By Robin Trott, Extension Educator
This summer has brought many challenges, not the least of which is the fungus gnat invasion in our extension office. This can be particularly troublesome for those who are squeamish about insects.
Fungus gnats are tiny, black flies that are commonly seen around lamps and windows. They are annoying, but harmless, and do not damage indoor plants. The larvae breed in the moist, rich soil in potted plants. Overwatering plants contributes to the growth of the fungi on which the larvae feed. If you have noticed tiny flying insects surrounding your house plants, lamps and windows, you might have fungus gnats. The good news is, there are many ways to manage these insects without getting rid or your indoor plants.
Monitor for fungus gnat adults using yellow sticky traps placed near a plant’s leaves. To monitor for fungus gnat larvae, place potato slices on the soil surface of potted plants. If there are fungus gnat larvae in the soil, they will come to the surface to feed on the potato tissue.
Keep the soil surface dry to eliminate favorable egg-laying sites for the insect. You can do this by allowing the top inch of the soil to dry out before you water. Alternatively, you can water from the bottom to provide moisture to the roots while keeping the soil surface dry. In addition, you can cover the soil with a ½ to one-inch layer of coarse sand or fine gravel, which will help keep the surface drier and make the soil less attractive for egg-laying.
Products containing Bacillus thuringiensis subsp. israelensis (Bti) are available to homeowners (e.g., Gnatrol, Mosquito Bits, etc.) and can be used to control fungus gnat larvae in soil. These treatments do not affect eggs, pupae or adult fungus gnats. Apply these products with adequate water to help the Bti filter through the soil to reach the larvae. Use several applications spaced five to seven days apart to control newly hatched larvae until the infestation is under control.
Unless a fungus gnat infestation is severe, chemical controls are not warranted. If adult numbers are excessive, insecticides containing pyrethrins or synthetic pyrethroids can provide temporary control. If you decide to use insecticides, select a product that is labeled for indoor use on houseplants, and read and follow all product label instructions. Apply insecticides to plants and to the surface of potting soil where adults typically rest. DO NOT spray the air with these products as such treatments are ineffective. Even in those situations where insecticide use may be warranted, keep in mind that chemical treatments should not be your sole management approach. Insecticides should always be used in combination with other non-chemical practices.
Hopefully these measures will work within a few weeks at the extension office. Stop by and check on our success, have a cup of coffee and let’s talk plants!
Until next time, happy gardening!
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“A beautiful plant is like having a friend around the house.” ~Beth Ditto