What Fungus Gnats Do to Your Plants

The flies are annoying. The larvae in the soil are the problem.

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Most people dismiss fungus gnats as annoying but harmless. That's only half right. Adult gnats don't damage plants or bite people, but big larval populations can hurt plants, especially seedlings, cuttings and young plants. Greenhouse growers treat them as a major pest.

Single fungus gnat larva with translucent body crawling through moss in potting soil
A single fungus gnat larva in growing media. If you can see one on the surface, check the soil below for more

Photo: zookanthos / iNaturalist, CC BY

Magnified healthy roots: white and firm, branching through the soil, tapering to fine tips covered in fuzzy root hairs. Healthy roots White and firm Fine, tapered tips Fuzzy root hairs take up water Magnified roots after fungus gnat larvae have fed: the tips are chewed back to blunt ends, the root hairs are gone, brown soft patches show where rot has got in, and larvae with clear bodies and shiny black heads sit on the roots. After larvae feed Tips chewed off, ends blunt Brown and soft where rot gets in No root hairs left to take up water Larva: clear body, shiny black head
Magnified. Tip a struggling plant out of its pot and look at the root ends: blunt, brown tips mean larvae have been feeding.

Direct Damage

Root Feeding

Larvae feed on root hairs and fine roots, interfering with the plant's ability to absorb water and nutrients. The result is stunted growth and an overall loss of vigor.

Stem Feeding

Larvae can tunnel into plant crowns and stems. Succulent-stemmed plants like poinsettias, begonias, geraniums and coleus are especially prone. The wounds give soil-borne pathogens a way in.

Seedling Death

Young plants and seedlings are much more vulnerable than mature plants. Larvae also feed on the callus of fresh cuttings, which slows rooting. Heavy infestations can kill seedlings and young plants.

Leaf Damage

Larvae may come up out of the soil to feed on leaves lying on the growing media surface.

Root Tip Destruction

Larval feeding leaves blunt root tips, which you can see if you tip the plant out. Healthy roots taper to fine points.

Indirect Damage: Disease Transmission

Diseases They Spread

Larvae have been shown to eat the spores of Pythium and Fusarium and carry them into healthy young plants as they feed. Pythium oospores can pass through a larva's gut and still germinate afterwards.

Adults can carry spores of Botrytis, Fusarium, Thielaviopsis and Verticillium on their bodies and spread them around a greenhouse. How much this matters in practice is still not clear.

Wound Entry Points

Feeding wounds give soil-borne pathogens a way into roots and stems.

If you are fighting root rot and you also have gnats, deal with both.

Symptoms of Fungus Gnat Damage

These symptoms overlap with overwatering, root rot and poor drainage, and UC IPM says wilting is more often caused by those than by larvae. If you see gnats and a struggling plant, tip it out and look at the roots and the soil before you decide which problem you have.

Plants Most Affected

While fungus gnats can infest nearly any plant grown in organic media, certain species and situations are especially vulnerable:

Plants named in extension guides: poinsettias, geraniums, begonias, coleus, sedum, cyclamen, impatiens, gerbera, gloxinia, peppers, annual vinca, cacti, young orchids, ferns, tulip bulbs, bedding plants, and vegetable transplants. Any seedlings or cuttings that are still forming roots are at higher risk.

Growing media high in peat moss, compost or composted bark attracts fungus gnats. Moist peat supports the fungi that larvae need to complete development.

Economic Impact

In commercial greenhouses and nurseries, fungus gnats are now recognized as major pests, because both larvae and adults can cause direct and indirect damage that leads to economic losses. For home growers, the cost comes in lost plants, wasted soil, and the frustration of watching a collection decline despite attentive care.

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Sources

  1. Fungus Gnat Management in Greenhouses and Nurseries (MF2937), Kansas State University Research and Extension
  2. Ecology of Fungus Gnats (Bradysia spp.) in Greenhouse Production Systems Associated with Disease-Interactions and Alternative Management Strategies, R.A. Cloyd, Insects (2015)
  3. Pest Notes: Fungus Gnats, UC Statewide IPM Program
  4. Managing Fungus Gnats in the Greenhouse, UConn Extension
  5. Biological Control of Fungus Gnats, UConn Extension
  6. Managing Fungus Gnats, Thrips, Aphids and Mites, UMass Extension
  7. Darkwinged Fungus Gnats, Bradysia spp. (EENY-215), University of Florida IFAS Extension