There are many factors that determine when a fly is attracted to an insect light trap, including the age of the fly, the time of day, the temperature and relative humidity and the levels of ambient light in the area where the trap is located. Testing has shown that flies are more rapidly attracted into light traps positioned at the fly’s usual flying height above the ground, suggesting that traps positioned below 6 feet (2 m) – where such installation is possible - will trap flies faster. Additional research has suggested that, in general, House flies ignore light traps that are over 100 feet (30 m) away, and are much more likely to move towards a light trap that is less than 15 feet (5 m) away, but neither of these figures has been suggested as a valuable way of calculating a specific trap’s area of coverage.
The largest variable in trying to determine the area of coverage for an insect light trap is the competition from other light sources in the immediate area. Sunlight is generally much stronger than artificial lighting and contains a wide range of light frequencies, so is a major competitor for the attention of flying insects, but artificial lighting will also play a role in reducing the area of attraction.
As there are so many local variables that impact performance, there is currently no practical way to determine the area of coverage of insect light traps with any meaningful accuracy.
There are a number of factors that will affect the performance of an insect light trap, depending mostly on the specific site of installation and its environment. In general, there are a few simple tips for installation that can ensure the best performance, but no “simple number” or “hard and fast” rules that apply under every situation.
- Your insect light trap requires regular maintenance to maintain its effectiveness. Dust and airborne grease will collect on the unit over time, reducing its efficiency, and glue boards will need changing or catch trays will need emptying, depending on the model. Site your insect light trap with access in mind to allow easy servicing.
- Most flying insect problems are due to pest insects entering the building from outdoors. Whenever possible, try to locate your insect light traps between the main entry points and the areas you are trying to protect so that you intercept the pests before they reach the critical areas.
- Insect light traps are designed to be attractive to flying insects and will draw them towards the trap. Don’t install your units close to the very areas you are trying to protect - site them to “pull” flying insects away from these areas.
- Sunlight will always outshine indoor lights and contains all colours of light. Where there is competition between an insect light trap and direct sunlight, the sunlight will win. Try to locate your insect light traps away from outdoor windows, and preferably in areas where there is shadow - or at least a darker background.
- High voltage insect light traps (“zappers”) should not be used in areas with exposed food. These units use a high voltage grid to kill the flying insects and have the potential to expose the food to insect particles. Only use glue board based traps in exposed food areas. However, even glue board traps should not be located directly over exposed food or food handling surfaces as they will require regular cleaning and maintenance.
- The most common pest fly species tend to fly at a height between 3 and 6 feet (1-2 m) above the ground. Installing insect light traps in or near this height range maximizes their speed of catch under most conditions, and improves capture efficiency.
- Always look for workplace activity that could impact your insect light trap location. Don’t install traps in narrow passageways or similar areas where they will be damaged by moving people or goods. Installing an insect light trap in an “ideal” location where it interferes with workplace activity will usually result in a damaged and inoperative unit.
- Your insect light trap is an electronic device and should not be placed directly above sources of extreme heat (ovens, deep fryers, etc.). Similarly, it should not be exposed to high moisture or wet conditions (unless the unit is specifically designed as “Jet Proof”). Lastly, you should avoid areas with high air movement (fans, air conditioning vents) as this will prevent flying insects from entering the trap.
Finally, remember that flying insects can only see an insect light trap that is in “line-of-sight”. If there are multiple areas you are trying to protect, position your traps so that at least one can be seen from each area.
The inverse square law is just a mathematical description of the spreading behaviour of a beam of light.
As a beam of light travels along a path the area that it illuminates is a “square” that grows on each side. As the area of illumination is the product of each side of the square, the area grows much faster than the distance travelled. This means that the same amount of light is covering a larger and larger area, and becomes proportionally weaker much quicker than the distance would seem to suggest.
For example, a beam covering a 1x1 square area at a distance of 1 would cover a 2x2 square area at a distance of 2 - a quarter of the intensity (2x2 equals 4 units), and cover a 3x3 square area at a distance of 3 - a ninth of the intensity (3x3 equals 9 units). At a distance of 4 units it would be a 16th intensity, and at 5 units it would be one twenty-fifth intensity, and so on.
