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It's often said that we have bees to thank for one out of every three bites we take of food. In addition to producing honey, honeybees literally criss-cross the United States, pollinating almonds, oranges, melons, blueberries, pumpkins, apples, and more. And while carrots are a biennial root crop that are harvested long before they flower, all carrots are planted from seed, and honeybees pollinate the carrot flowers that produce the seeds. Other species of bees, both social and solitary bees, pollinate other crops. And the populations of all these species of bees are in decline.

The decline of bees has been in the headlines for several years, and theories to explain their deaths abound. But perhaps there is not just one single cause. University of California San Diego professor of biology James Nieh studies foraging, communication and health of bees. "I would say it's a combination of four factors; pesticides, disease, parasites, and human mismanagement," says Nieh. Bees might be weakened by having a very low level of exposure to insecticides or fungicides, making them more susceptible if they are attacked by viruses or parasites. "It's kind of like taking a patient who is not doing so well -- very weak, poor diet, exposing them to pathogens, and then throwing more things at them. It's not surprising that honeybees are not very healthy."

One class of pesticides, neonicotinoids in particular has received a lot of attention for harming bees. In late 2010, the EPA came under fire from beekeepers and pesticide watchdog organizations. This happened when Colorado beekeeper Tom Theobald spoke out about how the EPA allowed clothianidin to be used without any proof it was safe and despite the fact that the EPA's own scientists believed it "has the potential for toxic risk to honey bees, as well as other pollinators."

At that time Theobald had reported losing up to 40 percent of his bees, and now, things are looking even worse. "As a business, I think it's over," he says. "I think my business is no longer viable. I'll continue to keep bees as best I can and may be able to pull off a halfway decent crop for another year or two but the trendline is down and over the edge of a cliff and that's typical of what's going on nationally."

A recently published study sheds a little more light on the impact of clothianidin. The study, which focuses on pesticide exposure in bees, looks at two pesticides that are used by treating seeds prior to planting. Each corn seed contains enough pesticide to kill 80,000 honeybees. Once the plant develops, all parts of the plant -- including the pollen collected by bees -- contain lower doses of the pesticide. One of the main revelations of the study is that bees get a hefty dose of these pesticides, clothianidin and thiamethoxam, during spring planting as the seeds are coated in talc to keep them from sticking together and then much of the talc enters the environment either with the seed or behind the planter through its exhaust fan.

The study found the pesticides on the soil of fields -- even unplanted fields -- and on nearby weeds, as well as in dead honeybees and in pollen collected by honeybees. Clothianidin is used on both corn and canola in the U.S., and while corn does not rely on honeybees for pollination (it is wind pollinated), the study found that "maize pollen comprised over 50 percent of the pollen collected by bees, by volume, in 10 of 20 samples."

Tucked in the middle of the study is a bombshell: "The levels of clothianidin in bee-collected pollen [from treated maize] that we found are approximately 10-fold higher than reported from experiments conducted in canola grown from clothianidin-treated seed."

This is significant because the pesticide clothianidin was deemed safe to bees by the EPA following a study of bees exposed to treated canola, a minor crop in the United States. However, according to the study, the pesticide dose bees are exposed to in the U.S. is usually ten times that, as corn (maize) covers more than 137,000 square miles in the U.S. -- an area larger than the state of New Mexico. So even though bees aren't pollinating corn directly, they still may be getting a toxic dose of pesticides from it.

To beekeepers, the news is not terribly surprising. Beekeeper Dave Hackenberg says, "You talk to more and more beekeepers across the country that don't really understand what's going on that are losing bees in the spring of the year when nobody's really spraying. You ask if there's corn there, and yeah, there's corn everywhere."

Hackenberg, a commercial beekeeper with 3,000 hives, somewhat unwittingly alerted the world to the mass deaths of bees after suffering major losses of his own bees in November 2006. "We were knocking our heads out trying to figure out what went wrong," he recalls. First he checked the hives for mites but found none. He called in experts from Penn State, who worked day and night, combing through the hives, and taking all kinds of samples. "Within four or five days they said 'We're finding stuff we've never seen before and you definitely don't have the virus we were looking for.' They saw paralysis in the bees, crystallization in their intestines, things that nobody had seen before, but nobody looked probably."

Several months later, Hackenberg's troubles were written up in the Philadelphia Inquirer. Within two days, his story appeared in 487 newspapers around the world. Hackenberg says that, for beekeepers, "Somebody changed the rules and forgot to tell us."

