The number of Monarch butterflies who fly south to spend the winter in Mexico has dropped to its lowest level since record keeping began more than 20 years ago. A new report finds the butterflies covered less than two acres of forest in December compared to nearly 45 acres during their peak in the mid-1990s. Researchers attribute the decline to factors including deforestation, extreme weather from climate change, and a rise in genetically modified corn and soybean crops designed to resist Monsanto’s Roundup weed killer. Widespread dousing with the herbicide has destroyed milkweed, the Monarchs’ food source. Experts fear the Monarchs’ annual migration from Canada and the United States to Mexico is at risk of disappearing entirely.