by David Moore
Manager of Technical Services and Board Certified Entomologist
with contributions by Eric Smith, PhD, BCE
For several decades pest management professionals (PMPs) have been advising their customers about the kind of lights to use and their proper placement on the exterior of their home or business to reduce the attraction to night-flying insects. When they became available, PMPs advised customers to change exterior white incandescent or fluorescent bulbs to less attractive yellow bulbs, and mercury vapor lights to less attractive sodium vapor lights. Recently, the lighting industry has brought LED (Light Emitting Diode) lights onto the market for outdoor use.
The question is, are LEDs a good choice for outdoor lights, and why or why not?
What light is attractive to insects?
Humans can see light wavelengths in the electromagnetic spectrum from 400-800 nanometers (nm), which ranges from violet to red in color, but does not include ultraviolet (UV) light at 350 nm. Insects can perceive light in the 300-650 nm range, but prefer light that is between 300-420 nm which includes UV light. A light’s UV output is probably the most important factor in its attractiveness to insects. Since most insects are attracted to UV light, this is why most ILTs (Insect Light Traps, including bug zappers) utilize UV/blacklight bulbs as their source of attraction.
Insects generally see 3 colors of light, Ultraviolet (UV), blue and green. Bright white or bluish lights (mercury vapor, white incandescent and white florescent) are the most attractive to insects. Yellowish, pinkish, or orange (sodium vapor, halogen, dichroic yellow) are the least attractive to most insects. When white incandescent bulbs were all that was available, the advice was to change them to yellow incandescent bug bulbs. Yellow and “warm white” bulbs tend to be more like sunlight and are less attractive to insects than “cool white” bulbs that have a more bluish tone. Red bulbs are even less attractive to insects than yellow, but red provides little visible light to humans and it carries an “undesirable” social stigma from decades ago.
In addition to the color or wavelength of light, insects are also attracted to the brightness and to the heat from lights. The greater the bulb’s wattage rating, the brighter the light and the greater the drawing distance. Also, the greater the wattage, lights that use glowing filaments (incandescent, halogen, etc.), generate an increasing amount of heat. Cool lights that generate light from flowing gas (LED, sodium vapor, mercury vapor, fluorescent, etc.) generate less heat.
Characteristics of LED light
LED bulbs can range in color from UV (350 nm) to infrared (700 nm). The light pattern emitted (angle of dispersion) can range from very narrow, like a laser pointer, to broadly diffuse like a flood/spot light. How bright a LED light is depends on the amount of current passing through the bulb. Unfortunately, while a higher current level will produce a brighter light, it also means the bulb won’t last as long. LED lights are much more energy efficient when compared to incandescent bulbs, using only 1/5 to 1/10 the power, and they last at least 10 times longer. LED bulbs are also smaller and stronger, have no filament, and usually have no glass to break.
Most LEDs don’t emit much or any UV light, but there are exceptions (see below). LED lights use a mixture of various light colors to produce their “white” light rather than producing white light. Some LEDs will show more of one color than the other colors. LED lights that are “cool white” or are more bluish may be more attractive to insects than the warmer or more yellowish lights.
Are LED lights attractive to insects or not?
Because most LED lights don’t emit UV light and generate little heat, they tend to have little attraction to insects. However, some insects may be attracted to one or more of the light colors used in the color mix used to produce the LED’s “white” light. While insects are attracted to light, LED lights give off little heat, and also emit the wrong colors of the visible light spectrum for most insects, resulting in that minimal numbers of insects are attracted to them.
Some LEDs are specifically manufactured to produce UV light, such as those used in mosquito light traps and as plant grow lights. Then there are those that are used for disinfection, sterilization, and curing of certain industrial coatings (e.g., dental tooth-colored fillings, also called composite resins).
Not all LED spotlights are invisible to insects. UV LED spotlights, cool white LED’s, and neutral colored LED’s may attract insects to the same degree as fluorescent or halogen bulbs. A better choice is a warm white or off white for LED spotlights.
Take home message
Most LED lights sold for residential lighting emit almost no UV light and are only slightly attractive to insects. LEDs that emit bluish or purplish light do attract some kinds of insects. So, LED lights are a good choice if you want to reduce flying insect problems. At present, the initial cost of LED bulbs is somewhat pricy but they are getting cheaper over time.
Choosing LED lights
The measurement of the amount of light produced by a LED bulb is done in units called lumens. For comparison, a 60 watt incandescent bulb produces 800 lumens and a 100 watt incandescent bulb produces about 1,600 lumens.
The color of a LED bulb is measured in degrees Kelvin (K). In general, the lower numbers of Kelvin have warmer, more yellowish color temperatures. A bulb of about 2500 Kelvin temperature would have a light resembling a candle, and that of about 3500 Kelvin color temperature would be comparable to a halogen bulb. At about the 5000 range, LED bulbs produce more scotopic lumens, which is the amount of light that is registered by the human eye, and while the light would seem brighter, it would not look like natural light. At over 6000 Kelvin results is a light with a lot of blues and purples that is bright but not very inviting. It’s advisable to use high Kelvin bulbs for outdoor lighting purposes and low Kelvin bulbs for indoor use.