Fungus Gnats

By Joe Merkelbach (April 2007)

Fungus gnats are more nuisances than actual threats to houseplants, including cacti and succulents. The small, 1/16- to 1/4-inch, dark insects with black to gray wings are members of the order Diptera and the family Scairidae. They have an affinity for houseplant media, a short life cycle and an omnivorous appetite, and thus are easily introduced into houseplant collections.

The gnat larvae feed on fungal growth, which occurs easily in potting media. Although media can be sterilized to prevent short-term fungus growth, airborne spores are so everpresent that soil is soon recolonized. To prevent the occurance of fungi, dryness and disturbance of the media surface are sufficient, which therefore prevents attraction of the gnats. Mature cacti and most succulents with the need to be on the dry side for proper culture do not present a good match for fungus gnat infestation.

The time when cacti and succulents are vulnerable to harm is during growth from seeds. The generation media can be sterilized, but fungus is still probable in a rather short period of time. Fungus gnats themselves can be a means of introduction, as the larvae can eat the initial root hairs or disturb their tentative media contact. More significantly, the fungi that gnats transfer are also cause for fungal damping, which can kill the seedlings.

Since drying the media and therefore the seedlings is also fatal, that cure is not possible. The best protection is the use of fine netting to prevent the gnats from reaching the media surface, thus slowing the growth of fungi. Use of chemicals is only feasible as a systemic for use in preparing the germination bed. A solution of fungicide can be used to initially wet the media used to sow the seeds.