Midwinter is a common time to notice fungus gnats. These little insects are so small they can be difficult to see but you may notice them flying up from your houseplants or resting on the leaves.
Adult fungus gnats are tiny, dark insects that resemble mosquitoes but are smaller – in the range of 1/16 to 1/8 inch long. Fungus gnats are not strong fliers so they tend to remain near potted plants and can frequently be found walking on the surface of the soil. While fungus gnats can occur anytime of year and are common outside, they are often noticed after houseplants have been brought back in after spending the summer outdoors.
Once established, the larvae of fungus gnats are found in the potting mix, feeding on the plant roots. The adults consume very little devoting most of their energy to mating and laying eggs.
The best way to control fungus gnats in houseplants is to modify the habitat to remove their breeding grounds. Fungus gnats require moist, organic soil so be careful to avoid overwatering your plants. The surface of the soil should dry out to the touch and the container should feel light for its size before watering. Do not allow any water to stand in saucers or decorative outer pots. Also avoid using incompletely composted organic matter in potting soil and remove dropped leaves, flowers and other plant debris as they fall on the surface of the potting mix.
There are several other control options available as well but they tend to be hard to find. The use of beneficial nematodes and the beneficial bacteria Bacillus thuringiensis subspecies israelensis (Bti) can help control fungus gnats. However, these options can be hard for homeowners to find. While adult fungus gnats can be controlled with an application of pyrethrin or pyrethroid based insecticides, this does not eliminate the larvae in the soil and will not effectively break the lifecycle of this pest.