Ever go to water your houseplants and see a cloud of tiny black flies skimming over the surface of the potting soil? These pesky critters are probably fungus gnats.
Fungus gnats are common pests of indoor plants. They are not only annoying but they can damage to your houseplants, vegetable seedlings, and greenhouse plants.
Fungus gnats are small flying insects that look sort of like mosquitoes but they don’t bite. In fact, adult fungus gnats live only about 7-10 days and do not feed. It is the immature larval form that can cause damage to your plants.
Fungus gnat larvae are slender maggots that thrive in moist potting media. They feed on organic matter in the soil such as decaying plant material and fungi but may also consume the fine root hairs that absorb water and nutrients to feed your plants. When these tiny root hairs are destroyed, it is harder for the plants to absorb water and they may become wilted. Fungus gnat larvae sometimes feed on the main plant roots as well and can eventually make their way into the stem causing small plants to fall over at the soil level. Young plants and seedlings are especially susceptible to damage from hungry fungus gnat larvae.
The life cycle of the fungus gnat from egg to adult fly is normally completed in about 4 weeks. Unfortunately, the larval stage, which causes the most damage, makes up the majority of the life cycle, lasting between 12 and 14 days.
They can be introduced when tropicals and houseplants that have been growing outside all summer are brought inside for the winter (fungus gnats are not just indoor plant pests). Eggs and larvae may already be in the soil and will continue to develop in the warmth of your home.
One of the best ways to prevent an outbreak of these pesky gnats is to avoid overwatering your plants. The larvae thrive in moist soil that is rich in organic matter – they cannot survive in dry soil. Allowing the top few inches of the soil to dry out between waterings will not only destroy existing larvae but will make your pots much less attractive to adult gnats looking to lay eggs.
It is important to alter your watering practices during the winter. Generally indoor plants enter a period of reduced growth during the winter and thus require less water. If you continue to water them according to your summer schedule, you will probably end up overwatering. Overly wet soil not only increases the likelihood of fungus gnat outbreaks, it also promotes disease problems.
In general, water your plants when the top inch or so of the soil has dried out and be sure to dump any standing water from pot saucers.
February and March is the time when many of us are beginning to think about starting vegetable and annual seeds indoors. Because young seedlings are particularly susceptible to damage and even death from fungus gnats, we must be particularly careful to guard against an outbreak. Always use a fresh, good quality potting mix when you start your seeds and be careful not to overwater the seedlings once the seeds germinate. This will also help to prevent fungal diseases that cause damping off of seedlings.
I already have an infestation! What can I do?
The trick to controlling an outbreak of fungus gnats is to disrupt their life cycle. If you destroy the eggs and larvae, you will be well on your way to eradicating the problem. You can do this by following the same cultural practices as you would to prevent an outbreak. Since the eggs and larvae are generally found in the upper two inches of the soil and both stages require moist conditions for survival, let the top few inches of the soil dry out before you water. This should help kill most of the eggs and larvae.
Another trick is to spread about ½” of sand over the surface of the potting soil. This not only discourages the adult fungus gnats from laying eggs, it also traps new emerging adults in the soil because they are not able to crawl out through the sand.
Yellow sticky cards can be used to trap the adult fungus gnats. These are effective but are used more often in a greenhouse situation than in the home because they aren’t very attractive – especially when they become covered with dead gnats!
For biological control, the naturally occurring soil bacterium, Bacillus thuringiensis subspecies israelensis (Bti) can be added to the soil as a drench. This bacterium is a different strain than the B. thuringiensis kurstaki strain (Btk) that is used for the control of foliage feeding caterpillars. Bti is used for the control of mosquito larvae in standing water in addition to its use as a soil drench for killing fungus gnat larvae. It is important to choose the correct strain of Bt and to read and follow the label instructions!
Chemical control is usually not warranted for fungus gnats unless the solutions mentioned above fail to control them. Bonide Eight Garden & Home RTU spray can be effective for controlling populations of fungus gnats when used according to the label directions. This product contains the active ingredient permethrin which provides some residual control.