Hundreds of millions of monarchs once spent the winter in Mexico. (PAT SULLIVAN/ASSOCIATED PRESS)

The cover of the August 1976 National Geographic turned a scientific breakthrough into a popular sensation: It showed a woman making her way through a previously unimaginable mass of monarch butterflies. After decades of seeking, scientists had discovered where America’s distinctive orange and black butterflies flew each fall, and the magazine featured stunning photographs of the monarchs “overwintering” by the hundreds of millions in colonies in Mexico.

Fascination with monarchs was fed by the burgeoning environmental movement, the entertaining nature of their story (they migrate astonishing distances; they eat milkweed, which is toxic — but they’ve evolved so it doesn’t kill them, though it sickens animals that want to eat them) and fears about the species’s future. Schoolchildren raised them from caterpillars. “Citizen-scientists” tracked them. By the end of the century, some people were calling the monarch “the Bambi of the insect world.”

The butterfly’s celebrity peaked when scientists recorded a worrisome decline in its numbers around 2014 or 2015, Anurag Agrawal writes in his new book, “Monarchs and Milkweed: A Migrating Butterfly, a Poisonous Plant, and Their Remarkable Story of Coevolution.” There was a widespread belief that they were suffering from a dearth of milkweed — which, besides being monarch food, is an agricultural pest that farmers were eradicating with the aid of genetically modified, herbicide-resistant crops. “Plant milkweed!” became something of an environmentalist mantra.

But Agrawal begs to differ. A professor in the departments of ecology and entomology at Cornell, he has been fascinated with monarchs for years. His book is a minutely detailed exploration of just about everything about them: sex lives, dining habits, internal flight guidance and, in particular, their coevolving-but-competitive relationship with milkweed. And he doesn’t think planting milkweed will solve the monarch’s problem.

His skepticism began when someone suggested to him that cell towers were killing the butterflies — a theory he didn’t credit. But then he ran some numbers and found there was as strong a correlation between monarch decline and cellphone subscriptions as there was between monarch decline and herbicide-resistant corn and soybeans. Then he found that he could get almost as strong a correlation by comparing monarchs to the stock market. Actually, he could find a correlation to a whole lot of factors.

He also came up with a lot of potential monarch menaces. For example: “In the state of Illinois alone, perhaps 2 million monarchs are killed by cars during the southern migration each year,” he writes. If those figures are extrapolated to the butterflies’ entire route, it could mean 20 million vehicular regicides a year.

Agrawal’s bottom line: Despite all the attention to monarch butterflies and all we know about them, the data are “messy” and inconclusive. Lack of milkweed may not be the culprit, but “it is hard to argue . . . that something isn’t right” with the Bambi of the insect world. “The onus is on us to understand the problem and to try and fix it.”

— Nancy Szokan