Insect light traps may seem like simple devices, but they have a surprisingly long and interesting history, particularly within the food industry where they play a vital role in fly control. For more than a hundred years, both households and businesses have relied on these traps to keep flying pests at bay. But where did it all start, and how has this technology progressed over the past century?
In this blog, we’ll take a closer look at the origins and evolution of insect light traps, exploring how they developed from the earliest basic designs to the advanced models available today, such as those offered by Rentokil. We’ll also unpack the science that makes these traps effective and review the different types that have emerged over time.
How have insect light traps changed over 100 years?
One of the first electrically powered fly traps was patented in 1902 by ER Greene from Rhode Island, USA (US Patent 698274). This design featured a vertical board made of wood or hard rubber, wrapped with alternating positive and negative wires placed close together. The concept is similar to the electric fly killers (EFKs) still used in the pest control industry today. When a fly landed on one wire, it would simultaneously touch another wire of opposite charge, resulting in electrocution. Greene’s design also included a spacer between the wires and the board where an attractant, such as a lump of sugar, could be placed to lure the flies.
At the top of the device was an incandescent bulb, not intended to attract insects but rather to prevent the fuse wires from burning out and to signal when a short circuit occurred. The inventor described this creation as “a new and improved electric fly killer,” specifically designed for places like shops, kitchens, and dining areas.
Although the inventor clearly understood the need to control flies, this particular device was targeted at domestic users. However, during that period, access to electricity was limited, meaning only wealthier households with a power supply or their own generator could consider using it. It is unclear whether the device was ever commercially produced or how effective it truly was at catching flies. No safety assurances were made either, and given its exposed live wires, it might have posed a greater risk to people than to the insects it aimed to eliminate.
Over the following decades, numerous devices were developed to control insect pests, and some of these early designs closely resembled the electric fly killers (EFKs) used in homes today. Many featured bait placed at the centre to lure insects, and some included a lamp to attract them at night or in dark areas. By around 1917, electrified wires were being enclosed within metal cages to prevent accidental contact with live wires (US Patent 1,247,488). Despite this innovation, there was still no clear reference to using light as the primary means to attract flies, although this particular model did include a small incandescent bulb inside the electrified grid, perhaps intended for night-time use. However, incandescent bulbs of that era emitted limited brightness, making them less effective during daylight hours.
In the 1930s, EFK designs shifted focus towards protecting crops from insect pests at night, offering an alternative to chemical spraying. The Folmer Chapin Corporation in New York filed several patents for an “insect exterminator” that initially used incandescent bulbs and later incorporated mercury vapour lamps to attract insects in outdoor settings. Mercury vapour lamps were preferred as they produced less heat, and it was noted that while insects such as house flies are drawn to light, they tend to avoid heat sources (US Patent 1,962,439). However, these outdoor traps did not become popular due to their high operational costs and limited effectiveness.
Advances in understanding fly vision and behaviour
Although mercury vapour lamps were invented in the late 1800s and it was known they emitted UV light, their potential for attracting insects was not yet understood. Initially, these lamps were used mainly for commercial lighting, such as streetlights, and for germicidal purposes using UV-C light, a benefit recognised even at that time.
It wasn’t until the 1970s that researchers discovered insect eyes are sensitive to UV light, particularly to wavelengths between 310 and 370 nanometres within the UV-A band, which is closest to visible light. This range was found to be especially attractive to house flies. Further studies revealed that UV vision plays an important role for insects in locating food, navigating their environment, and selecting mates. Interestingly, UV sensitivity is not limited to insects; it is also present in some vertebrates, including birds, fish, mice, and rats.
Indoor lighting sources, such as incandescent, fluorescent, and increasingly LED lamps, emit minimal UV light. While fluorescent tubes do produce UV light from the mercury vapour inside, this light is absorbed and re-emitted as visible light by the tube’s internal coating. In natural sunlight, UV intensity decreases significantly at shorter wavelengths because ozone absorbs UV-C rays, but sunlight remains strong in the UV-A range. Studies have shown that house flies and fruit flies have light receptors that peak at two UV-A wavelengths, as well as in three visible light ranges — blue, cyan, and green.
Flies have highly specialised eyes equipped with photoreceptors that detect light across a broad spectrum of wavelengths, making them naturally drawn to bright light sources. Research has shown that their neural circuits make them especially responsive to ultraviolet (UV) light compared to visible light. This behaviour, known as the open-space response, drives flies towards bright areas rich in UV light and away from dim, enclosed spaces.
Because indoor environments lack natural UV light, UV lamps stand out to flies and also attract other insects such as bees, butterflies, and certain beetles that can detect UV wavelengths. As a result, UV lamps have become the primary lure used in traps targeting house flies and biting insects.
