Although I’ve seen a lot of monarch butterflies in my years in Massachusetts, and even traveled to the forests of Michoacan in Central Mexico to watch millions of them congregate the winter, I’d never — surprisingly — come across a monarch caterpillar. But last month, at a roadside stop in the Adirondack Mountains of upstate New York, I finally saw one squirming on the leaf of a milk weed plant.
If everything goes well this caterpillar will metamorphose into a butterfly and will — all by itself — make the epic 2000-mile journey to Mexico.
The caterpillar sighting led me to check how monarchs have been doing in recent years. A good estimate of their numbers comes not from raw counts — it is very hard to count swarms of butterflies — but from the number of acres occupied by the migrant generation in Mexico at the peak of the winter. That’s the time of the year the butterflies are densely packed together on oyamel (fir) trees. So it’s a matter of identifying clusters of such trees, determining the perimeter of each cluster, and finally calculating the total area enclosed across all clusters. Mexican researchers, led by Eduardo Rendon Salinas, do this on an annual basis.
According to some recent references, the number of acres occupied by overwintering monarchs starting 2014-2015 (the year I visited) reads as follows:
2.79, 9.91, 7.19, 6.13, 14.95, 6.99, 4.9
A lot of ups and downs there with a seeming decline in the last couple of years. In contrast, the average acres monarchs occupied in the 1990s and 2000s was well over 15, and individual years frequently exceeded 20.