Gnats Are in My Terrarium

Moisture

• Let the terrarium soil dry. Instead of watering the plants, mist them lightly with a spray bottle so their foliage is moist but the soil stays dry. To deter egg-laying, add a layer of fine chicken grit or small decorative gravel about 1/2 inch thick to the top of the soil. If you use a cover, open it periodically to let condensation clear. If, when you first made the terrarium, you neglected to line the bottom with a layer of charcoal, you might need to take everything out, rinse the soil off the plant roots and remake the terrarium with a charcoal layer topped by a gravel layer topped by fresh soil. This will help drain moisture from the top layer of the soil.

Debris

• Fungus gnats feed on fungus in the soil and fungus that grows on plant material that is molding or has fallen to the floor of the terrarium to decay. Using a long tweezers, clean up all dead leaves and decaying plant matter from the terrarium. Remove and wash decorations and rocks. As the plants grow, trim them to allow air circulation among them. If a plant gets too large for the terrarium, it may be time to remove and replace it.

Sticky Traps

• You can trap the flying gnats with a commercial sticky trap – a yellow paper strip to which they are attracted and stick – or make your own with a strip of yellow cardboard or a large, yellow plastic plant marker coated with petroleum jelly. Insert the trap into the terrarium, being careful not to touch the sides of the container.

Carnivorous Plants

• Place a carnivorous plant such as a sundew or butterwort in a small dish of water near the opening of the terrarium to attract the gnats. The insects are lured by the plant’s sticky leaves, get trapped and dissolve into plant food. This method seems to work the best, according to terrarium aficionados. Because carnivorous plants like moist soil, you also could install one into the terrarium itself, to attract and kill the gnats.

Larval Traps

• Place a 1/4-inch to 1/2-inch slice of cut potato on the surface of the soil to attract the larvae. Remove and replace the potato slice each day until you see no more activity. You can also use mosquito dunks or chips, available at retail nurseries. Lay the dunk or sprinkle the chips on top of the soil. The moisture in the soil activates a bacterium that kills mosquito and gnat larvae.