A harsh winter storm has hit the world’s largest butterfly sanctuary in southern Mexico, killing upwards of 1.5 million monarch butterflies, ahead of their annual migration north. The Monarch Butterfly Biosphere Reserve, a designated World Heritage Site, has a comparatively cool climate due to it’s 2,700-meter (8,860-foot) elevation, typically seeing winter temperatures near the freezing mark, however this storm dropped temperatures to -12ºC (10.5ºF) and left up to a foot of snow in some areas.

El Rosario Sanctuary president Homero Gómez González reports that while the monarch population has been slowly rebounding from declines seen over recent decades, he estimates that roughly 3 percent of the colony’s 50 million mariposas Monarca succumbed to the storm, the worst the region has seen in 40 years. The storm also knocked down numerous trees in the reserve, blocking trails and preventing access to some areas of the sanctuary.

Gómez says that the remaining butterflies are due to embark on their northward journey within a few weeks. He is critical of the Mexican government’s role in his organization’s conservation efforts, saying that officials use them for photo-ops and political purposes. "In reality we receive nothing in the way of support. Proof of that is that we ourselves have reforested the areas [affected by logging].”