While a number of theories have been advanced for the recent disappearance of millions of honey bees, U.S. researchers say the reason is most likely overexposure to pesticides.

Walter Sheppard and other researchers at Washington State University say that rather than being driven off by cell phone frequencies as some have suggested its more likely that the bees have simply been poisoned.

The research team has been looking into what has become known as colony collapse disorder, when honey bees leave the hive and dont return.

I dont think we really know what were up against with colony collapse disorder, said Sheppard.

But the search for a cause is definitely narrowing. For the past decade, beekeepers have treated their hives with pesticides to combat two kinds of mites that parasitize the bees.

To keep bees, especially on a commercial level, beekeepers have needed to use some sort of chemical control of these mites, said Sheppard. Normally, Varroa mites will kill a colony within two years, if theyre not treated and the use of these pesticides brings with them a risk of accumulation in the wax.

Honey bees rear their young in waxy honeycomb which is re-used for several years. If pesticides used to control mites build up in the wax, over time they could reach a concentration at which they harm the bees as well.

Sheppard is testing whether something in the honeycomb of a failed colony will carry over and affect the health of a new brood of honey bees.

Weve gotten some combs that were from colonies that suffered from colony collapse disorder, and well be doing some experiments to compare them with combs from healthy colonies. Well have our [healthy] queens laying eggs on both the collapsed colony combs and the control combs at the same time.

Sheppard said the study should yield information about the potential role of pesticides in causing colony collapse by the end of the year.

Sheppard said honey bees could also be exposed to pesticides during their foraging flights, if they visit fields and gardens that were recently treated with the chemicals. That source of exposure has been a concern for beekeepers since pesticides came into wide use in the 1950s, he said.

Honey bee health is crucial to the nations farmers and fruit growers, who rely on honey bees to pollinate crops such as apples, cranberries and watermelons. Together, honey bee pollinated crops are worth more than $9 billion a year to the American economy.

While mass-produced crops like wheat and corn are pollinated by wind, some 90 cultivated flowering crops rely mostly on honeybees. According to a Cornell University study, honeybees pollinate every third bite of food ingested by Americans.