Review: Wegener et al. (2009). Insectes Sociaux

Written: April 24, 2009

Posted: 04/24/09

Word count: 767

Question: Are older nurse bees just as good as younger nurse bees?

Answer: Yes, but the brood they raise aren’t as good

Compared to other animals, humans are exceptionally adept at linking cause and effect. In fact, we are so good at it, we often assign cause when none actually exists. For example, we notice that our best beehive happens to have three pine cones on the lid, whereas our other beehives don’t have pine cones on top, so we conclude that putting pine cones on all of our hives will make them better. The old adage of “correlation doesn’t necessarily equal causation” is therefore important to remember when trying to make inferences about what causes what.

The flip side to our ability to link cause and effect is that we are equally bad at making such linkages when the two are separated by time or space. In other words, even if something directly causes something else, but the effect is only seen months afterwards, then we have difficulty in recognizing the association no matter how obvious it might be. Demonstrating cause, despite our inherent limitations, is the essence of scientific investigation.

The complexity of a honey bee society can make it difficult to identify cause and effect because events happen over such a long time period. It takes worker brood 3 weeks to fully develop from egg to adult, and their lifespan averages about 6 weeks. While 9 weeks to total existence may not seem like a long time, a lot happens to a bee during that period; she changes jobs as a function of her age, most notably feeding developing larvae (“nursing”) when she is young (<15 days old) and flying from the hive to collect food (foraging) when she is old (>21 days old). This age-based division of labor, however, is very plastic, where bees will respond to the needs of the colony no matter what their age. For example, nurse bees will start foraging much earlier if the field bees are removed from the colony, such as when you move a hive in the middle of the day during a nectar flow.

Nurse bees can also delay the transition to foraging if the colony needs it. This can happen for several reasons, such as starting a new colony after swarming, disease or requeening imposes a hiatus in brood rearing, or if a beekeeper robs a colony of all of its capped brood so that no new nurse bees are coming online. These older “prolonged nurses” might have a significant impact on the colony, since they may not be as good as normal-aged nurse bees, but we really don’t know what their impact might be.

It is this question that a German research team recently asked in a study investigating the cause and effects of prolonged nursing. They first compared various physiological parameters of normal-aged and older nurses, such as protein content in the hemolymph (particularly the protein vitellogenin, the main precursor to brood food), the development of the mandibular and hypopharyngeal glands (which make brood food), and the metabolic activity of a-glucosidase (an important enzyme in the hypopharyngeal glands for making brood food). They found that there were no significant differences among normal and older nurses for any of these measures.

Importantly, the researchers took the study a step further and measured the “quality” of the workers raised by normal and older nurses. They found that workers reared by over-aged nurses tended to have much more developed ovaries, which had a greater tendency to become active under queenless conditions, compared to workers reared by normal-aged nurses. What these results suggest is that while older nurse bees may be just as capable of producing brood food as younger nurses, they are not functionally equivalent in how well they can raise brood.

Ever notice how some colonies can become laying workers really easily, whereas others can stay queenless for weeks without a problem? These findings may partially explain why; subtle changes in age demographics within a colony, weeks prior to the event, may predispose workers to more readily develop their ovaries and become laying workers because they were raised by older nurse bees. It is not all that surprising that we have a difficult time seeing the cause of these effects, but it demonstrates how seemingly minor changes can have more than subtle consequences.

Reference

Wegener, J., M. W. Lorenz, and K. Bienefeld. (2009). Physiological consequences of prolonged nursing in the honey bee. Insectes Sociaux, DOI 10.1007/s00040-008-1042-1.