A coroner can't determine a cause of death without a body. That's why a mysterious malady called Colony Collapse Disorder (CCD) has put honeybee researchers in a tough position: Not only does CCD cause worker bees to die off in droves, but their bodies disappear without a trace.

CCD isn't a disease, but is described by researchers as a "set of symptoms." Since it started affecting honeybees in 2006, the number of colonies has declined by an average of 33 percent each year, according to the U.S. Department of Agriculture.

The disorder has been linked to a variety of contributing factors, including pesticides, parasites, habitat loss and, more recently, a study led by Jeff Pettis added fungicides to the list.

"Fungicides were previously thought to be safe for bees," says Pettis, a USDA Bee Research Laboratory scientist. "That's what was eye-opening about this finding."

Understanding bee declines

The study linked fungicide and pesticide exposure in honeybees to a greater vulnerability to Nosema ceranae, a fungal gut parasite that has been implicated in CCD.

"Nosema is a parasite that's always present in bees," Pettis says. "But occasionally it becomes a problem when it starts increasing in levels. Then bees can have trouble with it."

One theory is that these chemicals weaken the bees' immune systems, which makes them more vulnerable to Nosema. And while the parasite is but a piece of the puzzle in CCD, it's one of the most solid links researchers currently have to help dampen the disorder's spread in the United States.

Treating Nosema

Noah Wilson-Rich is a honeybee researcher who is currently working on treatments for Nosema. Wilson-Rich is the founder of Best Bees, a Boston-based company that installs and maintains urban beehives.

The company is currently testing a vaccine against Nosema, as well as a probiotic treatment to help boost the bees' immune systems.

"We call it bee yogurt, but it actually looks like peanut butter. It's the same concept as to why we would eat yogurt: to get the probiotics," says Wilson-Rich.

Best Bees maintains about 200 beehives, and a couple dozen of those are in Boston, which gave Wilson-Rich an interesting insight.

"The bees in the city do better than outside of the city -- they make more honey and have higher rates of surviving the winter," he says. "Urban beekeeping is more than a trend, it's good for the bees."

Clues from the city

Wilson-Rich doesn't know the exact reason why bees do better in cities, but he has a few theories, including fewer pesticides, plant diversity in community gardens or the warmer temperatures in cities caused by a phenomenon known as the urban heat island effect.

Pettis has observed the same phenomenon in his research.

"Bees tend to do really well in cities, and we don't know why. It may be because of less pesticides, or less competition," he says.

CCD is a complicated story with no clear answers, but the connection to pesticide and fungicide use is growing stronger every day. Last week, the U.S. Environmental Protection Agency announced new pesticide labels that ban the use of certain chemicals in areas where bees are present.

"What this paper, and what other research studies have evidenced, is that we need to be more cautious with our use of pesticides," says Pettis. "About 35 percent of crops depend on pollinators, so if we like diversity in our diets, we should all care about pollinators."

Editor's note: To learn more about cities and the convergence of sustainability and technology, be sure to check out VERGE SF Oct. 14-17.

This article originally appeared at the Txchnologist website.

Honey bees image by Tischenko Irina via Shutterstock.