Ever notice mosquito-sized gnats crawling across the soil surface of your plants or congregated in windows? You may be experiencing a fungus gnat infestation. Fungus gnats develop in the soil of houseplants and can become a nuisance in homes. Worm-like larvae are translucent in color with exception of their head, which is black. Larvae are usually located within the top two inches of soil, feeding on fungi, algae and decaying plant matter such as dead leaves. When other food sources are diminished larvae will also feed on plant roots and leaves that rest on the soil surface. Larvae are full grown within a short 2 to 3 week period and then pupate in the growing medium.
Adults emerge in approximately a week. Fungus gnats do not feed as adults, nor do the adults posses the ability to bite. Adults are a tiny 1/8 inch long with a prominent “Y” pattern on their forewings. Weak fliers, adult fungus gnats typically stay near houseplants where they breed, although they will attract around window frames if they are close in proximity. In their short 7 to 10-day lifespan, female fungus gnats are capable of laying approximately 200 eggs in surface crevices of the soil. Moist soil is the most attractive laying site and reproduction occurs year round. Initial infestations and increases in activity are common in cooler months when plant watering needs decrease, when watering schedules are not decreased as well. In addition, a plant that is normally outside during the summer and then brought inside for the winter commonly experience outbreaks of fungus gnat activity due to temperature and watering changes. Many planting soils age and degrade over time which may increase their ability to retain more moisture.
If you are uncertain if your plant is experiencing a fungus gnat infestation, you can confirm larval activity by placing potato wedges into the plant’s growing medium. Larvae will begin to migrate to the wedges within a few days. Inspection of the underside of the wedges will reveal larval activity when an infestation is present. Once an infestation has been determined, the first step many plant owners take is to change watering schedules. Watering should be decreased to allow the top 1-2 inches of soil to dry between watering. In addition, removal and replacement of the top 1-2 inches of soil may aide in reduction of the infestation. Repotting the plant may be necessary in high levels of infestation. Plant nurseries or greenhouses often have insecticides available to assist with both adult and larval stages of fungus gnats, consult nursery staff for information specific to the products they carry. Treatments that affect larvae often do not affect adults, therefore both life stages must be considered. Biological control options are available for the larvae in the form of specific insect parasitic nematodes, which may be available at garden centers, greenhouses and/or nurseries. Adults on the other hand, being that they are attracted to yellow, can often be mass-trapped utilizing yellow glue traps placed around plants or on the surface of the plant medium.
**Fungus gnats are considered to be ornamental pest control and are not typically covered by structural pest control applicators, particularly in the larval form.**