In addition to pesticides and diseases, using bees to pollinate monocultures and moving the bees around the country might be factors in their decline. Just as a person needs a varied diet, so do bees. According to Nieh, moving bees (as beekeepers like Hackenberg do several times a year) may cause them to lose some adult foragers. Bees begin their adult lives as nurse bees, become guard bees, and then spend the last few weeks of their lives as foragers. "Adult foragers learn where their home is based on solar and landscape cues." When they move, the adult foragers may leave the colony to gather honey and be unable to find their way home. "This may not be too difficult for the hive to weather, but it's just one more thing for an already weak colony," says Nieh. "The loss of a certain number of bees would not normally be fatal to the colony, but would not be good for a weakened colony."

"Monoculture and a homogeneous diet could be harmful, but the main thing is that bees are exposed to what we spray on crops, including fungicides that can harm bees because they have synergistic effects with insecticides and other toxins," says Nieh. "These factors are all combining in unexpected ways."

Nieh's research found that a very low dose of the pesticide imidacloprid, a relative of clothianidin that came onto the market in the mid-1990s, makes bees become, in essence, pickier eaters. "We find that it changes the sugar preferences of bees so that bees which previously would have accepted sugar solutions or nectars that were not very sweet, now will only feed at nectars that are much sweeter. And this is a problem because the amount of nectar out there that is very sweet is relatively limited. There is far more availability of nectar that has lower sugar concentrations ... Overall, this results in fewer calories flowing into the colony."

Hackenberg isn't doing as poorly as he was several years ago, but he attributes that to feeding the bees protein and supplements like brewers yeast and eggs and "kicking them in the pants with all kinds of nutrition because what they are gathering out there in nature is not what it's supposed to be." Hackenberg says, "We -- America or the world -- has messed up the bees' diet. Not only the bees' diet but everyone else's diet. We just don't have the nutrition that's out there in the food and bees are telling us this because what they are bringing home -- they can't make it anymore. We're supplementing them... and the bees are eating it... But go back 10-15 years ago, we didn't need this stuff."

A key question is whether the problem is simply a laundry list of unrelated factors (i.e. pesticides, disease, parasites, etc.) or whether those factors interact synergistically to kill bees. For example, does a sub-lethal dose of a relatively new pesticide make bees susceptible to die from a disease they would normally be able to recover from? This is important because it impacts the way the EPA should handle regulation of pesticides. If pesticides kill some bees, but parasites or diseases kill others, then the EPA's role is to merely ensure that the doses of pesticides used are low enough that they don't kill bees while scientists do their best to uncover how to treat parasites and disease. However, if low doses of pesticides weaken bees, making them susceptible to death by other causes (just like AIDS makes a patient susceptible to diseases that would not kill a healthy person), then the EPA will need to take more action.

Beekeepers see their bees as the canaries in the coal mine. All living beings are exposed to the cocktail of pesticides and other chemicals in our midst, each in sub-lethal doses but all mixing together and interacting in our bodies. Many Americans, like bees used to pollinate monocultures, do not eat very healthy or nutritious diets, and our stressful and sedentary lifestyles put us at even more risk of succumbing to illness. Are the bees giving us a message we should be heeding?

Dr. Nieh suggests that large growers could keep their own bees to give them "skin in the game." Currently, he says, one "focus is how much do I have to pay this year to rent honeybee colonies." If farmers kept their own bees, they would be "really invested in keeping these colonies healthy because these are the colonies that are pollinating their cash crop." Additionally, reducing the movement of bees around the country would slow the spread of diseases.

Nieh also sees a potential conflict of interest in the way that pesticides are approved by the EPA. "The approval process of pesticides would benefit from greater transparency and probably should undergo a more rigorous review process than it has in the past," he says. "It is a problem to require someone, a company like Bayer that has a vested interest in the approval, to pay for studies to show their pesticide is harmless."

For average Americans, in addition to eating foods grown without the use of pesticides, there are easy ways to support bees -- both honeybees and native bees. To attract and nourish pollinators, plant flowers in varieties of shapes, sizes and colors. Native plants are the best to attract native species of bees. Plant flowers in clumps instead of singly or in rows, and be sure that there is something blooming at all times during the year (at least, when your yard or garden is not covered in snow). Bees also need a water source, preferably shallow water so they will not drown. For more advice, visit the Xerces Society website.

Jill Richardson is the founder of the blog La Vida Locavore and a member of the Organic Consumers Association policy advisory board. She is the author of Recipe for America: Why Our Food System Is Broken and What We Can Do to Fix It..