Tis the season to start seed indoors, and with that effort, some of us undoubtedly will be bothered by unwanted organisms. Fungi in our fuchsias, bugs in our begonias, invertebrates in our impatiens.

As many windowsills (and other horizontal surfaces) in our homes are converted into miniature greenhouses, unwanted pests may view our indoor gardening attempts as a rolling out of the welcome mat. Tender young seedlings are particularly vulnerable to a range of common greenhouse pests. One of the most disturbing ones to affect indoor plants and seedlings is the fungus gnat.

A Machias reader recently wrote to inquire about an insect that commonly plagues houseplants, and he perfectly described a typical situation in which gardeners find fungus gnats.

“I bought several bags of potting soil from a local hardware store last summer. I stored the bags in my garage until near the end of the summer when I finally got around to repotting my plants,” he wrote. “I brought a geranium and a couple of other plants that had been outside during the summer inside after repotting them. I repotted the other plants I have in my home at the same time. I have since been plagued with very small flies that appear to live in the soil of all the plants. Whenever I water my plants, a few of these tiny flies seem to arise from the soil. Did they come from the potting soil? I would like to know the best way to rid my home of these pests without destroying my plants.”

Fungus gnats, moth flies and shore flies are minute species that often are mistaken for one another because they have a similar appearance and biological traits. Each of the species thrives in overwatered conditions frequently found among plants in home or greenhouse environments.

Fungus gnats are delicate-looking, with fine, fragile wings and a mosquitolike body that is slightly hunched. They have a set of long, wispy antennae. The insects hold their wings upright from their body when at rest. They are easily disturbed and often are visible when we water or work with plants. They also can be found scurrying across the surface of the soil, medium or, in an outdoor gardening situation, across organic matter or leaf litter, where they prefer to dwell.

Shore flies are a bit more robust than the fungus gnat. They have stronger, meatier bodies that are dark in color. They have a short set of antennae and pale, gray wings that display five lighter spots.

Moth flies are covered with fine hairs, and commonly are found around wet drains or sinks, but they also can thrive in the environment around waterlogged greenhouse or houseplants.

It’s possible that our gardening friend from Machias may have one of these three flies, but it’s probable that his pest is the dreaded fungus gnat. Of the trio, fungus gnats are the most common and of the most concern.

Fungal gnats undergo four stages of metamorphosis: from eggs to larvae to pupae to adults. We become irritated by the fly, which is the adult form of the fungal gnat, but the most detrimental phase of its life cycle is the larval stage. Fungal gnat larvae commonly are found in the unsanitized medium or soil around the roots of seedlings. The larvae also occasionally chew on leaves or stems that touch the soil surface. These pests have been found to reduce germination and inhibit plant growth. Both adults and larvae have been found to transport fungal pests on their bodies and in their intestinal tracks, including the ever-common botrytis, fusarium, pythium and verticillium species.

Cultural management, including proper watering and nutrition, environmental sanitation and eradication of the insects, is best. Sound watering practices – irrigating so that the soil or medium is moist but not saturated – is an important way to keep fungal gnats from thriving. Do not allow water to pool in trays beneath pots. Allow the medium to dry out slightly between waterings.

Sanitation is a critical preventative measure for keeping fungal gnats at bay. Repotting or propagating plants using contaminated medium or recycling used medium without sterilizing it should be avoided. If you have noticed the presence of adult fungal gnat flies, you might consider repotting your plants in clean, sterilized soil. When repotting, eliminate all soil from the roots of potted plants and then replant them in clean soil. Cut away roots that have an unhealthy appearance. For potted plants with fine, fibrous root systems, this may be a difficult task.

If you have access to the Internet, consider following the links at http://pmo.umext.maine.edu/factsht/links.htm to find information about fungal gnats and other pests.

Diana George Chapin is the NEWS garden columnist. Send horticulture questions to Gardening Questions, RR1, Box 2120, Montville 04941, or e-mail them to dianagc@midcoast.com. Selected questions will be answered in future columns. Include name, address and telephone number.