The result is that the intensity of the light reaching any one spot reduces very rapidly over distance.
While the actual dispersion of light from an artificial light source such as a bulb or fluorescent tube is somewhat more mathematically complex, the inverse square law is a useful approximation. For example, when the size of the light or UV source is less than one-fifth of the distance to the subject, the calculation error is less than 1%.
Light is a wave of electromagnetic energy, and the colours of light are just different wavelengths. White light is just a mix of all wavelengths. We detect (“see”) light using specialised nerve cells in our eyes that contain proteins that can absorb the energy from specific light wavelengths, and then use that energy to “trigger” the nerve to respond. Different animals have developed different proteins in their eyes for detecting light, and can therefore “see” different parts of the light spectrum.
The nerve cells in human eyes are sensitive and react to light wavelengths between about 400nm (that we perceive as the colour violet) and about 760nm (that we perceive as the colour red). The nerve cells in our eyes just do not respond to wavelengths shorter than 400nm (the spectra we call ultraviolet) or longer than 760nm (the spectra we call infrared), so we cannot “see” them.
Note: The wavelengths described are in nanometres (nm), which are 1,000,000,000 times smaller than a metre!
Our eyes are very well designed to allow us to see under a variety of light levels. The eye’s iris automatically closes and opens to ensure the light reaching the sensitive nerves of the retina are never exposed to too much or too little light. This makes us very poor at judging the apparent light intensity under normal situations.
As an example, a vehicle closely following you at night while driving with its headlights on high-beam is an annoyance, but during the day you would probably not even notice the vehicle had its lights on. This is because your eyes have adjusted for daylight, and are hugely reducing the amount of light they are allowing to reach your eye’s retina.
The reality is that sunlight is usually many times brighter than any normal indoor lighting. Sunlight also contains a mix of all light colours, including the ultraviolet range that is used as the attractant in insect light traps, so it will usually “overwhelm” the light output from an insect light trap in a “head-to-head” competition.
When situated in competition with sunlight, insect light traps will almost always loose.
The ultraviolet range of the electromagnetic spectrum is the part where the light wavelengths are just shorter than our eyes can detect. However, there are significant differences between the potential effects of human exposure to the light within the ultraviolet range, and it has been divided into sub-ranges based on this potential to assist in clarifying the safe exposure limits.
Ultraviolet light is normally considered to be the electromagnetic spectra between wavelengths of 100nm to 400nm. Note: There are 1,000,000,000 nanometres (“nm”) in a metre.
The energy contained within a single photon of light increases as the wavelength decreases. This makes light with shorter wavelengths more “energetic”, and so more capable of causing harm. Light in the human visible wavelengths can be absorbed by our eyes and skin without much potential for damage, but this potential increases in the light wavelengths shorter than we can see.
The UVA region is the longest wavelengths within the ultraviolet spectrum, with wavelengths from 400-315nm. UVA from the sun is not strongly absorbed by the Earth’s atmosphere, and is typically about 5-6% of the sunlight at ground level. Heavy exposure to UVA can cause damage to the eye lens and retina, as well as causing wrinkling of the skin. In general, UVA part of the ultraviolet range is considered to have the least potential for harm.
The UVB region contains wavelengths shorter than the UVA, from 315-280nm. UVB from the sun is strongly absorbed by the Earth’s atmosphere, and therefore only makes up about 0.05-0.5% of the sunlight at ground level. Its higher energy means that it does not penetrate human skin as deeply as UVA, but has the ability to cause more damage. Over exposure to UVB causes reddening of the skin (erythema).
The UVC region contains the shortest ultraviolet wavelengths, from 280-100nm. UVC from the sun is almost completely absorbed by the Earth’s atmosphere, with only traces remaining in the sunlight at ground level. However, UVC has sufficient energy to cause major damage to living cells, including damage to the DNA in the cell’s nucleus, which may result the cell losing its ability to replicate or carry out other vital functions.
