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Bees worldwide contribute an estimated $215 billion worth of work to agriculture, with industrial farmers often bringing in swarms of bees to pollinate many hectares of an individual crop. But does pollinating monocultures threaten the insects' health? A study published today suggests that forcing bees to feed on just one type of pollen can reduce their ability to synthesize an enzyme needed to protect hives from infection. "This is a really good first step," says behavioral ecologist Marla Spivak of the University of Minnesota, Twin Cities. "In the past, there have been studies on the protein and amino acid content of the pollen but no linking of that to the immune systems of the bees."

Honeybee populations in many countries, including the United States, appear to have declined markedly in recent years. Many commercial beekeepers have been devastated by a mysterious syndrome known as colony collapse disorder (CCD), in which many worker bees vanish. Some research has suggested that the bees' immune systems have been suppressed or that they are less able to defend the hive from parasites. Entomologist Cédric Alaux of the French National Institute for Agricultural Research in Avignon wondered if the diets of bees might be altering their immune systems.

"We know that in humans, protein is very important to the immune system, and beekeepers had been telling us that protein nutrition was very important to the bees, but no one had really tested it," says Alaux.

So Alaux's team divided newborn bees from local colonies into groups of 80 and reared them in cages. Each group was fed a preparation of pollen with a different protein content; some preparations comprised the pollen of only one type of flower, whereas others were a mixture of pollen from various flowers. After 5 days, the team measured four chemical indicators of immune system health of the bees. To Alaux's surprise, the bees fed with higher-protein pollen did not show any signs of superior immunity. But when the team compared bees fed a varied diet with those fed just one type of pollen, they found something more interesting.

Bees fed pollen from a variety of plants showed about 40% higher levels of a hormone called glucose oxidase, Alaux and colleagues report online 20 January in the Royal Society Journal Biology Letters. Workers bees use this hormone to sterilize the food fed to larvae, protecting the next generation and contributing to the collective immunity of the hive.

Next, Alaux's team plans to see if colonies resist infection in fields with diverse plants. They also want to find out whether chemicals in particular types of pollen can be linked to specific immune functions so that the beekeepers can optimize bees' diets.

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