway, fungus gnat larvae fit that pattern. Yellow sticky traps, or raw potato chunks pressed into the soil, are used to monitor and trap larvae and adults. Taken together, wilting despite adequate watering, slime trails and wet conditions make a pattern you can match against your own plant.

Treatments

Treatment follows an order here: cultural steps first, then organic options, then chemical ones.

Start with the cultural steps. Allow the surface of container soil to dry between waterings, and improve the drainage of the potting mix. Avoid fertilizing with excessive amounts of manure, blood meal or similar organic materials. Minimize organic debris around buildings and crops, clean up standing water and fix plumbing or irrigation leaks. Use only pasteurized container mix or potting mix, and keep growing media covered until you use it. Cover seedling flats with fine-mesh screens to exclude egg-laying adults. Yellow sticky traps or raw potato chunks pressed into the soil can monitor and trap larvae and adults.

The organic options come next. Bacillus thuringiensis subsp. israelensis, or Bti, is applied as a soil drench or spray to kill larvae. The nematode Steinernema feltiae is applied as a soil drench and is the most effective nematode species against fungus gnats. It works best in moist conditions between 60 and 90 °F (16 and 32 °C). Hypoaspis (Stratiolaelaps) predatory mites are released into growing media, where they feed on fungus gnat larvae and pupae. Azadirachtin is a botanical insect growth regulator that must contact the insect and is only effective on immature stages.

Chemical options are last. Pyrethrins or a pyrethroid insecticide can be sprayed on the surface of potting soil and plant resting spots for temporary, fast-acting control of adults. Do not fog indoors or spray adults in flight. Pyrethroids such as bifenthrin, cyfluthrin, fenpropathrin, permethrin or tau-fluvalinate are more persistent than pyrethrins but more toxic to beneficial insects. Cyromazine and diflubenzuron are insect growth regulators applied to growing media in commercial production. Pyriproxyfen is also applied to growing media and should not be applied more than once per crop cycle. Chlorfenapyr is a pyrrole insecticide for greenhouse use. Acephate is an organophosphate drench that must not be applied through any type of irrigation system. Neonicotinoids include acetamiprid, dinotefuran, imidacloprid and thiamethoxam, and imidacloprid should not go on waterlogged or saturated soil.

The product label is the legal instruction, so follow it for the crop and site you are treating.

Prevention

Prevention comes down to the conditions listed as causes. Let the surface of container soil dry between waterings, and keep the potting mix well drained. Avoid standing water and prolonged wet surfaces, and fix any plumbing or irrigation leaks. Excess humidity is a listed cause too.

Choose pasteurized container mix or potting mix, and keep growing media covered until use. Incompletely composted or unpasteurized organic matter in growing media is a listed cause. Avoid excessive amounts of manure, blood meal or similar organic materials as fertilizer, and minimize organic debris around buildings and crops.

Moist, decomposing grass clippings, compost and mulches are also listed causes. For seedlings, cover flats with fine-mesh screens to exclude egg-laying adults. In production areas, algal scum and weeds are listed causes as well. Yellow sticky traps are used to monitor larvae and adults.

Plants most often affected

• Monstera
• Peace Lily
• Porcelainflower