Why are insects attracted to light?

It doesn’t take a pest control professional to know that insects are attracted to light. Just think of all those times you’ve seen moths and other insects frantically flying into or around light fixtures when it’s dark out. But – unlike a pest control professional – you probably haven’t considered why, and how a greater understanding of this natural phenomenon can help to develop more effective fly killers (insect light traps).

Whilst it may not be of direct interest to consumers, extensive research into this topic has been undertaken to support food businesses such as food processing and food retail, to help ensure consumer food safety, and avoid outbreaks of fly-borne diseases such as Salmonellosis.

The team of scientists at Rentokil’s Global Research and Development Centre has looked into understanding the physics of how light impacts the biological attraction of flies to a trap. This research has helped uncover LED technology as an insect attractant far superior to conventional light sources.

Why are flies attracted to light?

There is no single scientific explanation as to why flies are attracted to light. There are several theories which offer a possible explanation, they are:

• They see light as an emergency beacon and a safety signal
• They use light to help with their flight and navigation
• They have a natural attraction to light known as phototaxis

Why do flies use light for safety?

For some insects, a bright light source may be seen as an emergency beacon. When in doubt, these insects instinctively head towards light sources, which are generally positioned on higher ground than the hazardous environment they are currently in.

Light, for some insects, act as a familiar safety signal; just as air bubbles lead the way to the surface of the water for some underwater creatures.

How do flies use light for navigation?

Another popular theory for attraction to light is that insects use it as a navigational aid. An insect flying north, for example, is able to judge its direction by keeping a natural source of light, such as the sun or moon, on its right. This method works well as long as the source of light remains both constant and at a distance.

If an insect encounters a round incandescent porch light, however, it becomes confused by its source. This explains the peculiar behaviour of a moth continuously encircling a light source – it instinctively wants to keep the light on a certain side of its body whilst navigating its route.

What is phototaxis?

The difference between insects that are attracted to light and those which are not, is a phenomenon known as phototaxis. Certain insects, such as cockroaches or earthworms, have negative phototaxis, meaning they are repelled by an exposure to light. Moths, flies and many other flying insects have positive phototaxis and are naturally attracted to it.

The scientific debate around a flies attraction to light

There is some debate in the scientific community over why a positively phototactic insect, like a fly, will continue to hover around an artificial light source even when natural light becomes available.

Some believe that the insect is not attracted to the light itself, but the darkness surrounding it.

Others suggest the insect’s eyes, which often contain multiple lenses, struggle to adjust from light to dark, leaving the insect vulnerable to predators whilst night-blind. In this case, the insect may find it safer to remain in the light rather than fly away and become too blind to react to threats and obstacles.

Why are flies attracted to LED?

As homeowners, we have been convinced for a number of years now that LED lights are a smarter, more energy efficient way to light our homes. More than that, LED technology also emits light in a different way and produces UV-A as intense beams of light, which penetrate further into the surrounding space than light phosphor lamps, for example.

House flies, in particular, can see and are attracted to UV light, which humans cannot see. Because we can’t see UV light, we don’t use it in our buildings and our environment. This means that any UV light that is used in urban buildings light up like a beacon, making UV light traps incredibly effective: they stand out in the human environment as the brightest source of UV-A light.

Using light for fly killing efficacy

Rentokil’s researches are able to prove the efficacy of fly killers using LED technology through a standard Half-Life measure test. The Half-life measure represents the time taken to eliminate 50% of flies released in a test chamber. The lower the Half-life measure, the more effective the insect light trap.

An effective fly control programme, however, must also consider the correct placement of fly trap units (given what we know about phototaxis). The position of fly killer units with respect to local light sources is of critical importance to fly control effectiveness. All this insight – and more – has been distilled into Rentokil’s leading fly control service solutions. Speak to an expert today to find out more.