Gnats' Eggs on the Leaves of My Houseplants

Lifecycle

• Fungus gnats live on decomposing organic matter, fungi and algae that are in the soil. Adults live 10 days, and females lay 200 eggs during this time. The entire life cycle from egg to adult is only three to four weeks. The gnats can reproduce all year indoors. Larvae develop quickly, pupate and turn into adults in just a couple of weeks. Mature larvae are 1/4 inch long, and adults are even smaller at 1/8 of an inch.

Damage

• Adult soil gnats will sip dew or moisture from the leaves but will not hurt the plant. They are busy cleaning up the soil, but heavy infestations could remove too much organic matter and cause soils to become sterile. Larvae burrow under the soil and dine on the roots of plants. A small number of the larvae are not damaging to healthy plants, but if the plant has undergone other stresses, its health may be compromised by these feeding activities.

Prevention

• Soil fungus gnats are found in moist soil. If plants are grown outside during summer, it may be wise to rinse and repot them before bringing them indoors. You can also quarantine them to minimize the risk of other plants contracting the gnats. All debris needs to be kept out of the pots, and the pot should not rest in a saucer of water. Let the soil dry out between watering, especially the top 2 inches.

Management

• If the gnats are in the soil, you can use sticky traps or tape to capture them. Mount the traps vertically so the flying adults will hit the trap and stick. Insecticidal methods need to have a residual effect so they continue to work even after the adults have been killed. Pyrethroids have been effective, as well as Bacillus thuringiensis, a naturally occurring bacteria. Beneficial nematodes are also used to control fungus gnats. Many chemical sprays are on the market that are also helpful, but be cautious about spraying such items in the home.