Some gnats are absolutely attracted to blood, but the word “gnat” covers a surprisingly wide range of tiny flies, and most of them have zero interest in biting you. The blood-feeders in the group are biting midges, often called no-see-ums, which belong to the family Ceratopogonidae and include over 6,000 described species worldwide.1ScienceDirect (Academic Press). Medical and Veterinary Entomology – Chapter 13 – Biting Midges (Ceratopogonidae) The gnats you see hovering around your houseplants or fruit bowl, on the other hand, are after fungi, rotting organic matter, or fermentation, not your blood. Understanding which type you’re dealing with changes everything about how to respond.
Not All Gnats Are the Same Insect
The confusion starts with the name itself. “Gnat” is a casual, catch-all label applied to several families of small flies that look broadly similar to the naked eye but lead very different lives. The ones most people encounter indoors are fungus gnats (family Sciaridae), which breed in damp potting soil and feed on fungi and decaying plant roots. These tiny flies are drawn to the chemical signatures given off by fungal substrates, using those volatile compounds to locate food and egg-laying sites.2PubMed Central. Fungal Volatiles as Olfactory Cues for Female Fungus Gnat, Lycoriella ingenua in the Avoidance of Mycelia Colonized Compost They have no mouthparts capable of piercing skin, and they couldn’t extract blood even if they wanted to. Fruit flies (Drosophilidae) and drain flies (Psychodidae) round out the list of tiny household flies people lump under “gnats,” and none of those bite either.
Biting midges are a different story entirely. These are the gnats that genuinely seek out blood meals, and they’re so small they can pass through standard window screens, earning them the nickname “no-see-ums.” The genus that matters most for people and livestock is Culicoides, the most medically and veterinarily significant group within the biting midge family.1ScienceDirect (Academic Press). Medical and Veterinary Entomology – Chapter 13 – Biting Midges (Ceratopogonidae) Only the females bite. Like female mosquitoes, they need the protein in blood to develop their eggs. Males feed exclusively on nectar and plant juices. So if you’ve been bitten by something gnat-sized outdoors, you were almost certainly the target of a female biting midge looking for a reproductive boost, not a random fly that stumbled into your skin.
How Biting Gnats Actually Find You
Here’s where the phrase “attracted to blood” gets misleading. Biting midges aren’t drawn to blood itself, because they can’t detect it from a distance. What they home in on are the signals your body gives off just by being alive: carbon dioxide from your breath, body heat, and volatile organic compounds released from your skin. The most powerful of these cues is CO2. Studies on Culicoides impunctatus, the notorious Scottish highland midge, found that this species is extremely sensitive to carbon dioxide and shows a strong dose-dependent response, meaning the more CO2 in the air, the more midges come looking.3PubMed. Field studies on efficacy of host odour baits for the biting midge Culicoides impunctatus in Scotland
But CO2 alone is only part of the picture. Biting midges use a layered detection system. Carbon dioxide acts as the long-range beacon that gets them flying in the right general direction. Once closer, skin-derived volatile compounds take over, allowing the midge to zero in on a specific host. Research on Culicoides obsoletus showed that these midges preferred humans, calves, and sheep as hosts, likely because larger animals exhale greater quantities of CO2, and that volatile organic compounds released from chicken and sheep skin had an independent attractive effect.4PubMed. Orientation behaviour of Culicoides obsoletus (Diptera: Ceratopogonidae), a relevant virus vector in northern Europe, toward host-associated odorant cues Field trials with C. impunctatus confirmed this synergy: combining CO2 with other host-associated chemicals like acetone or 1-octen-3-ol boosted trap catches several-fold compared to CO2 alone.3PubMed. Field studies on efficacy of host odour baits for the biting midge Culicoides impunctatus in Scotland
This means the factors that make one person more attractive to biting gnats than another are the same ones that matter for mosquitoes: how much CO2 you exhale, your individual skin chemistry, body temperature, and possibly even the bacterial communities living on your skin. People who swear they get bitten more than everyone else at the campfire aren’t imagining it. Their particular cocktail of skin volatiles and exhaled gases may genuinely be more attractive to these insects.
Why Some People React More Severely to Bites
When a female biting midge pierces your skin, she injects saliva containing anticoagulant compounds that keep your blood flowing freely into her mouthparts. Your immune system recognizes those salivary proteins as foreign, which triggers the familiar itchy welt. The severity of the reaction varies enormously from person to person. Some people get a small red bump that fades in hours. Others develop large, angry welts that itch intensely for days or even weeks. This isn’t about how much blood was taken; it’s about how aggressively your immune system responds to the midge’s saliva.
