Reference: Desouhant, E., Gomes, E., Mondy, N. and Amat, I. (2019), Mechanistic, ecological, and evolutionary consequences of artificial light at night for insects: review and prospective. Entomol Exp Appl, 167: 37-58. doi:10.1111/eea.12754
During the industrial revolution in Europe, manufacturing processes went through a transition from being handcrafted to being machine-built in factories. As part of this transition, industrial emissions filled the air with sulfur dioxide gas and black soot blanketed the surrounding areas. The tree bark became bare and darkened due to lichen dying. This series of events lead to a unique opportunity to study evolution in action. A particular moth (i.e. the peppered moth – Biston betularia) underwent what seemed a dramatic change in appearance in response to these conditions. It went from being light-colored to completely black. What caused this?
The peppered moth existed in two natural variations, a light and a dark bodied, but environmental conditions before the industrial revolution made the light-bodied variation predominant. The peppered moth with a light-colored body could camouflaged itself while it rested on the equally light-colored lichen covering the trees. With the trees becoming blackened by pollution, the light bodied variation became easily spotted by predators and eaten. Conversely, the darker colored peppered moths were now camouflaged and became more prevalent during this time by evading predation.
Analogous to what happened in the 1800s, in the last century our world has gone through a revolution of illumination. As the number of people on the planet increases, so does the size and number of cities and townships that emanate light at night. Cities never sleep. Businesses are open 24 hours. People are participating in activities at all hours of the night and day. The lights lining streets and roads are frequently amplified by clouds in the atmosphere that act to reflect the light. Skyglow increases the light levels over dozens of kilometers causing areas far beyond city limits to become illuminated. How will creatures respond to this kind of revolution?
Due to this explosion of light from urban centers, there have been increasingly more studies investigating the effects of artificial light at night on plants and animals. The light’s presence is a disruption to innate bodily functions and behaviors that are entrenched in the day-night cycle of all living things. In humans, it can cause hormonal imbalance that can lead to obesity, cancer, and depression. In wildlife, it can impact the ability to survive and reproduce successfully. For example, newly hatched sea turtles become disorientated by lights on the beach and head inland instead of towards the ocean making them an easy target for dehydration and predation.
What about insects?
The attraction of insects to light at night is not a new phenomenon, moths typically fly around streetlights. This scenario is both common and seemingly simple. However, understanding the relationship between insects and light at night is anything but. An insect’s attraction to artificial light at night varies according to sex, mating status, type of lighting, and kind of species. Nocturnal male insects are generally more attracted to light at night. There is a stronger attraction by insects to mercury vapour or compact fluorescent lamps when compared to LED lights. However, overall, LEDs attract a greater number of insects because it encompasses a broader range of light wavelengths (infrared to ultraviolet). Moreover, different insect species and types may be more or less attracted to certain lamps, which may be due to visual sensitivity to different wavelengths of light.
Artificial light at night is detrimental to insect reproduction and survival and is likely one of the factors involved in insect decline. A survey of the total number of insects in selected areas of Germany from 1989 to 2016 found an 80% decline in flying insects. Artificial light at night has been linked to alterations in feeding behaviours, mate selection, offspring development, and immune function. Four different species of moths demonstrated that feeding probability was higher in darkness and reduced by half when in the presence of artificial light at night. Insects such as fireflies and glow worms communicate and court a mate through bioluminescent flashes which is harder to observe in light polluted areas. Female fruit flies were shown to have a 20% reduction in the number of eggs laid when artificial light at night was present. In addition, suppression of the hormone melatonin due to artificial light at night could lead to pre-mature development of young insects and a reduction in immune system efficiency in adults.
From small to big
Streetlights directly affect the bodily functions and behaviour of insects, but indirectly affect other animals. As insects alter their normal patterns due to light pollution, the feeding behaviours of their predators (e.g. bats, spiders) may also change. For example, a spider may change its strategy and create a web near a streetlight to increase its chances of catching prey. In addition, competition for and obtaining resources may be altered. For example, species active at night (i.e. nocturnal) may now compete with those active during the day (i.e. diurnal) because in areas of continual light there is no differentiation between night and day. Changes to cycles of insect migration, hatching events and death due to artificial light at night can have widespread repercussions for plants and animals that rely on them for pollination and food. Humans are no exception. We heavily rely on insects to pollinate plants so that we can have a thriving agricultural industry that provides food to millions of people.
Light pollution has an immense influence on a myriad of insect behaviors resulting in their susceptibility to heightened predation and unsuccessful breeding. How will insects evolve in the revolution of illumination? Over centuries and millennia, people’s fondness for light will gradually cause certain genes to be selected for among insects in response to this unique stressor.
Will the insects of the future look and act the same as the ones we are familiar with today? Will some insects be unable to adapt and eventually become extinct? Will others adapt to being effective predators and evasive prey in continual light? Scientists of today will need to start focusing their efforts on these questions. The beauty of working with insects is that multi-generational studies can be performed in a matter of weeks to months due to their relatively short life spans. This would give scientists a better understanding of how certain traits are selected for under conditions of artificial light at night and the possibility to predict the evolution of insects.
Power in knowledge
Scientists and citizens need to understand the power that light has over insects. Discerning responses to different light characteristics (i.e. wavelengths, intensity) would be helpful in creating an insect friendly environment at night. This kind of knowledge could lead to the development of alternative light sources that minimize impact as well as the establishment of lighting laws that protect wildlife and its diversity. Further investigations comparing the responses of insects from varying geographical locations would reveal if naturally occurring fluctuations in day-night cycles (e.g. daylight hours in the northern hemisphere can range from 7 hours in winter to 16 hours in summer, whereas the tropics have 12 hours of daylight consistently all year round) are protective against the effects of artificial light at night or not. This would allow for the creation of regional guidelines to regulate light pollution.
For more information on how you can reduce light pollution, visit the website of International Dark Sky Association.