Published on Wednesday, 30 November -0001 00:00
A common call I receive this time of the year is in regard to small dark flies buzzing around the home. Often the homeowner suspects that these insects are fruit flies and begins checking all the exposed fruit in the home in search of an infestation of this pesky insect. Although these flies could certainly be fruit flies, at this time of the year they are most often fungus gnats which may be breeding in the soil of poinsettias or other houseplants that have been brought into the home. These gnats may also have hitch-hiked inside on those plants that were outdoors for the summer. Under magnification, fungus gnats are a slender dark-colored fly with long legs and are more mosquito-like than fruit flies. However, when they are flying about the home, they may look quite similar. Poinsettias, as well as most other indoor plants are now planted in soilless mixtures or other potting media with high organic matter content. These organic soils provide the ideal habitat for fungus gnats, particularly if the plants have been overwatered. The larvae are very small and worm-like, being about one-fourth inch long and pale white or translucent in color. They live in very damp conditions where they feed on decaying plant material, moist organic matter, and fungi. Fungus gnats rarely feed on healthy roots and typically do not injure the houseplants. Heavy infestations in the medium in which bedding plants are being grown may result in damage to the very small roots of immature plants. Although they may fly about the face, adult fungus gnats do not bite or attack people and are really only a nuisance. The first step to control fungus gnats is to reduce soil moisture by changing the plant watering schedule so that the plants receive sufficient moisture, but the soil surface is allowed to dry between waterings. In other words, water the plants when they need water, but do not keep the soil saturated. Do not, however, allow the plants to wilt. The only effective product for treating the larvae in the soil is Bacillus thuringiensis var. israelensis, also known as B.T. H-14 and usually sold as "Knock-Out Gnats"or "Gnatrol". This product is only effective on the larvae and does not kill the adult flies. There are yellow sticky cards or "sticky traps" available in garden centers that can be placed in the pots and are effective in reducing the number of adult flies. With persistence and patience, you can often eliminate the problem just with these traps. Sticky traps work well when fungus gnats are a problem in bedding plants that are being started indoors in which case the developing plants must be kept moist. There are some insecticides, such as pyrethrins, that will kill adult fungus gnats, but this is only a short-term solution. As long as there is a favorable site for the larvae to develop, adult gnats will continue to be present, even with repeated applications of the chemical.