They are those tiny “Y” shaped, two-winged dark colored flies that hover over the “old” soil medium that is left about uncovered. They look similar to mosquitoes. And they are bad news for your plants. Mostly these gnats are found in or around greenhouses, household potted plants, and those plants stored for over-wintering.
These gnats have a life cycle of about 35 days. The larval and adult stage are the two stages that cause the destruction. The female prefers to lay her hundreds of eggs in a damp medium such as peat moss and pine bark. When the eggs hatch, the larval damage begins. They live on the edges and bottoms of plant pots, and in the top (1-2 inch deep) section of potting soil.
Damage occurs as the larvae move about in the root system of both young and mature plants, and in seedlings and tender plant growth. Tunneling damage can occur at the growing surface area and below the growing surface area. The gnat will find a food source, likely a stem. It will tunnel into the stem interrupting the flow of nutrition to the plant. The plant then wilts, weakens, and dies. Larvae also transmit fungal pathogens directly to the plant with equally damaging effects. Wounded plants are susceptible to secondary soil-borne fungal pathogens, again resulting in a damaged plant. The larval stage inflicts the most damage, but the adult fungus gnat can also introduce fungal pathogens to your plant.
There are many ways to scout for larvae in your household plants, overwintered plants, and “left-over” soil. Take a sample of the soil in question, enough to make a thin layer across a flat surface. Slice a potato into a disc about one inch diameter and half inch thick. Place the potato slice on top of the soil sample and let set for 48 hours. Then look at the bottom of the potato and if you see larvae adhering to the potato, YOU HAVE FUNGUS GNATS!
There are unsuccessful known attempts at ridding your plant environment of these gnats, (Soap and water sprays and ineffective bug sprays.). Below freezing temperatures have little to no effect on their natural ability to survive the environment. They have the appearance of being dead and when the temperatures rise, so do the gnats.
The rainbow of success for eradicating fungus gnats, references three types of controls.
Cultural: Make your plant and plant work-space a non-friendly fungus gnat environment. Remove “old soil”. Remove weeds. A 20-30 foot weed clearance perimeter is recommended for greenhouse type areas. Avoid having free-standing water and over watering. Fungus gnats breed in algae. Keep unused pots and containers clean by using a bleach solution as one effective recommended method. Should you have an infested plant, remove as much soil from around the root system as possible if re-potting, and use an adequately draining pot with clean soil.
Biological: Two beneficial biological suggestions are nematodes and soil predatory mites. Nematodes are worm-like parasites that feed on the gnat larvae. Nematode applications have better chance of control success if the fungus gnats are spotted early. Soil-predatory mites (of the spider family) work differently and can be added at the time of planting. They feed on gnat larvae that inhabit the top layer of soil. Both suggestions are considered safe for the plants, users, and do not promote insecticide resistance
Chemical: Applications of labeled insecticides are used to target the larvae or adult stage of the life cycle. They are applied per label directions. Product recommendations are available at local garden supply stores and through Purdue Extension – https://extension.entm.purdue.edu/ publications/E-111/E-111.pdf
Send us your questions and comments to the Shelby Purdue Extension Office – 317 392 6460 ext. 0 and https://extension.Purdue.edu/Shelby