The epic 3,000-mile monarch butterfly migration may become a thing of the past. Each fall, monarchs travel from their summer homes in the northern U.S. and Canada to winter habitats in California and Mexico. But the 2018 Western Monarch Thanksgiving Count found that the number of west-coast monarchs spending the winter in California had plunged to only 20,456 butterflies—a drop of 86 percent since last year. And the number of eastern monarchs overwintering in Mexico this year has dropped 15 percent since last year, for a total decline of more than 80 percent over the past 20 years, according to the National Wildlife Federation.
Increasing carbon dioxide levels may be making milkweed—the only food monarch caterpillars will eat—too toxic for the monarchs to tolerate. And higher temperatures may also be driving summer breeding areas further north.
The creation of herbicide-resistant corn and soybeans meant that farmers could eradicate weeds and other understory plants, including milkweed, that competed with their crops. The advent of these new crop varieties meant the death of milkweed.
— source nationalgeographic.com | Dec 21, 2018