10 January 2024
Especially in the winter (because the house is closed up and pretty much recirculating the same air), houseplants can often start having gnats around the soil and, if unchecked, moving around the house. It is annoying especially if they try to fly up your nose or into your mouth. Let’s not even talk about the comments from your mother and/or mother-in-law about sanitation.
These gnats (Bradysia spp.) come from larvae that have found a home in the plant’s soil. As these larvae feed, they can damage or destroy the plant’s roots. If you are trying to grow from seed or it is an established plant, the plant will eventually die.
There is hope. Here are some products and techniques that you might want to look into. First, a word of caution. If you are growing edibles, be careful about products that could cause health issues.
Often these gnats are already in the potting mix. They infiltrated the bags as they sat outdoors and got wet. Mixes with peat and coir-based products will dry out better and will be less likely to encourage gnats. Those mixes with bark and compost are more susceptible.
Also if you had your houseplants outdoors during the summer, as you bring them into your home in the autumn, spray the soil with a broad spectrum insecticide and allow them to stay in a separate, enclosed area (like a garage) for a couple of days.
Watering
The larvae thrive because of moist soil and fungus from the organic matter. The organic matter is food for the plant but also nourishes the pest. Reduce the amount of water and things should get better, although you may need to apply some product. Note, if you are growing from seed, you need to keep the soil moist to germinate and, again, if you are growing something that will feed your family, be cautious of the product’s effects. Read all labels every time.
If you want, you can add a thin layer of sand or rice hulls to the top of the pot. This will cause the surface to dry quickly and the adults will not have a good base for their eggs.
Traps
These insects are attracted to the color yellow (go figure) so if you use a yellow sticky trap that is just above the surface of the soil, the adults will be stuck and unable to make it to the soil and reproduce.
There are also a couple of low tech options. Slice a potato in half or quarters and place it cut-side down on top of the soil. The larvae will gravitate to the potato to feed. Check the potato at least daily. When you dispose of it, don’t put it into your compost pile.
Place a small dish of water with one drop of dishwashing soap. The single drop of soap is necessary and only a drop will do ‘ya. Place it near your seedling germination area or your potted plant. The gnats hone in on the water in anticipation of laying eggs. The soap prevents the eggs from growing and the adults drown. You can gauge the amount of infestation by the number of dead females.
Products
The fungus larva can be thwarted with bacteria. Bacillus thuringiensis subsp. Israelensis (Bt) can be used even on wet soil. If so, be sure to allow the soil to dry completely before any further applications of Bt. Read all of the instructions and warnings before using and apply only per the manufacturers’ directions.
There are some natural deterrents like an insecticidal soap or neem oil. While not a effective as Bt, they can kill the larvae when watered into the soil. Spraying the foliage is not effective.
There are also natural predators like nematodes and predatory mites that feed on both gnats and their larvae.
Check with a local, respected nursery (not a big box store) for recommendations.
