Review: Wilson-Rich et al. (2008). Journal of Insect Physiology, 54: 1392-1399.

Written: 01/14//09

Posted: January 14, 2009

Word count: 642

Question: Does a bee’s immune system change as she ages?

Answer: Yes, in ways that are important for overall colony health.

Now that researchers have sequenced the honey bee genome—the genetic blueprint that codes for how bees become bees—there is an inherent promise that we will gain significant new insights into keeping bees healthy and productive. While knowing the genome is a starting point and not the end point, researchers have already learned a great deal about bees when it comes to colony health and their overall immunity. For example, it has been shown that honey bees have about one-third fewer genes for fighting off diseases and other infections compared to other insects. This has led some to speculate that the “social immunity” of honey bees is more important than bolstering individual bees’ immune systems.

But we should not discount the importance of how individual bees are able to stave off infection. After all, when living in a colony of 50,000 crowded sisters, if one bee gets sick it may quite easily spread throughout the hive. Understanding how this might be done, and how bees might prevent widespread disease outbreaks, is particularly important given the different life stages of developing bees in a colony at a given time.

A recent study, led by Noah Wilson-Rich at Tufts University in Boston, followed the immune responses of bees at different ages. Specifically, they studied bees at two developmental stages (larvae and pupae) and at two adult stages (younger nurse bees and older forager bees). They measured several different immune responses that help individual bees prevent or fight infections. First, they measured total hemocyte counts, analogous to our white blood cells that target foreign pathogens and kill them. Second, they measured the encapsulation response of bees towards sterile nylon threads. This immune response is a way that the insect’s body surrounds large objects with cells to “quarantine” the foreign object and keep it from doing harm. Third, they measured phenoloxidase activity, which is an enzyme in the bee’s blood that recognizes then attacks foreign particles. Finally, they measured the quantity of fat bodies (in the adult workers only). Fat bodies, located in the adult abdomen, is functionally analogous to our livers since it produces antipathogenic proteins and “cleans” the blood.

Wilson-Rich and his colleagues found that a bee’s immune system changes as they get older. Specifically, they found that while the encapsulation response does not significantly vary across time, developing bees (larvae and pupae) have higher hemocyte counts compared to adult bees (which makes sense since brood diseases are mostly bacterial or viral), whereas phenoloxidase activity increased directly as a function of age. Overall, they found that the immune systems of older foragers (those most likely to initially encounter a disease and bring it back to the colony) had the highest activity compared to younger bees. This suggests that by losing the foraging force from a colony—such as by moving a hive during the day or by their loss due to a pesticide, for example—it might force younger workers (who are more prone to contracting disease) to leave the nest before they are ready.

These findings show that it is very important to understand the dynamics of how bees might be able to withstand disease by using their own natural immunities. It also demonstrates that bees have a fairly well equipped disease-fighting arsenal at their disposal above and beyond behavioral mechanisms such as hygienic behavior. A greater understanding of these benefits might help us make more practical and effective management decisions.

Reference

Wilson-Rich, N, S. T. Dres, and P. T. Starks. (2008). The ontogeny of immunity: Development of innate immune strength in the honey bee (Apis mellifera). Journal of Insect Physiology, 54: 1392-1399.