Tuesday, January 13, 2015

Houseplant Problems: Fungus Gnats by Donna Duffy

Photo courtesy Organic Gardening
What are those annoying tiny black insects that hang out in your houseplants and fly around when disturbed? Most likely, you have fungus gnats.

Fungus gnats (Bradysia species) are small, mosquito-like insects often found in homes and offices, usually in the vicinity of houseplants. They are considered a nuisance when present in noticeable numbers, but the adults are harmless insects that do not bite. Fungus gnat larvae develop in the growing medium of houseplants and are considered minor pests of houseplants (but oh, so annoying to the indoor gardener). They are weak fliers and generally stay close to the infected plant. You may also see them on window sills.

Fungus gnats on window sill
The most important strategy to minimize fungus gnat problems associated with houseplants is to allow the growing medium to dry between watering, especially the top 1 to 2 inches. In addition, it is recommended to re-pot every so often, particularly when the growing medium has “broken down” and is retaining too much moisture. Be sure to remove any containers with an abundance of decaying plant matter such as decayed bulbs and roots, which provide an excellent food source for fungus gnat larvae.

Insecticides may be necessary if fungus gnat problems persist several weeks after watering practices have been adjusted. Typically, when fungus gnat populations are excessive, repeat applications are required to deal with adults. Apply insecticides to the growing medium surface since this where new adults will emerge.

An alternative approach to deal with fungus gnat adults, particularly when populations are abundant, is to strategically place yellow sticky cards (sold as Gnat Stix) underneath the plant canopy or on the edge of containers. The adults are attracted to yellow and will be captured on the sticky cards. This may be helpful in mass-trapping adult females, thus reducing the number of larvae in the next generation.
Fungus gnat on yellow sticky card
For more information, check out CSU Extension Fact Sheet Fungus Gnats as Houseplant and Indoor Pests.