In an immune mechanism known as “hygienic behavior,” adult honey bees (Apis mellifera) can detect and remove brood afflicted with parasites or pathogens. A new study shows that part of this process relies on chemical signals emitted by unhealthy brood, and brood coming from colonies bred to be more hygienic are more effective in signaling for their own removal. In this image, brood can be seen in some uncapped cells. (Photo credit: Kaira Wagoner, Ph.D.)

By Kaira Wagoner, Ph.D.

Pollinator health is a top priority these days, and everyone seems to be asking, “What can be done to save the bees?” Since most of the current challenges to pollinator health can be attributed to humans, there are several things we can do, from restoring pollinator habitat by planting pollinator-friendly natives to curbing our use of harmful pesticides.

Kaira Wagoner, Ph.D.

This work is both ecologically and economically important, as honey bees are the most agriculturally important pollinator worldwide, contributing over $15 billion to annual crop yields in the United States alone. But honey bees have flourished on Earth for over 100 million years, so perhaps it is also worth asking, “What can honey bees do to help themselves?”

As social insects, closely related honey bees live in crowded colonies with frequent physical contact, a recipe for the rapid spread of parasites and pathogens. As a result, honey bees have evolved some fascinating social immune mechanisms, which help mitigate the spread of disease between sisters in a bustling colony. One such immune mechanism is “hygienic behavior,” the ability of adult bees to detect and remove unhealthy brood from the colony. By sacrificing a few unhealthy young, the overall health of the colony, and thus the probability of colony survival, is improved.

Over the past few decades, numerous hygienic honey bee stocks have been selectively bred and studied. However, much of the hygiene-related research has focused on the detection capabilities of adult bees; we now know that adults from hygienic stocks are more sensitive to diseased brood odors than adults from unselected stocks. But does honey bee brood (larvae and pupae) also contribute to hygienic behavior? We decided to find out.

In our new study published last week in the Journal of Economic Entomology, we provide evidence that brood signaling in the European honey bee (Apis mellifera) plays a significant role in triggering hygienic behavior. To test the role of brood in hygiene communication, we quantified the removal of parasite-infested brood, cross-fostered between colonies with distinct breeding backgrounds. Our cross-fostering approach allowed us to disentangle the relative contributions of adult detection and brood signaling to hygienic behavior.

In short, we found that parasite-infested brood from hygienically selected colonies was more likely to be removed than brood from unselected colonies, regardless of where the brood was fostered. This suggests that hygienic brood more effectively signals stress, triggering its own removal for the good of the colony. The idea that altruism at the individual level may contribute to survival at the colony level is consistent with evidence of brood susceptibility and colony resistance to parasites in the Asian honey bee (Apis cerana).

Our study’s findings may have major implications for our understanding of intracolonial communication of social insects, as well as for honey bee management and breeding strategies. For example, hygienic response to chemical assays made from unhealthy brood compounds could be used to more accurately quantify colony hygiene level. Such a tool could guide management decisions (e.g., which colonies should be treated for parasites?) and could be used to breed honey bees with greater disease resistance. These healthier honey bees could not only improve crop and bee product yields but also reduce the spread of disease from honey bees to other, more ecologically important native pollinators.

The compounds that unhealthy honey bee brood uses to signal for its own removal could be used as a tool to aid in measuring the hygienic level of honey bee colonies and breed bees with greater resistance to parasites and pathogens. (Photo credit: Kaira Wagoner, Ph.D.)

As a follow-up to this study, we have already identified multiple brood chemicals associated with honey bee stressors. These compounds are capable of triggering hygienic behavior, and our most recent study indicates that they can be used to accurately assess a colony’s ability to limit infestation by harmful parasites. We are currently in the process of publishing these results and developing an improved tool for the evaluation and selection of hygienic honey bees.

One of my primary research goals is to develop practical (i.e., low cost, easy-to-use) and sustainable solutions to current honey bee health threats. By tapping into a social immune mechanism that honey bees have evolved over millions of years, we may just be able to help the honey bees help themselves.  And for us, as horticulturally minded hominids, healthy honey bees have big implications, from flower to farmer to fork.

Kaira Wagoner, Ph.D., is a post-doctoral fellow in the Rueppell Lab at the University of North Carolina at Greensboro in Greensboro, North Carolina, USA. For more information about Kaira’s research, contact Kaira at kmwagone@uncg.edu, or visit: https://biology.uncg.edu/rueppell-lab/team/.