Why Are Insects Attracted to Lights?

How Artificial Lights Affect Insects' Navigation at Night

Three moths flying around illuminated light bulb.
Artificial light confuses night flying insects, which navigate by moonlight. PIER/The Image Bank/Getty Images

Turn on your porch light after sunset, and you will be treated to an aerial display by dozens, if not hundreds, of bugs. Artificial lights attract moths, flies, crane flies, mayflies, beetles, and all sorts of other insects. You may even find frogs and other insect predators hanging around your porch at night, taking advantage of the easy pickings. Why are insects attracted to lights, and why do they keep circling around and around like that?

Night Flying Insects Navigate by Moonlight

Unfortunately for the insects, their attraction to artificial light is a cruel trick caused by our innovation moving faster than their evolution. Night flying insects evolved to navigate by the light of the moon. By keeping the moon's reflected light at a constant angle, insects can maintain a steady flight path and a straight course.

Artificial lights obscure the natural moonlight, making it hard for insects to find their way. Light bulbs appear brighter and radiate their light in multiple directions. Once an insect flies close enough to a light bulb, it attempts to navigate by way of the artificial light, rather than the moon.

Since the light bulb radiates light on all sides, the insect simply cannot keep the light source at a constant angle, as it does with the moon. It attempts to navigate a straight path, but ends up caught in an endless spiral dance around the bulb.

Is Light Pollution Killing Insects?

Some scientists believe light pollution is leading to a decline in certain insects. Fireflies, for example, have difficulty identifying the flashes of other fireflies where artificial lights are present.

For a moth that lives only a few weeks, a night spent circling a porch light represents a significant chunk of its reproductive lifespan.

Insects that mate between dusk and dawn may be drawn to artificial lights instead of seeking mates, thus reducing their chance to produce offspring. They also waste a considerable amount of energy, which can be deleterious in species that don't feed as adults and must rely on energy stores from the larval stage of the life cycle.

An extended line of artificial lights, such as street lights along a highway, can create a barrier to insect movement in some circumstances. Scientists refer to this as the crash barrier effect, because wildlife is effectively prevented from moving across the land by the lights impeding their navigation.

Another negative impact of artificial lighting on insects is called the vacuum cleaner effect, where insects are lured from their normal environment by the draw of the lighting. Mayflies spend their immature stages in water, and finally emerge and develop wings as adults. Their lives are brief, so anything that interferes with mating and egg laying can be disastrous to a given population. Unfortunately, mayflies sometimes swarm streetlights along bridges and waterways, and wind up depositing their eggs on road surfaces before dying en masse.

Which Artificial Lights Impact Insects the Most?

Mercury vapor lights are extremely effective at attracting night flying insects, which is why entomologists use them to observe and capture specimens.

Unfortunately, street lights that use mercury vapor bulbs also do an exceptionally good job of attracting insects. Incandescent bulbs also prove confusing to night flying insects, as do compact fluorescent bulbs. 

If you want to reduce the impact of your outdoor articial lights on insects, opt for either warm color LED bulbs or the yellow bulbs marketed specifically for reducing insect attraction.

Sources: