Fungus Gnats in Indoor Plants

A recent freeze forecast triggered bringing my outside houseplants indoors for the winter. That act can also bring inside unwanted hitchhiking pests. A few days before they come in, I closely examine houseplants for visible pests, such as aphids or scale, which like to hide under leaves or in leaf-stem intersections. If pests are present, I spray first with a blast of water (for aphids) or, as a last resort, with an insecticide labeled for a specific pest such as scale. Before using an insecticide, always read the label first to make sure you it is the appropriate insecticide for the pest you want to treat. Not all insecticides treat all pests. Read and follow the label directions to ensure efficacy.

There is one obnoxious hitchhiker who you may not detect until the plants are inside—the fungus gnat. These gnats thrive in moist soil and love potted houseplants. You will know you have them when you see very tiny delicate, grayish or black, mosquito-like flies (1/8 inch long) flying around your plants, especially when you are watering them. Fungus gnat larvae usually are located in the top 2 to 3 inches of the soil, depending on moisture level, and primarily feed on plant roots, fungi, and decaying plant matter.

Gnats are not strong fliers and emerge from soil when disturbed. Adult fungus gnats actually do not damage plants nor do they bite people. But the adult gnat's presence is considered a real nuisance and its larvae can actually damage a plant. In addition to seeing adult gnats flying around, you may see symptoms of this infestation in your plants, including sudden wilting, loss of vigor, poor growth, and yellowing. Any plant is a target, but African violets, geraniums, carnations, cyclamens, and poinsettias are especially prone to attack.

While fungus gnats can come in from the outside with the plants you bring in for the winter, they also hitchhike home in newly purchased plants. Before purchase, turn up soil near the base of a potential new plant and look for glossy larvae or flying gnats. Regardless of whether you see the gnats, it is a good thing to isolate any new plant for a few weeks and especially those coming in from outside.

Adult gnats live about one week and can lay up to two hundred eggs during their short lifetime in moist potted plant soils. Within four to six days, tiny larvae emerge and feed on plant roots during their two-week stage as larvae. Their pupal stage lasts only three to six days. Then young adults leave the soil and begin the next generation. The entire life cycle from egg to adult may be completed in as little as three weeks, depending on temperature. Because of this, plants can host each stage—egg, larval, pupal, and adult—in multiple generations at once.

Treatments aimed at the adults, eggs and larvae require different approaches. Yellow sticky card "traps" are very effective at reducing a flying gnat infestation. These small cards are relatively inexpensive and perch in the plant's pot on a stick. One card is good for a large area and can trap numerous gnats. They are a very good approach to capturing the adults, particularly adult females, thus reducing the number of larvae in the next generation. I use them all year to monitor for their presence, since I have many indoor plants.

Another strategy to minimize fungus gnat problems is to allow the soil to dry between watering, especially the top one to two inches. A drier growing medium can decrease the viable population of eggs and larvae as well as reduce the attractiveness of the soil to egg-laying adult females. Cultivating the top layer of soil to expose eggs and larvae to air also can also be effective.

For severe infestations, contact insecticides can be used on the soil's surface, since this is where new adults will emerge. The most effective treatments are those that are persistent for up to several days. Read the insecticide label to determine if it treats fungus gnats. Be sure to follow directions. Insecticides such as those containing soaps, oils, and neem do not provide sufficient long-term control of fungus gnat adults. Insecticides should be a last resort. It is much easier, and more environmentally friendly, to let the soil dry sufficiently before watering and to use yellow sticky traps to catch these unwanted hitchhikers.