How many insects get trapped by streetlights in cities? How many get trapped by rural yard lights and porch lights? I couldn’t find a reliable number that applies to every light, but something so simple isn’t something I often give much thought.
City Lights
A single rural light may seem insignificant compared with a city full of streetlights, but it can sit beside a pond, pasture, woodland edge or garden where nocturnal insects are concentrated. A full moon provides about 0.1 to 0.3 lux; lux is a unit used to measure how much light reaches a surface. I know a full moon is typically enough for me to stumble around at night. Maybe it isn’t enough to read by, but I can find my way. Artificial outdoor lighting can provide 10–30 lux, which can be around 100 times brighter than moonlight. A global survey of night-sky brightness concluded that 23% of the world’s land surface, including almost half of the United States, experiences light-polluted nights. The study defined a light-polluted sky as one with artificial brightness more than 8% above the natural level. This light increase can help us move around in the dark and maybe read or see something we might otherwise trip over but who else sees this light?
Brightness is only part of the issue. The color, or wavelength, of the light also determines which animals can see it most strongly. Light intensity is measured in lux, but light is typically composed of multiple wavelengths. The human visual range is commonly described as about 380–700 nm. This range comprises the colors of the rainbow, or the portion of sunlight we can see. Cool or neutral white LEDs typically emit from about 400–700 nm, with a blue peak near 440–460 nm and broader green, yellow and red output. High-pressure sodium bulbs emit mostly yellow, orange and red light, with strong emissions between about 569 and 616 nm. Metal-halide lights have a broader spectrum that can include ultraviolet, blue, green, yellow and red light. White LEDs generally contain more blue light than sodium lamps, creating greater overlap with the vision of many blue- and green-sensitive animals. Some animals also detect portions of these wavelengths. Nocturnal birds and mammals are often particularly sensitive to blue-green light, while some rodents can also detect ultraviolet light. Many moths are especially sensitive to ultraviolet, blue and green wavelengths, so their vision is strongly affected by LEDs and metal-halide lights that emit within those ranges.
Studies of different moth species have found sensitivity to ultraviolet, blue and green light, commonly with receptor peaks near 350, 440 and 530 nm. This means that blue and white LEDs can be particularly visible and disruptive to many of these insects. So they are sensitive, but why does this matter? For years, a common explanation was that moths used the moon as a navigation reference and mistook artificial lights for it. More recent research suggests something different: many flying insects normally keep their backs toward the brightest part of the sky. A nearby point of light confuses that response, causing them to orbit, stall, invert or crash. Because the light is nearby rather than spread across the sky, the insect keeps adjusting its body toward it. As we’ve all seen, it may circle the light repeatedly and become trapped around it.
Who Cares About Moths
When most people picture a flying insect around a porch light, they probably call it a moth whether it actually is one or not. Moths are the most familiar example, but mosquitoes, fireflies and many other nocturnal insects are affected by artificial light too.
Nocturnal insects like moths, mosquitoes and fireflies are adapted to naturally dark nights, and artificial light can interfere with their navigation, feeding or reproduction. They are important to native ecosystems as pollinators, predators, prey and links between different parts of the food web. Moths are also important nighttime pollinators of native wildflowers and plants in our gardens, including plants that open their flowers or release scent after sunset. Moth caterpillars are an important food source for many birds, insects and other animals.
The dreaded mosquitoes also contribute to the ecosystem. Adult mosquitoes primarily feed on nectar and other plant sugars. Females of many species also take blood meals to produce eggs. They can transfer pollen even though not many of our native Oklahoma species rely on mosquito pollination. The aquatic larvae consume microorganisms and organic particles, transferring nutrients from bodies of water to terrestrial food webs. Both larvae and adults are eaten by fish, dragonflies, spiders, frogs and birds as well as bats. Mosquitoes are one part of the food web, particularly around ponds, wetlands and temporary pools, although most animals that eat them also rely on other prey.
Fireflies emit chemically produced light to communicate and attract mates. Reports of declining firefly populations in recent years have alarmed both scientists and observers, although long-term population data are limited for many species. The suspected causes include habitat loss and degradation, artificial light at night, pesticide use, poor water quality and climate change. Light pollution is especially important for flashing species because it can interfere with their ability to find mates. Oklahoma is home to at least 26 firefly species. None of Oklahoma’s firefly species are currently federally listed as threatened or endangered, but several North American species are considered at risk. Read more here.
Turn the Lights Out
One of the most direct things we can do for these nocturnal species is turn out unnecessary lights at night. Turn off unused outdoor lighting like porch lights, yard lights or decorative lighting. We still need light to see, and there is no way to stop using it altogether, but reducing our effect on native species can give them a better chance.
When I built my house in the country, I had to have a new electric pole installed. As part of the installation, they offered a yard light. I chose not to add the yard light to help these nocturnal species and retain as much of a native landscape as I could. I still have a porch light, and I have floodlights on the greenhouse for working in the garden on hot summer nights. I keep them off when I’m not actively using them, or I put them on a timer so they automatically turn off. When light is necessary, timers, motion sensors, shields that aim light downward and warmer-colored bulbs can reduce the amount that spreads into the surrounding landscape.
Our native landscape will continue to change as we encroach on it. There is no doubt we have an impact, and the less unnecessary disturbance we cause, the more room native species have to continue their natural relationships. Our goal isn’t to improve nature but just to let it be natural.