Each year as winter descends on the United States and Canada, millions of monarch butterflies migrate as far as 4,500 kilometers to the oyamel forests in central Mexico.
But thousands do not make it to their destinations in Michoacán and the state of México as they end up killed by cars on the Saltillo-Monterrey highway.
Researchers in 2018 counted 11,280 dead monarchs in five 500-meter sections of the highway, leading them to believe that as many as 127,689 monarchs were killed in a 14-kilometer stretch of road over a 19-day period.
The group of researchers from the Autonomous University of Nuevo León (UANL) and the University of Western Ontario in Canada also studied another area of the highway known as La Muralla where they studied six sections of the road over 14 days and counted 601 dead butterflies, leading them to extrapolate that 23,520 butterflies were killed in that area.
Between the two areas, scientists estimate that at least 151,200 monarchs did not manage to complete their journey.
In an interview with the newspaper Milenio, Rogelio Carrera Treviño, coordinator of the wildlife laboratory at the Faculty of Veterinary Medicine and Zootechnics at UANL, noted that the deaths of butterflies on highways is a phenomenon that researchers are just beginning to examine. The only previous study that exists was carried out in Texas, but it is expected that monarch mortality in Mexico is much higher.
“In a previous study that was done with data from 2016 and 2017 in the state of Texas, they found 580 butterflies run over in those two years. With that they made a model to determine or predict how many monarch butterflies are killed in total on all roads (in that country), they estimated that depending on some variables, between 3 and 10% of the total population of butterflies was killed.
“However, that estimate was made only with data from Texas, precisely what we are trying to do is to do it with data from Mexico,” Carrera said.
According to Carrera, when crossing into Mexican territory, butterflies are concentrated in a funnel-shaped swarm and fly low to the ground so they can rest and feed, leaving them particularly vulnerable to being killed by cars.
Carrera said there is still a long way to go to understand the phenomenon of butterfly roadkill, and there may be actions that can be taken to mitigate mortality, such as the installation of infrastructure in sections of the road where the highest number of butterflies are killed to force them to fly at a higher altitude to avoid colliding with vehicles.
In Taiwan, nets have been erected along a highway in the migratory path to direct purple crow butterflies away from cars and deaths have dropped 80% as a result.
Another proposal is that during the migration, which only lasts a few days, the speed limit on the highway be reduced to 60 kilometers per hour with police enforcement.
“If they travel [at speeds of] 60 kilometers or less, even if the butterfly is in front of the car, it is able to avoid it,” Carrera said.
That speed limit is already in effect in some areas of Mexico, where signs have been posted advising that the limit is 60 km/h when monarch butterflies are present.
Soon he will be launching a new study in collaboration with Texas A&M University to study the monarch mortality rate on other highways in Mexico, including those of Coahuila, Nuevo León, Tamaulipas, San Luis Potosí and Zacatecas.
Monarch populations have dwindled 80% in the past two decades mainly due to loss of habitat, but roadkill may also be playing a part, scientists now believe.