The number of monarch butterflies that traveled to Mexico for the winter declined for the second consecutive season, according to a report released yesterday.
The Natural Protected Areas Commission (Conanp) and the Mexico office of the World Wildlife Fund (WWF), which jointly completed the study, said they counted nine butterfly colonies in the most recent migration compared to 13 in 2016-2017.
Five of the colonies were inside the Monarch Butterfly Biosphere Reserve on the Michoacán-México state border and covered a total area of 1.5 hectares.
The other four were outside the reserve and extended over 0.98 hectares. There was an estimated density of 37.5 million butterflies per hectare.
The combined area of the colonies this season was 2.48 hectares, 14% less than the 2.91 hectares covered by the 13 colonies a year earlier. In 2015-2016, the butterflies clustered in pine and fir trees covering 4.01 hectares.
At a press conference, Conanp chief Alejandro del Mazo explained that “extreme meteorological events” could be the reason why fewer monarch butterflies arrived in Mexico this winter.
WWF Mexico director Jorge Rickards agreed that a busy hurricane season across the almost 5,000-kilometer migration route from Canada to Mexico was a factor.
“These climate phenomena without a doubt have an impact on the migration,” he said.
The increased use of herbicides in the United States has also reduced the amount of milkweed, which monarch caterpillars eat, further depleting numbers.
Just over 20 years ago, in 1996-1997, the monarchs covered an area of 18 hectares in Mexican forests but declined to a low point of just one hectare in 2014-2015 before recovering the following year.
Despite the reduction in numbers, butterflies arrived at protected areas in Valle del Bravo, Nevado de Toluca and the Iztaccíhuatl-Popocatépetl National Park this season, del Mazo pointed out.
Scientists have discovered that the black and gold insects use a kind of internal solar compass to guide them on their journey, during which four or five generations of butterflies are born and die.
In more positive news, Rickards announced that illegal logging was reduced by 94% last year in the forests where the monarch butterflies arrive due to the work of gendarmes who patrolled the areas.
“On one hand, Mexico has done its work as far as controlling logging, but on the other hand Mexico is not free of the global phenomena implied by climate change and we have to be ready to confront it and take measures to be able to adapt to this reality,” he said.
One program designed to promote the safe and healthy migration of monarch butterflies as they traverse Mexico has planted 67 gardens where the insects can rest and feed.