While studying pesticides in pollen, I was always curious about the potential for pollen to carry disease organisms as well. Indeed, a new study that appeared in the December 22 PLoS ONE confirmed my worst fears—that pollen may be a major route of viral infection from managed honey bees to wild native bees.
The authors of the study examined the four viruses that are most commonly found in North American honey bees—deformed-wing virus, sacbrood virus, black queen cell virus, and Kashmir bee virus—plus Israeli acute paralysis virus, which is often found in conjunction with colony collapse disorder. They asked a number of questions about bee-to-bee disease transmission and then set up a series of experiments to answer those questions.
They found eleven species of wild pollinators in Pennsylvania, New York, and Illinois that carried at least some of the viruses. These viruses were much more likely to show up in wild pollinators that lived near apiaries known to be infected with the various pathogens.
Tests on both the pollen and the bees themselves showed that in many cases disease-free foragers were carrying pollen loads that contained viral diseases—especially deformed-wing virus and sacbrood virus. This finding indicates that the pollen, itself, may be capable of transmitting the disease from one bee to another—it may not be necessary for an infected bee to pass the virus directly to another bee. Similar to human viruses that survive on door knobs, these bee viruses appear to survive on pollen grains.
In other experiments, honey bees and bumble bees kept in greenhouses were shown to transmit Israeli acute paralysis virus among themselves by simply foraging on the same flowers. The disease moved freely in both directions, from honey bees to bumble bees and from bumble bees to honey bees.
The authors point out that the exact mechanisms of disease transmission via flowers and pollen are not understood and more study is needed to see if host plants have a greater role in disease transmission than just as physical carriers. In the meantime, it is important for beekeepers to understand the impact diseased honey bees may have on wild pollinator populations. Honey bee health needs to be a priority if we are to maintain the health—or perhaps the very existence—of wild pollinator populations.
For more information, you can download the complete paper for free at http://www.plosone.org/article/info:doi/10.1371/journal.pone.0014357.