Why are insects attracted to light?

For thousands of years, humans have observed that flying insects are attracted to lamp and candle flames at night and, at some point, realised that light could help control them. Even back in Roman times, there are records of lamps used at night to protect bee hives by attracting and trapping wax moths. An almost identical trap was used in the 1500s, and one of the earliest American patents was issued in 1847 for a beehive incorporating a light trap.

So, insect light traps are nothing new, but no one knew why flying insects were attracted to the light. You might think it doesn’t matter, but how can we design the most efficient insect light trap – to trap pest insects – if we don’t know why the light helps to catch them?  

Trapping pest insects to prevent contamination and loss of food products is essential for many businesses, including restaurants and food processing, retail and storage – and that’s not to mention people also wanting to be protected from biting insects in their homes and leisure places. Finally, after thousands of years of exploiting the insect-light phenomenon, we think we’ve got an answer.

What do insects see?

Insects have several types of photosensitive cells in their bodies that give them a different view of the environment compared to mammals.

Compound eyes

Most adult insects have compound eyes, one on either side of the head, consisting of 100s to 1000s of tiny elongated hexagonal cell structures called ommatidia. Each one collects a simple image from a small part of the surroundings and when they are combined in the insect’s brain form a mosaic-like image across a wide angle of vision.

Different species of insects have from one (eg Heliconius butterflies, Aedes albopictus mosquito) to 15 (eg Graphium sarpedon, a butterfly species) types of colour photoreceptor in their ommatidia, detecting different ranges of wavelengths of light. Some butterfly species can detect up to 10 ranges of colour, with peaks of sensitivity spread across the spectrum from near UV light to red. This means some of the photoreceptors in those with 15 could be redundant or detect the same wavelengths, but also could have other functions, such as detecting different intensities. 

The common food pests, house flies and fruit flies, have five colour photoreceptors, with two detecting near-UV (UVA), and one each for blue, cyan and green wavelengths. Flying insects with UV vision are attracted to UV more than visible light — many insect species cannot see UV and some only have one photoreceptor, limiting them to a narrow band of visible light. As daylight has high levels of UV, their preference for it is thought to drive an attraction to open, bright spaces and away from dim places. No insect can see UV below around 300nm because the chitin that their bodies and eye lenses are made of absorbs UV with shorter wavelengths. This helps in designing insect light traps and ensures that the UV light used is safe for humans.

Insect eyes are highly adapted to detecting movement. The thousands of ommatidia arranged over a wide angle allow the detection of movement all around the insect. Combined with neural circuits in the insect brain that process visual information from the ommatidia at a high frame rate, this ability gives insects quicker reactions to movement than humans. 

The eyes of nocturnal Insects are also highly adapted to night vision. Their eyes sacrifice detail for light sensitivity enabled by their ability to capture and process photons more efficiently. They can manoeuvre in complex surroundings and distinguish colours at night at light levels that would appear dark to humans. They can also analyse the pattern of polarised light around the moon for navigation.

Ocelli

These are simple eyes found in many adult insects, usually on the top or front of the head. They do not form an image but sense the brightness and direction of light. They are thought to help insects to maintain their flight stability and to navigate using the Sun’s position.

Polarization-sensitive photoreceptors

These are found in some ommatidia of insects in the dorsal or ventral regions of their eyes. They are specialized for the detection of sky light, distinguishing from reflected light and helping insects to orient themselves and navigate using the polarization pattern of the sky.

Extra-ocular photoreceptors

These are light-sensitive cells located on other parts of the insect body, such as the antennae, abdomen or legs. They sense the intensity and wavelength of light and are believed to play a role in regulating circadian rhythms and body temperature. 

The structure of ommatidia in a compound eye

Old theories about why insects are attracted to light

Insect vision has evolved to help them survive in their environments and for flying insects the skills needed to survive require more complex abilities. Vision helps insects find food sources, such as leaves and flowers, use visual clues, including polarised light, to orientate and navigate, detect shelters and landmarks, find and choose a mate, detect and escape from predators, find the best place to lay eggs, such as young leaves.

Various theories to explain the phenomenon of insects gathering around artificial lights at night have been proposed for years, including:

• Navigation using celestial objects. Nocturnal insects use the moon and stars to navigate at night by keeping them at a specific angle to their flight path, and artificial lights, which they can get close to, confuse them. This explains why they seem to circle lamps.

• Feeding behaviour. Moths and other insects that fly at night may associate light at wavelengths that are attractive to them with their food sources, such as flowers.

• Getting warmth. Some insects may be drawn to sources of heat, which is emitted by older sources of artificial light – but obviously, LEDs emit far less heat.

• Escape from danger. Insects head towards light as a mechanism to escape from predators that may be hiding in dark areas such as plant foliage.

• Open-space response. Most flying insects are attracted towards open, bright spaces and away from dim places. They have what is called a positive phototactic response and show a preference for UV light, which has high levels in sunlight. 

• Blinded by the light. Insect eyes are sensitive to the low levels of light that exist at night and artificial light blinds them, causing them to crash into the light or objects near it.

Some of these are little more than guesses, while others are based on research into fly behaviour.

Is this the real reason why insects are attracted to artificial light?

New research published in 2023 used high-speed cameras to film insects in flight as they responded to different artificial light sources in low ambient light levels. This was done both in a lab in controlled conditions and in a Costa Rican rainforest. The insects studied included dragonflies, butterflies, moths, several fly species, and bees.

When diffuse UV light was shone from above in a container in the lab, honeybees and flies flew upwards in stable flight, as in normal escape flight. When the diffuse UV light was shone from below, neither type of insect was able to fly normally. They tilted and inverted, causing them to crash into the floor. This did not occur when flies were tested with white light, so is an effect unique to diffuse UV light — for the fly species used in the study. This didn’t affect fruit flies, however, which can see similar UV wavelengths to other flies, and nobody knows why!

The same tumbling and crashing effect with UV light below insects’ flight was observed in the Costa Rican rainforest, and normal flight was observed when illuminated from above. This shows the behaviour is caused by the disturbance of the insects’ sense of upwards and downwards by the UV light being below them instead of from the sky. 

High-speed photography showed the insects tilt their bodies towards nearby point light sources at night, causing them to fly orthogonally (at right angles), so they end up circling the lights. This explains the behaviour of nocturnal insects gathering around a light source.

Although this answers some of the questions about insect behaviour to artificial light, there is still much we don’t know and can’t explain about the remarkable vision abilities of insects. But this knowledge will likely help us find more efficient ways to catch pest insects.