Monarch butterfly deaths due to road collisions during fall migration

Insects around the world are disappearing at a troubling rate, and as they do, we lose the important roles they play in ecosystems. These include serving as the base of most food chains, pollinating plants, and decomposing dead plant and animal material. 

Just as shocking, until recently this trend has mostly gone unnoticed. It is now called the “windshield phenomenon” by entomologists who note that over time, cars are colliding with fewer insects and windshields are cleaner. This isn’t because something is changing about our cars, but because there are simply fewer insects to hit.

String of Monarchs by Steve Corey from San Luis Obispo, CA, USA

While loss of some insect species has passed by unnoticed, the loss of butterflies receives more attention. Butterflies are among the most charismatic and beloved groups of insects, and so particular attention is paid to the loss of these species. 

One of the most familiar butterflies is the Monarch. This species is a common sight in backyards, and is so visible because they are a migratory species – each year, hundreds of millions of them make their way from the United States and Canada to Mexico for the winter. In the United States, scientists discovered that the population of monarch butterflies fell by 90% in the last 20 years. This translates to a loss of 900 million individuals!

There are several reasons the Monarch populations are starting to decline. One major cause is the widespread use of glyphosate herbicides, which kills milkweed, the butterfly’s only host plant. Another lesser-known cause is collisions with cars along the migration route. A new paper out in Biological Conservation looks into the role that road mortality causes during the Monarch migration.

Monarch roadkill autumn 2016 and 2017 survey results for 100 m transects along major road classes within the background evaluation extent in the monarch Central Funnel in Texas, including previously reported locations of high monarch roadkill. Reproduced from Kantola et al. 2019.

The scientists monitored Monarch roadkill along Texas roads in 2016 and 2017. The area surveyed is called the Central Funnel, a point where the main migration route of Monarchs narrows as it passes through Oklahoma, heading southwards. They focused their efforts during the month of fall migration. Along a 100m stretch, they found an average of 3.4 dead Monarchs, with some hotspots having levels as high as 66 individuals. After field data collection, they used models to extrapolate their findings, and calculated about 2.1 million individuals might be killed by collisions with cars within the Central Funnel each year — that’s 3% of the overwintering population.

While destruction of milkweed plants is the largest contributor to Monarch deaths, these roadkill numbers are still troubling. Even just a small percent of Monarchs lost to cars can have important impacts on this dwindling species.

Conservation efforts aimed at protecting Monarchs would likely protect other insect species as well. The authors recommend several effective mitigation strategies, such as (1) slowing traffic when Monarchs are present in the area, by installation of signs or closing of road lanes, and (2) installation of nets to direct butterflies upwards to fly over the height of traffic. Their identification of roadkill hotspots allows for the targeting of efforts, making mitigation much more effective than if deaths were evenly spread.


Tuula K., Tracy J.L., Baum, K. A., Quinn, M. A., and Coulson, R. N. 2019. Spatial risk assessment of eastern monarch butterfly road mortality during autumn migration within the southern corridor. Biological Conservation 231: 150-160.

The Insect Apocalypse Is Here: What Does It Mean For the Rest of Life On Earth? (2018). The New York Times.

2019-09-05T15:43:28-04:00 July 15th, 2019|

About the Author:

Elizabeth Schultheis
I am a postdoctoral researcher at Michigan State University. My work includes research in plant biology and science education, professional development for scientists on science communication, and teacher professional development on data in the classroom. I am a co-founder of Data Nuggets, an innovative approach to bring cutting edge research and authentic data into K-16 classrooms.