If you're seeing white gnats, you could be dealing with either of two completely unrelated types of insect, though they may cause similar damage to cultivated plants. One group, the whiteflies and woolly aphids, are members of order Hemiptera, the "true bugs." The other, white fruit flies, are from order Diptera, the "true flies."
Gnaturally, It's Not So Simple
"Gnat" isn't a terribly scientific term. Like "bug" it originally meant a specific type of insect, but has since been broadly applied to many unrelated species. To be accurate, a gnat is any of several tiny dipteran species that aggregate into clouds, usually over water. However, in common usage it refers to almost any tiny, flying insect -- much like the word "bug," commonly used interchangeably with "insect," but which officially only means hemipterans, an order of soft-bodied insects with a specific type of forewing.
Your white gnats may be whiteflies, identified primarily by their truly bizarre nymphal stages. When they first hatch, these animals look like minuscule white worms -- but not for long. They settle on the underside of their host plant's leaves and transform into something that looks like a cross between a jellyfish and possibly a landing vehicle from another planet. They stay in this form for four molts, called instars. Although their last juvenile instar is completely immobile, it is a true nymphal stage, not a pupa. They hatch as small, white or yellowish, moth-like flies. There are more than 50 known species in at least two genera, but you can identify all by the waxy or powdery white coating on their bodies and by their wings, whose ends overlap at rest.
If your white gnats are fuzzy and furry, they're probably woolly aphids of the Shivaphis genus, most likely the species commonly known as Asian woolly hackberry aphids. These introduced animals have naturalized across the eastern and southern portions of North America, where they feed on native trees but are not known to cause significant damage. Flying aphids are the sexually reproducing males and females, born from asexually reproducing wingless mothers. All are identified by their fuzzy coats of white fur.
White fruit flies are another potential culprit. At least two species are flapping around the U.S., Neaspilota floridana and Neaspilota alba. They're from family Tephritidae, completely different from the common fruit flies, Drosophilidae. You're unlikely to see them in your kitchen -- most tephritids are wild-flying, wildly patterned and colored critters with impressive territorial and breeding displays. Unlike some of their showier cousins, though, white fruit flies have fairly uniform white, very light tan, or grayish bodies and clear wings, but may have strikingly bright green, blue or red eyes. No one can seem to agree on whether or not they're harmful to cultivated plants, but they're often found in gardens and fields. You can identify them by their clear wings, which only overlap over their backs.
- University of Missouri Extension: Managing Whiteflies on Indoor and Outdoor Plants
- University of California Agriculture and Natural Resources: Whiteflies
- The Florida Entomologist: A New Species of Neaspilota (Diptera: Tephritidae) From Florida
- BugGuide.net: Genus Neaspilota
- The University of Georgia College of Agricultural and Environmental Sciences: Hackberry Pest Has Homeowners Seeing "Snowflakes" Falling
- University of California, Riverside, Center for Invasive Species Research: Asian Woolly Hackberry Aphid
- BugGuide.net: Genus Shivaphis
- University of Florida, Department of Entomology and Nematology: Common Name - Sweet Potato Whitefly B Biotype or Silverleaf Whitefly
- USDA, The Diptera Site: Fruit Fly (Diptera: Tephritidae) Databases
- BugGuide.net: Species Shivaphis celti - Asian Woolly Hackberry Aphid
- George Doyle/Stockbyte/Getty Images