But an equally alarming source of the decline, both Mr. Taylor and Mr. Vidal said, is the explosive increase in American farmland planted in soybean and corn genetically modified to tolerate herbicides.
The American Midwest’s corn belt is a critical feeding ground for monarchs, which once found a ready source of milkweed growing between the rows of millions of acres of soybean and corn. But the ubiquitous use of herbicide-tolerant crops has enabled farmers to wipe out the milkweed, and with it much of the butterflies’ food supply.
“That habitat is virtually gone. We’ve lost well over 120 million acres, and probably closer to 150 million acres,” Mr. Taylor said.
A rapid expansion of farmland — more than 25 million new acres in the United States since 2007 — has eaten away grasslands and conservation reserves that supplied the monarchs with milkweed, he said.
The monarchs’ migration is seen as a natural marvel and, for Mexico, a huge tourist attraction. But naturalists regard the butterflies as a forward indicator of the health of the food chain. Fewer butterflies probably means there are fewer other insects that are food for birds, and fewer birds for larger predators.
Here on my ranchito I have seen no monarchs this year. It is a little early for their migration through central Texas (at least here in north Erath County, Texas), and I will hold off making any conclusive statements about their pattern for several more weeks.
I have only a few sprouts of milkweed on my 53 acres. I know precisely where the milkweed is and seek to keep it flourishing for the butterflies.