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10/28/02

Contacts:
Donald Lewis, Entomology, (515) 294-1101, drlewis@iastate.edu
Jean McGuire, Continuing Education and Communication Services, (515) 294-7033, jmcguire@iastate.edu

Yard and Garden Column for the Week Beginning Nov. 1, 2002

Gnats That Drive You Nuts

By Donald Lewis
Extension Entomologist
Iowa State University

Big annoyances often come in small packages, especially insects and certainly insects in the house. "Gnat" is an all-purpose word that is misused as often as it is misspelled but it does accurately convey the connotation of a diminutive, big pest.

Household Gnats

There are three species of gnats commonly found inside the home. Fortunately all are completely harmless. They cause an annoyance because of their presence, but they cannot bite or sting, they do not transmit diseases and they do not attack the house furnishings or structure.

The familiar fruit fly (also known as vinegar fly or pomace fly) is generally easy to recognize as the gnat seen around overripe fruits and vegetables in the fall of the year. It is also the fly used in genetic experiments in school. Each fly is approximately 1/8th inch in length. The body is gray or tan except the abdomen, which may have crosswise alternating light and dark stripes. The eyes are bright red in most individuals.

Fruit flies hitchhike indoors on fruits and vegetables in the summer, but fruits and vegetables are not the source of fruit flies in late fall or winter. Eliminating fruit is not the solution to a fruit fly problem. The most likely source of fruit flies in the home is a slow-moving or seldom-used drain (sink, bathtub, shower or floor drain) in which a layer of slime has built up above the water line. Cleaning slime from inside the drainpipe is the only effective management alternative.

Drains are also the source of another aptly named insect, the drain fly. These small, fuzzy black flies are also known as moth flies. Drain flies spend a great deal of time resting on walls where they are easy to observe and study. The triangular shape and fuzzy body are distinguishing characteristics. Control is the same as mentioned for fruit flies: locate and eliminate breeding conditions within slow-running drains.

Fungus Gnats

Fungus gnats are another small pest with the potential to be a big aggravation. Fungus gnats are about the same size as fruit flies but have a more slender body. Fungus gnats are black to dark gray with no markings or stripes. The wings on a fungus gnat are prominently held to the side and are smoky gray. Entomologists confirm fungus gnat identification by looking for a letter "Y" formed by the veins at the tip of each wing.

Like all flies (and like beetles, butterflies, ants, wasps and bees), gnats have a four-step, complete life cycle. The four steps are egg, larva, pupa and adult. For flies, the larva is a legless, worm-like juvenile that is also known as a maggot.

Fungus gnats are frequently plentiful outdoors in fungi, damp soil and decayed vegetable matter. Although fungus gnats occasionally wander in from outdoors, a persistent problem with this nuisance in the house indicates an indoor breeding site. The indoor source of fungus gnats almost always turns out to be houseplant potting soil.

When houseplants are infested, it is often because the plants are being overwatered. I am not accusing anyone of improper houseplant management, but fungus gnats are often a sign your plants are receiving just a little too much love and attention.

Fungus gnats cannot survive in houseplants if the soil dries almost to the leaf-wilting point between waterings. The good news is that fungus gnat larvae living in moist potting soil are feeding on fungus and decaying organic matter. They are not feeding on the houseplant and do not cause the wilting, yellowing, dieback or other symptoms.

Houseplant insecticide sprays can be applied to the surface of the soil and around the edges of a potted plant to provide some relief. However, spraying does not solve the problem and is a temporary control at best. Greenhouse growers may use insecticide drenches to control fungus gnat larvae in the soil, but drenches are seldom practical for homeowners. Your best bet is to check the soil moisture with the end of your finger, water less often and only when needed. Of course, you could learn to love fungus gnats or at least tolerate them. Your houseplants have.

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ml: isugarden


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