The monarch and light pollution

Over the last century, the celestial landscape has undergone significant changes due to increasing light production in towns and villages. While artificial light may blur the night sky, its effect goes well beyond depriving us of stars to gaze at. Light pollution disrupts various essential biological mechanisms in living things, including plants and animals. The impact of this type of pollution has been studied with regard to a number of nocturnal insects.¹ For example, in fireflies, artificial light interferes with their ability to find a sexual partner.² But what about diurnal species, like monarch butterflies? Does light pollution have an impact on them as well?

Regaining strength overnight

Active during the day, the monarch takes advantage of the night to rest. Indeed, periods of darkness are crucial for the monarch. That’s when the butterfly’s organism synthesizes the proteins needed for its internal compass to work properly.

This period of nighttime rest also allows the butterfly to keep from burning its energy reserves needlessly. These are necessary both for its migration to central Mexico and for surviving the winter. Hence, maintaining a balance between these periods of waking and sleep, defined as circadian rhythm, is essential to the monarch’s survival.

The dark side of light

Monarchs deal with different light intensities during their fall migration as they pass through urban and suburban areas. To study the impact of this light pollution, researchers at the University of Cincinnati submitted monarchs to artificial light during their nocturnal rest period. They determined that nighttime light disrupts monarchs’ circadian rhythm,³ with effects resembling jet lag. That disruption destabilizes the butterfly’s guidance mechanisms, more precisely its internal compass, which synchronizes to the circadian rhythm.

The study also points out that artificial light can create signals similar to those of the sun.³ In their laboratory experiment, the researchers noticed that in being exposed to nighttime light, the monarchs woke up prematurely during the night or started early flight, which entails unnecessary energy expenditure.³

Constant exposure to this sort of disruption lowers the monarch’s energy reserves and may increase the mortality rate.³

Research still to be done

We’re only in the very early days of research on light pollution and its impact on monarch migration. Although this study raises important questions, bear in mind that it’s based on simulations carried out in the laboratory. It remains to be seen how these observations translate to a natural environment.

In any case, if you’re planning to create a garden for monarchs, it’s better to do so far from artificial lighting. You’ll be surprised at the beneficial effect that the dark of night has on a variety of other species!

Read also

• Gentle or devastating light? The impact of urban light on nocturnal insects
• The impact of LED technology on light pollution in Montréal

References

1. Owens et al. (2020). Light pollution is a driver of insect declines. Biological Conservation, 241. doi:10.1016/j.biocon.2019.108259
2. Lewis et al. (2020). A Global Perspective on Firefly Extinction Threats. BioScience, 70 (2). doi:10.1093/biosci/biz157
3. Parlin et al. (2022). Oriented migratory flight at night: Consequences of nighttime light pollution for monarch butterflies. iScience, 25(5). doi: 10.1016/j.isci.2022.104310