The little black flies in your plants are called fungus gnats, and although they won’t harm you or your plants, they multiply rapidly and can be a nuisance. They are related to fruit flies, which are larger, and sewer flies, which thrive in the bathroom. The entire life cycle of a fungus gnat, from egg to larva to pupa to adult, lasts four weeks and takes place in fungus-rich, moist topsoil, such as that in which you grow your plants. Fungus gnats are weak fliers and won’t stray far from your plants. They aren’t difficult to control.
Keep the soil dry. The larvae cannot survive in dry soil. Alternatively, use high concentrations of perlite or vermiculite in the soil instead of organic materials such as peat moss.
Treat the soil with an insecticide to kill the larvae. One of the safest is pyrethrin, which is derived from chrysanthemum flowers. You can also use BT/Bacillus thuringiensis-based larvae killer. You can purchase either product at most garden centers.
Take the plants outside to treat the soil. Dilute the larvicide according to the instructions on the container, and saturate the surface of the topsoil. Leave the plants outside for about a day, and then take them back inside. Repeat the treatment every seventh day for four or five weeks to make sure that no more viable eggs are present in the soil.
Saturate pieces of yellow construction paper with sticky material, such as a thin layer of honey, and spread them around the room to catch the flies while the larvicide is working on the soil. Alternatively, buy manufactured fungus gnat traps at a garden center.
Things You Will Need
- Perlite or vermiculite
- Pyrethrin or BT/bacillus thuringiensis insecticide
- Yellow construction paper
- Fungus gnat traps
- In most cases, drying out the soil should kill the flies. Use an insecticide only as a last resort, because it also kills beneficial organisms.
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