Effects of Light Pollution on Monarch Butterflies

Cara Piergiovanni

Monarch butterflies are a very vital pollinator among our gardens here in North America. According to the U.S. Fish and Wildlife Service, the Monarch population has dropped 88% since 1996. One reason for that is the invasion of non-native plant species and consumer dislike of the native milkweed plant. Commercially it is not a favorite because it grows like a weed, even though it serves a vital role in monarchs’ life cycle. It is the only plant they will lay their eggs on, as they have developed an immunity to its toxic properties from evolution. While creating gardens and landscaping for pollinators it is important to consider the light pollution that they will be exposed to at night. Anything like a street lamp can cause the butterfly to be disrupted. Another adverse effect on their numbers is the light pollution that has increased with population growth and urbanization. 

According to research done by The University of Cincinnati Monarch butterflies, other wildlife, and even people are affected by light pollution. Monarchs travel through various regions and different landscapes throughout their long journey from Canada to Mexico, passing through the United States in the warmer months. Lights as small as a porch light can affect their circadian rhythms. These rhythms guide them on their journey and tell them when to rest and when to travel. Monarchs treat these artificial lights as a source of sunlight. So when it is nighttime and these lights are on, they will struggle to rest. If Monarch butterflies do not have time to rest they will have difficulty processing proteins that guide them on their long migrational journey. This raises the concern that light pollution may affect their destination. They must reach their final destination near the southern hemisphere by winter, in time to survive. Monarch Butterflies’ internal clock is shifted because of lights at night, and their sense of time is disrupted. They will perceive the time of day differently, resulting in less resting time or too much. It sets off the natural and delicate balance necessary to survive such a long trip. The travel can consist of around 2,500 miles in total. This is similar to a road trip in which you need to stop at a rest stop or change drivers due to sleepiness. For us, we see the sun setting and we know the difference between the sun and artificial lights, but sadly most creatures do not. Monarchs are not the only ones affected by light pollution. 

Like those living in urban areas, Monarchs pass through cities with many lights on at all hours. We are also affected by this. New York is known as the city that never sleeps and it is ironic because people may suffer from unhealthy sleep cycles due to the continuous lights at all hours. Light pollution is not the only thing to worry about in city life. There is a reason the sky does not fully darken at night. 

Creatures such as baby turtles are affected by light pollution as well. It is interesting to see our effect on the natural world. Hatchlings can be led astray away from the ocean by lights like the monarchs which are led to flight at the wrong times of the day. To mitigate light pollution you can learn to live with natural darkness and use light with a purpose. Utilize smart lighting to have timers and only be in use when necessary. Shut off porch lights where monarchs may be resting nearby. Shield your lights and use lower path lights rather than high ones. Lastly, by using dimmer lights instead of the strongest and brightest option, you lower the pollution effect.

Sources: Science Daily, Study Finds Organization, University of Cincinnati News