The simple act of lighting a candle often introduces a complex biological question: does the flame attract flies, or does the smoke and scent send them away? The answer is not a simple yes or no, but rather a dynamic balance between two competing forces—the programmed instinct to navigate by light and the powerful deterrent effect of volatile chemical compounds. A fly’s final behavior near a candle depends entirely on which signal proves stronger: the physical pull of the flame or the chemical push of the burning wax.
Flies and the Physics of Fire: Light and Heat Attraction
Flies, like many flying insects, possess a strong behavioral tendency known as positive phototaxis, which is the movement toward a light source. This behavior is often a catastrophic navigational error caused by artificial lights. In nature, insects use distant, constant light sources like the sun and moon to maintain a straight flight path, a process called transverse orientation. When an insect attempts to orient itself by keeping a close, artificial light source, such as a candle flame, at a constant angle, this maneuver forces it into a disoriented spiral that ends near the light.
The light quality of the flame is also significant, as insect vision is highly sensitive to the shorter wavelengths of the electromagnetic spectrum. House flies show the strongest phototactic response to ultraviolet (UV) light, which falls in the 300 to 400 nanometer range. While a candle flame emits a broad spectrum of light, it includes some of these attractive shorter wavelengths, reinforcing the insect’s tendency to approach. Beyond light, the heat generated by the flame is a secondary attractant, especially for species that thrive in warm environments. The small thermal signature of a flame can contribute to the overall physical pull toward the candle.
The Chemistry of Repulsion: Scents and Smoke
While the light of the flame exerts a physical pull, the chemical output of the candle frequently overrides this attraction, acting as a powerful deterrent. The most effective repelling factor is the volatile organic compounds (VOCs) released by scented candles, particularly those infused with essential oils. Citronella oil, for example, contains compounds such as citronellal, citronellol, and geraniol. These molecules interfere with the fly’s highly specialized olfactory receptors on their antennae, effectively jamming the chemical signals they use to navigate and find food.
Other natural scents commonly found in candles, such as peppermint, lavender, and eucalyptus, contain similar volatile compounds that flies find irritating. These chemicals create an aversive environment that the fly instinctively avoids. Furthermore, the physical output of the burning wax, specifically smoke and soot, functions as a mechanical and respiratory irritant. Incomplete combustion produces minute airborne carbon particles that can physically foul the insect’s delicate chemosensory hairs (sensilla), compromising their sense of smell. These irritating particulates prompt an immediate avoidance response, making the area surrounding a smoky candle uninhabitable for the fly.
The Verdict: Do Candles Attract or Repel Flies?
The final behavior of a fly is the result of a sensory competition between the physical attraction of the flame and the chemical repulsion of the candle’s emissions. Unscented candles, typically made from waxes like paraffin or beeswax that lack repellent VOCs, tend to fall into the attractant category. In these cases, the insect’s natural phototactic response to the light and the minor thermal signature of the flame dominate, drawing the fly into the immediate vicinity. This is why simple, unscented tapers can sometimes appear to lure insects.
Conversely, strongly scented candles, especially those engineered with high concentrations of essential oils like citronella, are primarily repellents. The powerful chemical signal from compounds like citronellal and geraniol, combined with any smoke and soot produced, creates a defensive perimeter that overwhelms the light attraction. For practical purposes, a smoky, citronella-based candle is the superior choice because the chemical deterrents are potent enough to override the insect’s instinct to orient toward the light.