Stupid Question ™
July 26, 2001
By John Ruch
Q: Why are bugs attracted to bright lights? And why only at night? Why do yellow lights deter them?
A: It is presumed that nocturnal moths and beetles mistake bright artificial lights for the Moon, which they probably use as an orientation device while flying at night.
Day insects aren’t fooled because there are very few artificial lights as bright as the Sun (and there are many other visual cues in daylight).
Yellow, orange and red lights (including sodium-vapor street lamps) are essentially invisible to most insects, which can’t see into the red portion of the spectrum.
Insects do see blue and ultraviolet light very well, including those wavelengths in moonlight. Candle flames and campfires contain weak amounts of these wavelengths, and have attracted insects in small numbers since ancient times. However, the common modern light bulb, and especially the mercury-vapor bulb, put out lots of UV and can attract huge numbers of insects.
The Moon theory says that moths know that the brightest light at night—the Moon—is always “up.” But if they mistake a street lamp for the Moon, they find themselves in the utterly confusing position of flying above or past the “Moon.” They can no longer navigate. Also, the brightness of the light at close quarters makes them think it’s daytime, so they go to sleep.
This theory is not airtight, but the evidence is strong. Lights attract insects best on cloudy nights (when there’s no real Moon) and worst on clear, moonlit nights. Also, lights attract mostly migratory species, or males that are traveling to find mates.
Similarly, night-migrating birds have been thrown off course by artificial lights on cloudy or moonless nights.
There’s experimental evidence that moths fly in a straight line by keeping themselves at a regular angle from the Moon, using it as a reference point. Some moths maintain this same angle from artificial lights before spiraling into them. This spiral could be the result of the confused moth trying to maintain the proper angle as it flies past or above the “Moon.” (Some dive straight into the light, which could be the result of an overflying moth thinking it’s upside down, and diving to keep the “Moon” above it.)
More evidence comes from the surprisingly short range of attraction.
The 125-watt mercury-vapor bulb used in insect-collecting traps has the same brightness as the Moon at a distance of 115 to 1,703 feet (depending on the phase of the Moon you compare it to). But a mercury street lamp 30 feet high will attract insects from only 33 to 56 feet away. Why? The standard mercury bulb has the same apparent size as the Moon at a distance of 43.5 feet—right in the middle of the attraction range. It’s the right brightness and the right size—a perfect fake.
However, the same bulb placed 2 feet off the ground will attract moths from only about 10 feet away. It’s got the right brightness and size, but the wrong elevation, and fools relatively few moths.