I have several houseplants and I’ve noticed the plants have gnats living in the soil. I have used two types of insecticide, which hasn’t work. Should I repot all my plants?
The Gardener’s Answer
Hi, Sandra in Kentucky: Fungus gnats are quite common but the good news is that they are more of a nuisance than they are harmful. When we bring our tropical plants indoors for the winter months the fungus gnats are carried in with the soil. There are many generations per year and the adults only live for seven to 10 days. As adults the females lay eggs by the hundreds into the moist soil, so eliminating them in the adult stage will stop the reproduction process. Sprays are effective only if they come into contact with the insect. The most effective means of getting rid of them is sticky fly traps. They are not very attractive but placing them on the soil or hanging them in that space will help keep the numbers down. The sticky traps are just that, stick traps, the gnats are attracted to the yellow color and once they touch the trap they are not able to break away. You will want to keep them up for a few weeks just to make sure that all the overlapping generations have reached the adult stage. Fungus gnats thrive in shaded moist soils. It is important that your containers have good drainage and the soil is never sopping wet. If your soil seems to be on the moist side, allowing it to dry out will kill them in the larval stage. It is a good idea to cut back on your watering during the winter months as the plants do not need as much moisture. Remove any plant debris that has fallen and replace traps as they become covered. It really is best to wait until spring to repot but if the sticky traps do not get the gnats under control you can change out the soil and that should take care of the problem. Even if you change out the soil still use the sticky traps to catch the flying adults that may want to lay eggs in the new soil.