What are dark-winged fungus gnats?

Diptera: Sciaridae are also known as root gnats.

The name “fungus gnats” refers to the fact that many species feed on fungi. Some species can be major insect pests of greenhouse plants and can cause economic loss across a wide range of crops during stock plant, propagation and finished plant production. These species damage the roots of plants, leading to their other common name of “root gnats.” While Sciaridae comprise 137 different species in North America (north of Mexico), they’re often referred to as “dark-winged fungus gnats” because of their non-transparent wings.

Habitat and distribution

Dark-winged fungus gnats live in forests, swamps and moist meadows, where adults are often found resting within the foliage of plants. Some species live in extreme environments, such as Antarctic islands, on mountains and in deserts, where members of the genus Parapnyxic burrow into the sand during periods of high temperatures. Several species live exclusively in caves and are commonly referred to as cave-dwellers.

Around 1,700 species of these gnats have been described worldwide, of which more than 600 are from Europe. It is believed there are more than 20,000 undescribed species. It is speculated that in moist and shady areas, up to 70 percent of the Dipterans in such environments could be Sciaridae. Sciarids are ancient insects that are common
in amber deposits. The earliest known fossil of a Sciarid is from the Cretaceous period.

Identification of adults to species level is based primarily on males, which have to be “cleared” with sodium hydroxide (NaOH) or potassium hydroxide (KOH) and subsequently mounted on slides. High levels of microscopic magnification are necessary for identification. It is very difficult for non-experts to identify these insects even to genus level. If you have an interest in the systematics of Sciaridae, consult Mohrig et al. 2012.

Dark-winged fungus gnat adults have characteristic wing venation that makes it somewhat helpful to identify them to family level. Borror, DeLong and Triplehorn (1976) on page 577, Fig. 580 C, presented a good depiction of the wing venation of a dark-winged fungus gnat. Another helpful identification characteristic is that the compound eyes of the adult meet above the bases of the antennae, except in the genus Pnyxia. Refer to Borror, DeLong and Triplehorn (1976), page 547, Fig. 447 A, for an illustration of the compound eyes touching above the bases of the antennae. Arnett (1985) on page 649, couplet 26(25), provides useful information for the differentiation of members of the families Sciaridae and Mycetophilidae. Insects belonging to these two families are very similar in appearance, and the taxonomic information outlined above is critical to separating them. Small insects seen flitting about inside structures should not be arbitrarily referred to as “gnats.”

Basic life history

Eggs are laid in moist soils containing organic matter like the potting media of some container grown plants. The length of each stage of development and the life cycle itself are dependent on temperature. A single female can lay hundreds of eggs, which hatch in about four days.

There are four larval instars. Larva and pupal development can take from seven to 14 days depending on food availability, moisture and temperature. Generation time has been listed as 21 days at 72°F and 40 days at 61°F. (Source: ipm.ucanr.edu/PMG/r280300811.html.) When the moisture content of soils of potted plants dry out and temperatures are high, the life cycle is shortened and these conditions often cause smaller adults to emerge.

Adults are tiny insects, ranging in size from about 2mm to 5mm long. Adults live up to 12 days when fed with 10 percent table sugar solution, with about 50 percent dying by the eighth day.

Dr. Hanif Gulmahamad, BCE-Emeritus, PCA is an urban and structural entomologist and consultant based in Ontario, Calif. He may be reached at entodoc@verizon.net.