DEET (N,N-Diethyl-meta-toluamide) is a chemical compound considered the gold standard in personal insect repellents, particularly for warding off disease-carrying mosquitoes and ticks. It creates a protective barrier on the skin, allowing outdoor enthusiasts to reduce their risk of bites. However, “gnat” is a vague label for many small flying insects, making DEET’s effectiveness complex. This article clarifies DEET’s relationship with the various insects people call gnats.
How DEET Repels Insects
DEET does not kill insects; instead, it confuses their highly sensitive olfactory (smell) system, effectively making a person invisible to them. The compound interferes with the neurons and receptors on an insect’s antennae designed to detect host cues. DEET masks chemicals like carbon dioxide (\(\text{CO}_2\)), which humans exhale, and lactic acid, found in sweat, that biting insects use to locate a host.
Recent research suggests that many insects, such as mosquitoes, also possess specialized olfactory receptor neurons that detect the DEET molecule itself, causing them to actively avoid the treated area. This dual mode of action—masking attractants while simultaneously acting as a repellant—is what makes it so effective against blood-feeding insects. This chemical interference prevents the insect from landing and attempting to bite.
The Importance of Defining “Gnat”
The term “gnat” is not a formal scientific classification but a general, colloquial name applied to various small, two-winged flies (Diptera) that often swarm. DEET’s effectiveness depends entirely on the specific insect encountered, as different species are attracted to different things. Common insects referred to as gnats include biting midges, black flies, fungus gnats, and fruit flies.
Biting midges and black flies are blood-feeding insects, similar to mosquitoes in their host-seeking behavior. Conversely, non-biting species like fungus gnats are attracted to moist soil and decaying plant matter, while fruit flies are drawn to fermenting fruit. Since these species are not seeking a blood meal, the chemical cues DEET is designed to mask are irrelevant to them.
DEET’s Effectiveness Against Different Small Flies
DEET is highly effective against insects commonly called “biting gnats,” including biting midges (“no-see-ums”) and black flies. These insects rely on detecting the carbon dioxide and skin odors of a host to feed. By disrupting these specific chemical signals, DEET successfully prevents them from locating and biting the treated skin.
The compound is ineffective against non-biting species like fungus gnats and fruit flies. These small flies are attracted to environmental factors such as high humidity and organic decay found in houseplants or overripe fruit. Since their primary attractants are not related to human scent, applying DEET to the skin will not deter them from swarming. The repellent is only successful when the insect is motivated by the same host-seeking behaviors as mosquitoes and ticks.
Proven Alternatives to DEET for Gnats
When dealing with biting gnats like midges and black flies, two effective alternatives to DEET are Picaridin (icaridin) and Oil of Lemon Eucalyptus (OLE). OLE contains the active ingredient p-menthane-3,8-diol (PMD) and is the only plant-based ingredient recommended by the Centers for Disease Control and Prevention for its proven efficacy. Picaridin, a synthetic compound resembling a natural pepper derivative, is comparable to DEET in efficacy against biting insects and is known to be particularly strong against flies.
For non-biting gnats, personal repellents are not the solution; control methods must focus on eliminating the source of the infestation. Fungus gnat problems are best solved by allowing houseplant soil to dry out and using physical barriers like yellow sticky traps to catch adults. Fruit fly infestations require removing overripe produce and using simple traps, such as a small dish of apple cider vinegar covered with plastic wrap with small holes, to trap the adults.