However, while UV electric fly killers (EFKs) are popular in households for controlling mosquitoes and other biting flies, they are largely ineffective for this purpose. Instead, they often harm beneficial insects when used outdoors, negatively impacting local ecosystems. For example, one study found that out of 13,789 insects killed by EFKs around homes, only 31 were biting flies, including mosquitoes. This aligns with current understanding that many fly species do not possess UV light receptors.
Due to the Minamata Convention on Mercury, fluorescent lamps are being phased out globally, with European countries starting the ban in September 2023. This has driven the adoption of LED lamps in insect light traps (ILTs). LEDs not only provide effective UV wavelengths for attracting flies but also consume less energy. The first commercially available ILTs using LED technology were launched in 2017.
In sectors such as food production and pharmaceuticals, which demand high standards of hygiene and pest control, EFKs only became common from the 1960s onwards. Early models for these industries were adapted from existing designs used in homes and outdoor settings, typically featuring a cylindrical shape with a fluorescent tube positioned vertically at the centre.
Due to the need for greater effectiveness, safety, and adherence to regulations, the traditional EFK design proved unsuitable for certain environments. In the 1980s, the FDA’s retail food protection guidelines specified that only wall-mounted units could be used, all electrically charged components had to be safely enclosed to prevent accidental electrocution, the devices needed certification from a recognised testing laboratory, and they had to be installed at least five feet away from any exposed food items.
Today, it is understood that contamination risks extend well beyond the visible debris created when insects are electrocuted. Research evaluating EFKs has shown that the risk of spreading fly-borne pathogens increases significantly when these devices are in use. Tiny particles can remain suspended in the air for long periods, enabling them to travel further on air currents, much like airborne diseases. As a result, EFKs that kill insects by electrocution are not suitable for high-hygiene environments, as they may actually facilitate the spread of harmful pathogens carried by flies. The only truly safe and effective solution in such settings is to capture the flies rather than destroy them.
What are the essential design features of a modern insect light trap?
After decades of development, some basic design features of ILTs were fixed by scientific findings
and regulations. The latest ILTs don’t look very different from early models from the outside, but
there are multiple design features that are hidden to the untrained eye that make them more
sustainable, efficient and effective for insect pest control.
1. Attractiveness to flies
To attract flies, there must be a high UV light output at the wavelengths that fly eyes are most
sensitive to. The light must be brighter than its surroundings, so in bright light the UV output must
be sufficient to attract flies, but in low ambient light the output of the lamp can be reduced while
keeping effectiveness.
2. Protection of food from contamination
Capturing the insects whole, containing them and being able to dispose of them easily and safely is
an essential feature for an ILT in high-hygiene business environments. Glueboards and rolls capture
and contain flies, and glueboards can allow easy monitoring of the insect catch. Safety for food
handling environments can be certified, such as with HACCP certification, to give assurance that the
design is suitable for a business.
3. Free of toxic substances
LEDs are safer than fluorescent tubes, with no mercury, glass and several other toxic chemicals that
could contaminate the area and cause a hazard on shattering and require specialised waste
disposal.
4. Low energy costs
LED lamps give high UV output and use less energy than fluorescent lamps. LED lamps vary in UV
output and energy use, so using high-efficiency and low-energy lamps is critical even for LEDs.
5. Low waste
LED lamps last at least three times longer than fluorescent tubes, reducing the number of lamps
going to waste, but LED lamps also need designing to optimise their life. LED lamps used in ILTs use
less materials, which reduces waste, and they do not require specialised waste disposal treatment.
6. Exposure to UV radiation
UV-A light from ILTs is generally considered safe for typical exposure levels, being far less than
would be experienced outdoors, but the effects of long-term exposure are not known. In the
interests of caution, it is better for an ILT to shield people from direct exposure to UV radiation by
directing the light above head height using louvres.
7. Safe from ingress
An electrical device should be designed to be safe from damage by liquid or dust ingress, and there
should be no risk of electrocution to people nearby in case of accidental water ingress. A rating of
IP65 will give protection from dust and sprayed water.
8. Sustainability
Sustainability issues include whole-life use of resources and the effect on the environment, from
manufacture to disposal. All components of the device, consumables, servicing and whole-life
carbon footprint should be considered. There is a range of certifications that give assurance to
sustainability claims, such as the Planet Mark.
9. Servicing convenience
Glueboards need to be changed periodically for monitoring insect pests and replacing them. The
unit’s design should make this a safe and easy operation, including accessing and opening the unit
and replacing the glueboard or roll.
10. Visual attractiveness
This is important in environments such as hospitality front of house, where the ILT needs to fit into
the decor of the surrounds and not stand out as a piece of industrial hardware that advertises there
is an insect problem.
Contact Rentokil for commercial fly control solutions.
