Nathalie M. has asked what the “bugs” are that are flitting around the soil in her houseplants.
Following her description, these are fungus gnats, which are attracted to houseplants that have been overwatered, causing potting soil to become soggy. Fungus gnats don’t do much damage to houseplants or to humans, but they can be a huge nuisance. They seem to appear out of nowhere and persist if care is not taken to eliminate them.
Fungus gnats are drawn to the fungus that grows in soggy, moist, decaying organic matter. Adults lay eggs near the surface of potting soil and within a few days larvae emerge from the eggs, which burrow down and commence to feed on roots and decaying organic matter under the surface of the potting soil. Multiple generations quickly become established, as the life cycle takes only a week or so to complete. Populations quickly explode into a full infestation.
Fungus gnats are in the fly family and resemble fruit flies in size and form. Adult fungus gnats are brown or black with light colored legs. They will be hovering over the soil surface, where they are quite happy.
Fungus gnats are a problem especially for seedlings, as they can cause “damping off” in seedlings. Damping off is caused by the plant pathogen Phythium, resulting in weakened stems of seedlings, and eventually death of the seedling.
Getting rid of fungus gnats is not difficult ,but there does need to be a plan of attack. Following are some suggestions that do not require chemicals.
• First is to alter the watering regime. Since fungus gnats populate moist, organic matter reduce the amount of water given the plants. Allow the surface to dry before watering. Discard the water that has percolated through the potting soil. Do not let it accumulate.
• Remove all surface organic debris.
• Since fungus gnats lay eggs in the top inch or so of potting soil, replace the top inch with sand or gravel. This discourages adult from laying eggs.
• Spray the top of the potting soil with soapy water. The soap will kill the larvae. repeat every few days to kill the next generation.
• Apply beneficial nematodes to the soil (arbico-organics.com).
• Add diatomaceous earth, natural mineralized fossil dust. Disturb the top inch of soil and mix the diatomaceous earth into the top inch.
Ellen Peffley Harp, a retired professor of horticulture at Texas Tech University, writes about gardening for several Texas newspapers.
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