How to Kill Tiny House Plant Flies

Isolate new houseplants from other plants to determine if they suffer from fungus gnats.

The fungus gnat belongs to the Sciaridae fly family. The pest occurs around houseplants. Homeowners also notice the flies on windowsills, curtains and other surfaces near indoor plants. The flies appear grayish or black in color and measure only 1/8 inch in length. The wings of the insect have a translucent appearance. The fungus gnat lays its eggs in the decomposing soil around the houseplant. The larvae of the fungus gnat often damages the plant's root system by consuming the small feeder roots of the plant.

Things You'll Need

• Bacillus thuringiensis
• Parasitic nematodes
• Spray or drench
• Yellow insect sticky traps
• Potatoes

Instructions

1. Cease watering the houseplant. The eggs and larvae of the fungus gnat requires moist soil to survive and grow. Allow the top 2 inches of the plant's soil to dry out completely to kill the larvae and eggs.

2. Drench the soil using a microbial insecticide that contains Bacillus thuringiensis to kill the larvae. Consider drenching the soil with a parasitic nematode, Steinernema feltiae, to naturally kill the fungus gnat larvae. Follow the directions on the label of the product for application instructions.

3. Spray or drench the soil around the houseplant using a pesticide that contains lambdacyhalothrin, cyfluthrin, bifenthrin or permethrin. Follow the directions on the label for application instructions.

4. Hang yellow insect sticky traps near houseplants. The yellow will attract the adult fungus gnat which will become ensnared in its sticky surface and perish.

Tips & Warnings

• Cut up a potato and place a wedge on top of the soil. The fungus gnat larvae will be drawn to the potato wedge and help you determine the severity of the infestation.

• Isolate newly purchased houseplants to determine if they have fungus gnats.

• Pyrethrin-based pesticides rarely offer adequate control of the fungus gnat.

References

• Colorado State University; Fungus Gnats as Houseplant and Indoor Pests; W.S. Cransaw; April 2009
• Washington State University; Fungus Gnats; A. L. Antonelli; Sept. 2007
• Texas A&M University; Fungus Gnat Management; Bastiaan M. Drees; 1994
• Colorado State University; Managing Houseplants; W.S. Cranshaw; Nov. 2006

Resources

• University of Minnesota Extension; Houseplant Insect Control; Jeffrey Hahn, et al.