Flies, like many insects, are often seen congregating around light sources. Their attraction to light involves exploring their visual systems and natural instincts.
Understanding Fly Attraction to Light
Flies are indeed attracted to light, a behavior known as positive phototaxis. However, this attraction is not universal across all light types or all fly species. While some insects, like cockroaches, exhibit negative phototaxis and move away from light, many flying insects, including flies and moths, are positively phototactic. This attraction is influenced by various factors, including the light’s wavelength and intensity. Flies, for instance, have compound eyes adapted to detect movement and light patterns, which they use for navigation.
The Science Behind Light Attraction
The attraction of flies to light is rooted in their biological need for navigation. Flies, like many insects, use natural light sources, such as the sun and moon, for orientation. This navigational method, transverse orientation, involves maintaining a constant angle relative to a distant light source to fly in a straight line.
Artificial light sources, however, disrupt this natural mechanism. Because human-made lights are much closer than celestial bodies, maintaining a fixed angle causes flies to fly in spirals or circles, drawing them ever closer. A recent study suggests insects don’t fly directly toward light but rather tilt their backs toward it, a “dorsal light response” that helps them maintain flight attitude. When faced with an artificial light source below them, this instinct can lead to disorientation, causing them to fly in confused patterns or even crash.
Different Types of Light and Their Effects
Flies, like many insects, perceive a different spectrum of light than humans, being particularly sensitive to ultraviolet (UV) light and shorter wavelengths of visible light, such as blue and green. Their compound eyes are well-equipped to detect UV-A light, with peak attraction often observed at wavelengths between 310 and 370 nanometers. This sensitivity is due to their photoreceptors being tuned to these specific wavelengths. Conversely, flies show less interest in longer wavelengths, such as yellow, orange, and red light; for many insects, red objects may even appear black.
Traditional incandescent bulbs emit a broad spectrum of light, including UV and a significant amount of heat, making them highly attractive to flies. Modern LED lights, while energy-efficient, can still attract flies if they emit light in the UV or blue spectrum. However, LEDs that produce warmer, yellowish light or lack significant UV emission tend to attract fewer insects. The intensity of UV light also correlates with the number of flies attracted, with greater intensity drawing more flies.
Using Light Knowledge in Daily Life
Insect light traps, commonly known as bug zappers, leverage this knowledge by using UV light to attract flies and other light-sensitive insects. These devices typically employ UV-A light, which is highly attractive to flies, to lure them to an electrified grid. Some newer bug zappers also incorporate additional attractants like carbon dioxide or specific pheromones, as not all insects are solely attracted to UV light.
For outdoor lighting, choosing yellow or amber-colored “bug lights” can help reduce the number of flies attracted. These lights emit wavelengths that are less visible or attractive to flies. While yellow lights do not repel insects, they simply do not attract them as much as white or blue lights do. Using LED lights with warmer color temperatures or those specifically designed to minimize UV emission can also be an effective strategy to reduce insect presence.