The humble, hardworking bee, which pollinates most of the world's crops, is rapidly disappearing from the planet, and nobody's sure why.

As entire colonies of dead bees litter the fields, in a condition called colony collapse disorder, more and more evidence is pointing to a class of pesticides called neonicotinoids -- the agricultural industry's best-selling type of pesticide.

What Are Neonicotinoids?

Neonicotinoids are chemically related to nicotine, an insect repellent that's found in many plants including tobacco.

Both nicotine and neonicotinoids act on the nervous system of insects and, to a much lesser degree, mammals.

Initially developed in the 1980s by chemical companies like Shell and Bayer, neonicotinoids like clothianidin and thiamethoxam were developed in part because they're less toxic than other kinds of pesticides.

Neonicotinoids, or "neonics," are now the world's most widely-used class of insecticide; one neonic known as imidacloprid is the world's most popular insecticide.

One reason for their popularity is the way neonicotinoids are applied: Instead of being sprayed over a field, neonics are applied to the outer shell of seeds before they're planted. As the seedlings sprout, the insecticide spreads throughout the entire plant, protecting it from aphids and other herbivores, NPR.org reports.

But neonics don't stay on the seeds when they're planted in the soil, research suggests. Christian Krupke, a professor of entomology at Purdue University, found that the machines used to plant corn seeds (they use air pressure to shoot the seeds into the ground) also blow neonicotinoids into the air.

Krupke's research found that the air expelled from these planting machines contains a whopping 700,000 times more neonicotinoids than what is needed to kill a honeybee, NPR reports.

Colony Collapse Disorder: It's Getting Worse

Colony collapse disorder has been in the news since about 2005, when farmers and other agricultural workers noticed that bees were dying. And not just a few: Entire colonies of thousands of bees were being obliterated across North America and Europe.

This was much more serious than an ecological curiosity: The U.S. Department of Agriculture says bees are responsible for pollinating about 25 percent of the American diet, from onions to apples, the New York Times reports. And when bees don't pollinate crops, food prices soar.

And despite some of the best minds in agricultural science working on the disaster, recent reports from the field find it's only getting worse: Beekeepers who had suffered through years of losing about one-third of their bees are now reporting losses of over half of their bees.

Something is very wrong.

"They looked so healthy last spring," Bill Dahle of Big Sky Honey in Montana told the Times, referring to his bees colonies. "Then, about the first of September, they started to fall on their face, to die like crazy. We’ve been doing this 30 years, and we’ve never experienced this kind of loss before."

Will We Save the Bees in Time?

Fed up with a lack of federal action on the growing crisis, early in 2013 several environmental groups and beekeepers sued the EPA, demanding that the agency revoke its approval of two popular neonicotinoids, clothianidin and thiamethoxam.

"The EPA should immediately take these two neonicotinoid pesticides off the market," Paul Towers of the Pesticide Action Network told NPR.

Towers claims the problem with neonics is that they also show up in the pollen of corn, canola and sunflowers that grow from treated seed. After bees feed on that pollen, Towers says they can suffer from "Disorientation; reduced ability to gather food; impaired memory and learning; and lack of ability to communicate with other bees."

Manufacturers claim otherwise, but some neonicotinoids are now banned in France, Italy, Slovenia and Germany, and there's an ongoing fight in the European Union to ban the entire class. Will America follow suit? Time will tell -- but only if there's time enough before all our bees are gone.