Home & Garden: Fungus gnats, a mighty pest for spider plants | PostIndependent.com

Home & Garden: Fungus gnats, a mighty pest for spider plants

Curt Swift
Free Press Gardening Columnist
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Upon arriving home one evening, my wife informed me we had little flies everywhere. After examining my houseplants I discovered a hatch of fungus gnats from a recently repotted pair of Chlorophytum (spider plant, airplane plant). The adult fungus gnat, about 1/8 inch long, is a fragile grayish-to-black fly with long, slender legs and thread-like antennae.

I’m not sure if this pest arrived with the spider plants or it came in with the potting soil. The plants were in decrepit shape when I found them; their tops were melted down and mushy, but the roots and crown of the plants were in good shape. Not wanting to toss these plants, I located a pot and potting soil, removed some of the old soil from their roots, and potted the two plants into one large pot. The spider plant is best grown in hanging baskets as it forms long runners, upon which small plants develop. Plants that are pot-bound and under-fed tend to develop the most runners.

The soil for the spider plant should be kept moist. The temperature needs to be above 45 degrees, preferable between 55 and 70 degrees. These conditions are also ideal for the fungus gnat.

The fungus gnat larval stage feeds on healthy roots, stunting or killing young plants. These larvae are clear to creamy white maggots that can grow to about a quarter-inch long. They have shiny, black head capsules.

The introduction and spread of plant diseases such as Pythium, Verticillium, Cylindrocladium, Scelerotinia and Theilaviopsis is common with this pest. The larvae ingest fungal pathogens that remain in the insect during its metamorphic change to the adult. The fungal pathogen is thereby spread to the next pot of plants as the female lay eggs.

Yellow sticky cards placed near houseplants will trap adult flies and thus help you determine if an infestation exists. Small pieces of potato placed on the surface of soil for about four hours will attract larvae of the fungus gnat. The latter technique pinpoints the pot that is infested and needs to be treated.

An excellent treatment is Bacillus thuringiensis var. israelensis, sold under the name Gnatrol. This is a bacterium toxic only to flies (Diptera). Parasitic nematodes, sold under such trade names as BioSafe, BioVector, Scanmask, Exhibit, Oti-Nem, and Guardian are also effective in controlling fungus gnats.

I had neither nematodes nor Gnatrol on hand, and the problem was becoming worse over the course of an evening. Since I did not want to use a toxic treatment in my home, I placed the pot with plants and fungus gnats alike on my back step. The plants and maggots were frozen by morning. The morale of this story is that sometimes the best way to control an infestation is by discarding the plant.

Free Press columnist Dr. Curtis E. Swift is a retired horticulture agent with the Colorado State University Extension. Reach him at Curtis.Swift@alumni.colostate.edu, 970-778-7866 or check out his blog at http://SwiftsGardeningBlog.blogspot.com. He owns Swift Horticultural Consulting and High Altitude Lavender.

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