The same mechanism causes major problems in horses. Insect bite hypersensitivity, sometimes called sweet itch or Queensland itch, is the most common itchy skin condition in horses, and it’s most often triggered by sensitivity to the saliva of Culicoides biting midges.5PubMed Central. Insect Bite Hypersensitivity in Horses: Causes, Diagnosis, Scoring and New Therapies Affected horses can develop severe itching, hair loss, and thickened skin, particularly along the mane and tail. The parallel with human reactions is informative: in both cases, it’s the immune response to saliva, not the blood loss, that causes the real misery.
If you’re someone who reacts strongly, repeated exposure over time can go one of two ways. Some people gradually desensitize, with reactions becoming milder after years of seasonal exposure. Others develop increasingly severe reactions. There’s no reliable way to predict which direction your immune system will go, though people with existing allergic tendencies (hay fever, eczema) sometimes report worse reactions to biting midges.
Can Biting Gnats Transmit Diseases to Humans?
Culicoides midges are confirmed vectors for several pathogens, and this is an area of growing concern. The most prominent human disease linked to biting midges is Oropouche fever, caused by the Oropouche virus. Biting midges in the Culicoides genus are the primary vectors during human outbreaks of this disease.6PubMed Central. Enhanced infection and transmission of the 2022-2024 Oropouche virus strain in the North American biting midge Culicoides sonorensis Oropouche fever has historically been concentrated in South America, particularly Brazil, but outbreaks have expanded in recent years, raising alarms about the virus spreading into new regions.
The veterinary picture is even more alarming. Culicoides species transmit bluetongue virus to cattle and sheep, African horse sickness virus to horses, and Schmallenberg virus to ruminants, among others. Climate change may be expanding the geographic range of these midges, potentially increasing the incidence of these diseases in regions that were previously too cold to support large Culicoides populations.7eClinicalMedicine. Oropouche virus: transmission, epidemiology, genetic diversity, and public health implications For most people in temperate regions, the disease risk from biting gnats remains low compared to mosquitoes, but it’s not zero and it’s trending in the wrong direction.
It’s worth noting that the non-biting gnats people encounter indoors, the fungus gnats and fruit flies, do not transmit diseases to humans. They can be annoying and, in the case of fungus gnats, damaging to houseplants, but they pose no health risk to you.
Keeping Biting Gnats Away
Because biting midges track you primarily through CO2 and skin volatiles, the most effective prevention strategies work by either blocking those cues or creating chemical signals the midges find repulsive. DEET-based repellents remain the standard recommendation, just as with mosquitoes. Permethrin-treated clothing adds another layer of protection, particularly in areas with heavy midge activity.
For those looking beyond conventional repellents, there’s promising evidence for certain essential oils. A study testing Culicoides biting midges found that lavender, lemongrass, and eucalyptus oils all acted as repellents, with roughly three-quarters or more of the midges in test chambers actively moving away from these oils.8PubMed Central. Attraction and Repellent Behaviors of Culicoides Biting Midges toward Cow Dung, Carbon Dioxide, and Essential Oils The midges that did wander into the essential oil chambers showed markedly reduced activity, suggesting the oils may have a sedating or disorienting effect beyond simple repulsion. That said, essential oil-based repellents typically need more frequent reapplication than DEET and haven’t been tested as extensively in real-world field conditions.
Environmental management also helps. Biting midges breed in moist, organic-rich soil, marshes, and muddy areas near water. Reducing standing water and wet soil around your home cuts down on local breeding habitat. Fans are surprisingly effective too, because biting midges are weak fliers. Even a moderate breeze disrupts their ability to fly and track your CO2 plume. Sitting near a box fan on a porch can do more than any citronella candle.
For fungus gnats indoors, the approach is completely different since they aren’t after you at all. Letting potting soil dry out between waterings eliminates the moist conditions their larvae need. Yellow sticky traps catch adults effectively. Bottom-watering plants instead of top-watering keeps the soil surface dry where these gnats like to lay eggs.
