Whether fire keeps bugs away is a complex question involving chemistry, thermal physics, and insect biology. For biting insects like mosquitoes, fire is an effective deterrent, though not for the reason most people might assume. However, for other types of insects, the flickering flame can act as a powerful beacon, drawing them in with disastrous consequences. Understanding the specific mechanisms of repulsion and attraction reveals how to best utilize a fire for comfort during an outdoor gathering.
The Primary Repellent: Smoke and Chemical Irritants
The primary repellent effect from a fire comes not from the flame’s light or heat, but from the incomplete combustion that generates smoke. Wood smoke is an aerosol containing hundreds of compounds, which includes phenols, acids, and carbonyls that are irritants to many insects. These volatile organic compounds overwhelm an insect’s highly sensitive olfactory system, especially the chemoreceptors located on their antennae.
Biting insects like mosquitoes are strongly attracted to the carbon dioxide and volatile chemicals released in human breath and sweat. Smoke creates a chemical barrier that masks these attractive host cues, confusing the insects’ ability to navigate toward a target. The smoke itself physically irritates the insect’s chemoreceptors, causing an avoidance reaction that makes them fly elsewhere. This effect relies on the smoke being thick enough to remain concentrated in the air around people, often diminishing quickly in windy conditions.
The Role of Heat and Thermal Avoidance
Localized heat from a fire contributes to keeping some insects away through a mechanism known as thermal avoidance. Many insects, particularly those that are small and thin-bodied like mosquitoes, are highly sensitive to sudden changes in air temperature. The immediate zone around a fire pit or campfire generates temperatures significantly higher than the ambient environment.
While intense heat is lethal to insects, the ambient heat surrounding the fire acts as a deterrent, causing them to steer clear of the immediate perimeter. The upper thermal tolerance limit for many insects is around 50°C, and they will avoid areas approaching this temperature. However, this thermal effect is confined to a relatively small area, as heat dissipates rapidly through convection and radiation, making it a much shorter-range defense than the chemical fog of the smoke.
Why Some Insects Are Drawn to Fire
Counterintuitively, fire can be a strong attractant for many nocturnal flying insects, a phenomenon explained by their innate behavior known as positive phototaxis. Moths, beetles, and certain midges are genetically programmed to move toward a light source. Historically, these insects evolved to use celestial light, such as the moon, for orientation and navigation during flight. A theory suggests that insects use a “dorsal-light-response” to maintain their flight attitude by keeping their back facing the brightest visual hemisphere. When an artificial light source like a fire is nearby, this mechanism becomes confused, causing the insect to constantly turn its back toward the flame, resulting in the erratic, spiraling flight path that often leads them directly into the flame.
An even more specialized group, known as pyrophilous insects, are actively drawn to fire. Certain fire beetles, such as those in the genus Melanophila, possess specialized infrared receptors that can detect the heat of a forest fire from miles away. These beetles are attracted to the fire because their larvae only develop in the wood of recently burned trees. They also have olfactory receptors that allow them to sense smoke, further guiding them toward the source.
Maximizing Repellency Using Specific Fuels
Since smoke-borne chemicals are the most effective repellent mechanism, selecting certain fuels can maximize a fire’s bug-deterring properties. Woods rich in volatile organic compounds produce smoke that is more irritating to insect chemoreceptors. Cedar and pine woods are effective options because they release aromatic oils, with cedar smoke containing thujone, a compound that mosquitoes find unpleasant.
Adding fresh or dried herbs to the fire can further enhance the effect by releasing volatile chemicals that act as natural insect repellents when smoldered. To best utilize these materials, they should be placed on the embers to create a slow, smoldering burn, maximizing the release of these compounds over time. Effective herbs include:
- Sage
- Rosemary
- Lavender
- Mint
- Eucalyptus leaves and wood, which release cineole and other compounds known to have significant repellent properties against mosquitoes.