Light Pollution Can Interfere With Butterfly Migration

Porch lights and streetlights throw off the monarchs' circadian rhythm.

monarch butterfly at dusk

Gagandeep Singh / 500px / Getty Images

Want to help butterflies? A new study suggests turning off the lights.

Light pollution at night can throw off a monarch butterfly’s internal compass, interfering with its navigational abilities, researchers have found. Artificial light, such as a lone porch light or streetlight, can disrupt their circadian rhythms. That can make the monarchs disoriented when they try to fly the next day.

“Darkness is important to butterflies since the internal circadian clock of butterflies functions normally when they are exposed to natural day: night lighting cycles,” study author Patrick Guerra, an assistant professor at the University of Cincinnati, tells Treehugger.

“Without darkness, the circadian clock of butterflies cannot function properly.”

Monarch butterflies migrate by the millions each year, making the trip from northern areas of the continent to California and Mexico and back. Because most butterflies only live for a few weeks to a few months, it takes several generations of butterflies to complete the migration.

But too much light at the wrong time can urge the butterflies to take off when they should be resting instead for their grueling voyage.

“We use monarch butterflies as a model system to understand long-distance animal migration. Our goal was to understand how contemporary environmental stress, such as different types of stress induced by urbanization, impacts animal migratory phenomena,” Guerra says. “As monarchs are a threatened species with their population numbers in decline, we wanted to see what types of environmental stress might be linked to their decline.”

While monarch populations have fluctuated annually, there has been a general decline. Most recently, the number of monarch butterflies in Mexico’s forests this past winter was 35% greater than the year before, according to a survey led by World Wildlife Fund (WWF) Mexico. The increase is a sign of a “fragile” recovery, according to the WWF.

Like a Butterfly Treadmill

For their study, researchers conducted lab studies where they reproduced the effects of artificial light pollution with butterflies using a flight simulator.

“The flight simulator is like a butterfly treadmill, in which we can study the flight behavior of insects in controlled, experimental conditions. We first tested if monarchs treated a single, artificial light source as if it were the actual sun during the day,” Guerra says.

“Once we established that they did, we then tested how the monarchs would behave when exposed to this same artificial light source during their night.”

Butterflies stayed quiet and unmoving when they were first placed in the flight simulator in the dark. But as soon as scientists turned on the light, they started flying. The light made them believe that night was day and this can cause them to fly longer at night or start flying too early.

“When exposed to light pollution, the light can perturb the normal functioning of the animal's circadian clock,” Guerra says. “Light pollution, particularly nighttime light pollution, causes animals to experience light at a time when they are not supposed to, e.g., at night when they are normally in the dark, and are quiescent and at rest. Exposure to such light can hamper the behavior and biological processes of animals which rely on or that are regulated by the circadian clock.”

The findings were published in the journal iScience.

Other Impacts of Night Light

Porch lights, streetlights, and other artificial night lights can do more than mess up the insects’ wake/sleep cycle and migration.

“Beyond impacting the migration abilities of butterflies, nighttime light pollution can negatively impact various aspects of the health of butterflies, such as metabolism, immune response, and motivation,” Guerra says. “These negative effects of light pollution exposure have been seen in many other animal species.”

Artificial light at night can have an impact on animals’ hunting, breeding, and sleeping patterns.

“These findings are important since they show that nighttime light pollution not only affects animals that migrate at night, but also affects animals that migrate during the day such as the monarch,” Guerra says. “These findings demonstrate how urbanization can negatively affect important biological phenomena, and therefore allow us to understand how human activity impacts the rest of nature.”

Using these findings, researchers hope to now find ways to mitigate the negative impacts of nighttime light pollution, particularly for species that are active during the day and may not have been considered before.

“These findings are fascinating, since it shows how susceptible animals are to environmental perturbation and stress,” Guerra says. “Moreover, these findings further tell us how biological phenomena, such as circadian clocks, function.”

View Article Sources
  1. Parlin, Adam F., et al. "Oriented Migratory Flight At Night: Consequences of Nighttime Light Pollution for Monarch Butterflies". Iscience, vol. 25, no. 5, 2022, p. 104310., doi:10.1016/j.isci.2022.104310


  3. "Eastern Monarch Butterfly Population Shows Signs of Recovery." World Wildlife Fund.

  4. "Light Pollution Effects on Wildlife and Ecosystems." International Dark-Sky Association.