What Makes Some Gnats Blood-Feeders and Others Not
The evolutionary roots of blood-feeding in insects are deep and surprisingly convergent. Blood-feeding, or hematophagy, has evolved independently dozens of times across the insect world, in mosquitoes, sandflies, biting midges, horse flies, bed bugs, and others. Each time, it has required a similar suite of genetic adaptations: enzymes to digest blood, anticoagulants to keep the meal flowing, sensory apparatus to detect hosts, and immune defenses to deal with the pathogens that come along with drinking blood. Research comparing the genomes of mosquitoes and sandflies, two groups that evolved blood-feeding independently, has identified convergent patterns of gene expansion in areas related to neural signaling and sensory perception.9Oxford Academic (Genome Biology and Evolution). Hematophagy Generates a Convergent Genomic Signature in Mosquitoes and Sandflies Biting midges followed a similar evolutionary path, arriving at blood-feeding through their own independent lineage within the Diptera.
Fungus gnats, by contrast, never went down this road. Their ancestors specialized in feeding on fungi and decomposing organic matter, a nutritional strategy that provides everything needed for reproduction without the risks and costs of piercing a host’s skin. From an evolutionary standpoint, blood-feeding is expensive: it requires specialized mouthparts, salivary chemistry, and the ability to evade a host’s immune defenses and physical swatting. Insects only evolve toward it when the protein payoff for egg production outweighs those costs. For fungus gnats, the fungal diet was apparently sufficient, so they never developed the toolkit for biting.
Why Biting Midges Are Worse at Certain Times and Places
If you’ve spent time near marshes, coastal areas, or highlands at dawn or dusk, you already know that biting midges aren’t evenly distributed in time or space. Most Culicoides species are crepuscular, meaning they’re most active in the low-light periods around sunrise and sunset. Some species extend their activity into the night, but midday feeding is rare. This is partly a moisture issue: their tiny bodies lose water rapidly in dry, sunny conditions, so they confine their host-seeking flights to cooler, more humid periods.
Geography matters enormously. Scotland’s highlands are legendary for their midges. Coastal wetlands, tropical forests, and areas near livestock operations all harbor dense populations. The presence of livestock is a major attractor on its own. The same study that demonstrated essential oil repellency also showed that Culicoides were strongly attracted to cow dung and CO2.8PubMed Central. Attraction and Repellent Behaviors of Culicoides Biting Midges toward Cow Dung, Carbon Dioxide, and Essential Oils If you live near a farm or stable, you’re living near a year-round breeding ground and feeding station for biting midges. The midges don’t care whether the CO2 and volatile compounds are coming from a cow or from you sitting on your porch afterward.
Seasonality varies by region but follows a general pattern. In temperate areas, biting midges emerge in late spring, peak in summer, and decline in autumn as temperatures drop. In tropical regions, they can be active year-round, with peaks often tied to rainy seasons that create the moist soil conditions larvae need. Climate change is shifting these patterns, pushing midge-active seasons earlier in spring and later into autumn in some areas, and allowing midge populations to establish in regions that were previously marginal for them.7eClinicalMedicine. Oropouche virus: transmission, epidemiology, genetic diversity, and public health implications For people in northern Europe and the northern United States, this means encounters with biting gnats may become more frequent in the coming decades.
The Blood Type Myth and Other Misconceptions
A persistent popular belief holds that certain blood types attract biting insects more than others. While there is some limited evidence for this with mosquitoes, the research on biting midges specifically is thin. What the evidence does support is that host detection relies overwhelmingly on CO2, skin volatiles, and body heat, not on anything circulating in your bloodstream. A biting midge can’t “smell” your blood type from the air. By the time she’s close enough to your skin to detect anything blood-related, she’s already committed to biting based on other cues.
Another common misconception is that eating garlic, taking vitamin B supplements, or drinking apple cider vinegar will repel gnats. No rigorous study has demonstrated that dietary changes meaningfully alter your attractiveness to biting midges. Your skin chemistry does vary based on genetics, the microbial communities living on you, and to some extent your diet, but the effect of specific dietary supplements as repellents remains unproven.
Perhaps the biggest misconception is treating all gnats the same. People who see tiny flies hovering around their kitchen and worry about being bitten are usually looking at fruit flies or fungus gnats that couldn’t bite them if they tried. And people who get chewed up outdoors at dusk and blame “gnats” sometimes underestimate what they’re dealing with. Biting midges in the Culicoides genus are legitimate medical and veterinary pests that transmit real diseases, and dismissing them as mere “gnats” can lead to inadequate protection in high-risk areas.