Author Anurag Agrawal argues that planting milkweed alone won't solve a beloved butterfly's problem in "Monarchs and Milkweed: A Migrating Butterfly, a Poisonous Plant, and Their Remarkable Story of Coevolution."

The book, which comes out early next month, explores just about everything about the insects, including their relationship with milkweed. Farmers have long fought to keep that plant out of their fields, in part with the aid of genetically modified, herbicide-resistant crops.

Agrawal, an ecology and entomology professor at Cornell University, said his milkweed skepticism began when someone suggested that cell towers were killing the monarchs — a theory he didn't credit. But 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, and a whole lot of other factors.

Agrawal 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, it could mean 20 million deaths a year.

Agrawal's bottom line: Despite all we know about monarchs, the data are messy and inconclusive. Lack of milkweed may not be the main culprit threatening the butterflies.