The amount of ultraviolet in sunlight outdoors varies greatly according to time of day, time of year, altitude and latitude, with more of present at high altitudes, closer to the equator, and at mid-day in summertime.
The amount of any light (the “Irradiance”) is measured as the concentration of energy received over a specific area in a given time, and usually represented as:
Energy per second (watts) per area (square metre) – or W/sq m
Because the amounts of energy are usually quite small, this is often shown as milliwatts per square centimetre (mW/sq cm) or microwatts per square centimetre (µW/sq cm).
Another term commonly used with regard to measuring ultraviolet light is the “Effective” or “Actinic” UV. This is a measurement of the entire ultraviolet spectrum, with more emphasis given to those wavelengths that have greater potential to cause damaging effects on human skin and eyes. In order to determine the Effective or Actinic spectrum, the actual UV spectrum under investigation is multiplied wavelength by wavelength by an internationally agreed weighting curve which was created from the results of studies of UV damage to human skin and eyes. Known as the ACGIH curve, it has value 1.00 at 270nm (the wavelength most damaging to humans), and falls to less than 0.1 below 215nm and above 305nm. The Actinic value provides a better measurement of the actual potential for harm from any given source of ultraviolet light.
In Cornwall, Southern UK, latitude 50 degrees N, altitude 87m, the peak UVA irradiance observed over three years was 45 W/sq m. This fell to 5 W/sq m in the winter. The ‘effective’ UV irradiance at the same location peaked at 0.13W/sq m in June and fell to about 0.005W/sq m in the following December.
In Coimbatore, India, at latitude 11 degrees N, altitude 411m, UVA irradiance peaked at 66 W/sq m during 1999.
Ultraviolet light can have the potential to cause damage to both human skin and human eyes, dependent on the specific wavelengths involved and the exposure (see section 6 above), and there are agreed international standards for electrical products and systems that emit visible, ultraviolet and infrared light specifically designed to protect against damaging exposure. Certification standards such as UL (formerly Underwriters Laboratories) include testing to these standards as part of their safety certification procedure.
An important quantity in this measurement is the concentration of energy (the dose) to which workers are exposed. Standard EN62471 (2008) “Photobiological safety of lamps and lamp systems” requires that workers are not exposed to a dose of more than 30 Joules per square metre of “Actinic” ultraviolet light in a working day of 8 hours. To meet this requirement, the actinic ultraviolet level to which they are exposed throughout a working day must not exceed 1mW/sq m. “Actinic” ultraviolet measurements are specifically weighted based on the potential for the wavelengths involved to cause harm (see section 7 above).
You may note that, under these standards, a worker would be limited to less than 4 minutes of unprotected exposure to the midday summer sun outdoors in Cornwall, Southern UK (see section 7 above).
Years of research has shown that a large number of insects have the ability to detect light in the ultraviolet range, and many routinely use this ability in their activities. For example, both butterflies and bees can see patterns in flowers that are only visible in the ultraviolet range (butterflies and bees both use nectar from flowers as a food source). While many flies also have the ability to detect light in the ultraviolet range, there are differences between species to both the ultraviolet sensitivity of their eyes and their reaction when ultraviolet light is detected.
Most of the common flying insect pests found within structures are known to both see ultraviolet light and show a positive movement response towards ultraviolet light, so they are attracted. This includes House flies, Blowflies and Bottle flies, Fruit flies, Yellow Jacket wasps, and many varieties of moths.
When light is absorbed by a molecule within a living cell the energy within the light is transferred to the molecule. In most cases this results in minor heating, but if the energy transferred is high enough it can cause the molecule to change or “break”. This is critical if the molecule is part of the organisms’ genetic code (the DNA), where damage or changes usually result in the inability to replicate or a failure to support the many cellular functions needed for survival.
The short wavelength ultraviolet light in the UVC range contains more energy per “packet” (photon) of light - enough energy to cause critical molecular damage to the genetic material in living cells. This is used in a number of applications as a cost effective non-chemical method of sterilization, where the UVA light is able to safely eliminate a wide range of pathogens (bacteria, viruses and fungi).