By Richard Valdmanis

BOSTON (Reuters) - Bee keepers' use of corn syrup and other honey substitutes as bee feed may be contributing to colony collapse by depriving the insects of compounds that strengthen their immune systems, according to a study released on Monday.

U.S. bee keepers lost nearly a third of their colonies last winter as part of an ongoing and largely unexplained decline in the population of the crop-pollinating insects that could hurt the U.S. food supply.

A bee's natural food is its own honey, which contains compounds like p-coumaric acid that appear to help detoxify and strengthen a bee's immunity to disease, according to a study by scientists at the University of Illinois.

Bee keepers, however, typically harvest and sell the honey produced by the bees and use substitutes like sugar or high-fructose corn syrup to feed them.

"The widespread apicultural use of honey substitutes, including high-fructose corn syrup, may thus compromise the ability of honey bees to cope with pesticides and pathogens and contribute to colony losses," according to the study, which was published on May 28 in the Proceedings of the National Academy of Sciences.

The U.S. Department of Agriculture and the Apiary Inspectors of America said in May that more than 30 percent of America's managed honeybee colonies were lost during the winter of 2012-13, up sharply from around 22 percent the previous winter but still close to the six-year average. The losses vary year to year, but a huge and prolonged multiyear decline threatens the species and crop pollination.

Honeybees pollinate fruits and vegetables that make up roughly one-quarter of the American diet, and scientists are split over whether pesticides, parasites or habitat loss are mainly to blame for the deaths.

Similar losses have been recorded in Europe where lawmakers have moved to ban three of the world's most widely used pesticides for two years amid growing criticism from environmental activists.

Agrichemical and pesticide makers like Monsanto, Bayer AG and Syngenta are also launching projects to study and counter colony collapse.

Few deny that pesticides - particularly a class of commonly used insecticides called neonicotinoids - can be harmful to bees in the laboratory. It is unclear what threat the insecticides pose under current agricultural usage. Some scientists say habitat decline and disease-carrying parasites, such as the Varroa mite, are the chief cause of bee deaths.

(Writing by Richard Valdmanis; Editing by Kenneth Barry)