Bugs in Indoor Tropical Plant Soil

Fungus gnat larvae are the most common insect found in tropical houseplants.

Fungus gnat larvae are the most common insect found in the soil around tropical houseplants. Adult fungus gnats don't do any significant harm to a plant, but they are a terrible nuisance to people. Fungus gnat larvae feed on the roots of the plant.

Fungus Gnat Identification

Fungus gnats are tiny flying insects about 1/8 inch long. They have long legs and antennae and one pair of wings. Fungus gnat larvae are very thin, white legless maggots with shiny black heads. The pupae are found in the soil wrapped in cocoons.

Life Cycle

The complete life cycle of a fungus gnat lasts about a month, and there are many overlapping generations throughout the year. Adult fungus gnats live seven to 10 days, and during this time they lay up to 300 eggs in moist, decaying organic material such as potting soil or compost. The eggs hatch in four to six days, and the larvae that hatch from the eggs feed heavily for 12 to 14 days. Once they have eaten their fill, the larvae wrap themselves up in a silk-like cocoon. This stage is called the pupal stage, and adult fungus gnats emerge from the cocoons after five or six days.

Signs and Symptoms

These tiny, mosquito-like adults are sometimes seen flying around the plant and nearby windows where they are attracted to the light. The larvae feed on the roots of the plant and cause significant damage. Plants lose vigor, cease to grow, and leaves turn yellow and drop off.

Prevention

There are several things you can do to keep fungus gnats from invading your tropical houseplants. Inspect new plants before bringing them into your home. Purchase plants that look healthy and vigorous. Keep extra potting soil in sealed containers such as plastic tubs. Allow the soil to dry out as much as possible between waterings, and remove and discard plant debris and dead leaves as soon as possible.

Insecticides

Adult fungus gnats are easily killed with insecticides containing pyrethrins. Read the label carefully to make sure the product you buy is safe for use on houseplants. Because new adults are constantly emerging, frequent application is necessary. Insecticides that contain Bacillus thuringiensis berliner var. israelensis, such as Gnatrol and Vectobac, kill the larvae.

Other Insects

Tropical houseplants that are taken outdoors during warm weather can get infestations of pillbugs and millipedes in their root balls. They cause minor feeding damage, and usually stay around the exterior of the root ball. To check for these insects, gently remove the plant from the pot.

Springtails are tiny insects about 1/5 inch long that live in the soil. They jump, much like a flea. Finding springtails in the potting soil usually means that you are overwatering your plants. They cause minor damage, and the best solution is to allow the soil to dry out as much as possible between waterings.

References

• Ohio State University Extension: Ohio State University Extension--Fungus Gnats
• Clemson University Cooperative Extension: Common Houseplant Insects & Related Pests
• Mississippi State University Extension: Bug-Wise