Crop herbicides play role in shrinking monarch butterfly population; agribusiness pledges help

A butterfly being considered for federal protection is emblematic of the plight pollinating insects face partially because farmers enticed by ethanol mandates grow more herbicide-resistant crops.

Herbicide makers say they're committed to boosting the population of monarchs, which have seen their numbers plummet by more than 90 percent in the past two decades.

Some populations migrate thousands of miles, across multiple generations each year from breeding and wintering grounds. But there's less of the milkweed they depend upon to nourish them along the route because corn and soybeans that withstand herbicides are wiping it out. Logging, construction and a drought that peaked in 2012 also have harmed habitat.

The number of managed honey bee colonies in the United States also has declined.