Fungus Gnats Identification Guide

Fungus Gnat Identification Guide

Physical Characteristics

Fungus gnats are small flies that measure 2 to 5 millimeters long. They have thin bodies with long legs and antennae. Many people think they look like tiny mosquitoes, but fungus gnats are smaller and do not bite.

Their wings are clear to light gray. A Y-shaped vein pattern on the wings helps tell them apart from other small flies. At rest, they hold their wings flat over their bodies. Adults are black, dark gray, or brown. Some have yellow or orange marks on their bodies.

Fungus gnat larvae are thin, legless maggots about 5 to 6 millimeters long. They are white and see-through with a shiny black head. You can find larvae in the top few inches of moist potting soil where they eat fungi and rotting plant matter.

Fungus gnats look sleeker than drain flies, which are fuzzy and moth-like. They are weak fliers and stay close to where they breed. You will often see them hovering around plant pots or sitting on nearby surfaces.

Common Species

The most common fungus gnats in houseplants belong to the family Sciaridae. The species Bradysia coprophila and Bradysia impatiens appear most often in homes and greenhouses across North America.

Another family called Mycetophilidae lives outdoors in forests and gardens. These gnats breed in rotting wood, mushrooms, and fallen leaves. They sometimes enter homes but rarely cause houseplant problems.

Dark-winged fungus gnats (Sciaridae) are the main indoor pest. They do well in the warm, humid air found in homes with plants. Greenhouses and nurseries often have large numbers because potted plants give them perfect places to breed.

Fungus Gnat Behavior and Biology

Fungus gnats go through four life stages: egg, larva, pupa, and adult. The full cycle takes about three to four weeks in warm conditions. In cooler weather, it can take several months.

Female gnats lay 100 to 200 eggs in small groups on moist soil or in cracks near rotting matter. The eggs are tiny, oval, and yellowish-white. In warm conditions, eggs hatch in four to six days.

The larval stage lasts about two weeks. Larvae live in the top two to three inches of soil where there is the most moisture and organic matter. They mainly eat fungi, algae, and rotting plant material. They also eat root hairs, small roots, and stems of young plants.

After the larval stage, gnats pupate near the soil surface. This stage lasts four to five days before adults come out. Adult gnats live only about one week, but they reproduce fast. Populations can grow quickly when conditions are right.

Fungus gnats are weak fliers and stay near where they breed. Adults like light and often gather near windows during the day. They do not bite. As adults, they eat very little, mainly nectar and water.

Where They Live

Fungus gnats live anywhere there is moist organic matter. Common indoor breeding sites include:

• Overwatered houseplants and pots with poor drainage
• Potting soil high in peat moss, bark, or other organic materials
• Indoor compost bins and worm farms
• Greenhouses and indoor growing areas
• Damp moss or mulch used for decoration
• Leaking pipes or areas with water damage
• Floor drains with built-up organic debris

Outdoors, fungus gnats breed in forests, gardens, and anywhere rotting plant matter builds up. They are common in shaded, moist spots with old leaves, mulch, or rotting wood.

Treatment Methods for Fungus Gnats

To control fungus gnats, you must fix the moist soil conditions that let them breed. Adults are annoying, but the real key is killing larvae and stopping new gnats from growing.

Watering Changes

The best way to manage fungus gnats is to change how you water plants. Let the top one to two inches of soil dry out fully between waterings. This simple step kills larvae and makes the soil a bad place for egg-laying.

Do not leave water sitting in saucers under plant pots. This keeps soil too wet. Try bottom watering, where plants soak up water from below. This keeps the top soil drier while roots still get enough moisture.

Use potting mixes and containers that drain well. Add perlite or sand to soil to help it drain faster. This reduces moisture that fungus gnats need. Stay away from potting mixes that stay soggy or have lots of organic matter.

Monitoring and Trapping

Yellow sticky traps work well to watch adult gnat numbers and cut down on them. Place traps flat on the soil or standing up near plants to catch adults. Traps alone will not end an infestation, but they help you see how many gnats you have and catch females before they lay eggs.

To check for larvae, put thin slices of raw potato on the soil surface. After 48 hours, look at the bottom of each slice. Larvae like the potato and will stick to it if they are in the soil.

Biological Control

Several natural products work well against fungus gnat larvae. Bacillus thuringiensis var. israelensis (Bti) is a soil bacterium that makes toxins that kill fly larvae. When you pour it into infested soil, Bti kills fungus gnat larvae within days. Products with Bti are sold at garden centers and can be used around people and pets when you follow the label.

Nematodes, especially Steinernema feltiae, are tiny worms that hunt and kill fungus gnat larvae in soil. You add them to soil as a drench and they find larvae on their own. This makes them work even in thick root systems. They do best in moist soil between 50 and 80 degrees Fahrenheit.

Predatory mites like Stratiolaelaps scimitus eat fungus gnat eggs and larvae. These mites live in potting soil and give ongoing control. The rove beetle Dalotia coriaria is another predator used in greenhouses.

Soil Treatment

For plants with lots of gnats, remove and replace the top layer of soil. Take out the top two inches where most eggs and larvae live. Replace it with fresh, clean potting mix.

In bad cases, repot plants in all new soil. Wash the old soil off the roots before replanting. This gets rid of larvae that may be feeding deeper in the roots.

Letting soil dry out fully can kill larvae, but be careful not to hurt your plants. Most houseplants can handle short dry periods, but too much drought will damage them. Focus on drying the top layer of soil where larvae live.

How to Prevent Fungus Gnats

Stopping fungus gnats before they start is easier than getting rid of them once they move in. Try these steps:

• Check new plants closely before you bring them home
• Let new potting soil dry before use, or pick sterile mixes
• Do not overwater and make sure all pots drain well
• Clean up dead leaves and plant debris on soil surfaces
• Store unused potting soil in sealed containers
• Keep new plants separate for a few weeks to watch for pests

References

• Fungus Gnats - University of California
• Fungus Gnats Indoors - NC State Extension
• Fungus Gnats and Shore Flies - Purdue Extension
• Fungus Gnats - UMass Extension
• Fungus Gnats Fact Sheet - UNH Extension