Question: There are little black gnats flying around my house, and I’ve tracked them back to my houseplants. How can I control these annoying creatures, and are they harming my plants?
Answer: The little black insects are most likely fungus gnats. They breed in overly damp soil, where both the larvae and adults feed on abundant soil fungi. If the population of gnats is very high, the larvae may run out of fungus to eat and turn to your plants’ rootlets for food, but this rarely causes plants distress. To rid yourself of these pests, allow the soil surface to dry out a little between waterings. Most houseplants do better when the soil is not overly wet, so this should not hurt them. To break the breeding cycle and eliminate the pests you can also spread a thin layer of sand or gravel over top of the potting soil — this works well if you have plants that do actually require a constantly wet soil. This layer will dry out very quickly (faster than the potting mix) and the gnats will have no place to breed. Each generation lives only a short while so once the surface moisture stops they should disappear in a few weeks. There are also commercial products containing a special strain of the biological control Bt that help control this pest.