Wednesday, December 22, 2004

The Bee Fountain of Youth

One of the primary ways to distinguish honeybees in a colony is by the type of work the bee usually does. Young worker bees typically stay in the nest and help rear the brood (larvae) of the colony, as well as performing other in-nest colony maintenance activities. As the workers age, they get tired of staying at home with their mom and sisters, so they head out into the world to explore, free from the confines of the nest (i.e. they become foragers, and focus on out-of-nest activities).

This change in general behavior has motivated a great deal of study, both to document when the changes occur, and to determine the factors that regulate the changes. The change is physiologically mediated by levels of juvenile hormone in the worker bees and influenced by genetics (this American Scientist article has more background). Overall colony characteristics have been observed to change the age at which bees mature into foragers, so we know that the change in behavior is not completely controlled by genetic factors. For instance, in colonies without many foragers, young worker bees mature into foragers faster than in colonies with lots of foragers.

Considering that bees use pheromones for many other colony-level tasks (see my earlier post on bees and pheromones for more background), it seemed to make sense that pheromones might be involved in this process. However, no pheromone controlling this change in behavior had been identified until this month, when Leoncini et al. (2004) published a paper reporting that ethyl oleate appears to be a chemical (pheromone) produced by foraging bees that inhibits young bees from maturing into foragers.

Since the presence of foraging bees appeared to inhibit the maturation of young bees into foragers, the researchers looked for a chemical present in foragers at high concentrations but not in workers, and tested those chemicals to see if they inhibited worker maturation (see table 2 online for some examples; Science News reports that the research took more than 10 years).

Leoncini et al. ended up finding ethyl oleate at higher concentrations in foragers than in nurse bees (3x the level when their entire bodies were analyzed). When they analyzed the amount of ethyl oleate in specific body parts, they found an even larger difference: the crops of foraging bees had ~30 times the amount of ethyl oleate than the crops of nurse bees (levels in all other body parts were statistically similar).

But what are bee crops? A foraging bee collects nectar from flowers to bring back to its hive, and stores the nectar in its crop, a large sac near the front of its digestive system designed for temporary storage of fluids. When the foraging bee returns to the hive, it regurgitates the nectar from its crop, both feeding it to other bees and placing it in cells in the hive so it can be turned into honey.

The finding that foragers have high ethyl oleate levels in their crops suggests an extremely simple mechanism of transfer for the pheromone: when returning foragers transfer collected nectar to other workers, they also transfer the pheromone. Thus, if there are lots of foragers collecting lots of nectar, young bees will be exposed to high levels of ethyl oleate and will delay their maturation into foragers. However, if there are not lots of foragers, or those foragers are not successfully collecting nectar, then young bees will not be exposed to high levels of ethyl oleate and will more quickly turn into foragers to remedy the problem.

To directly test whether ethyl oleate controlled the maturation of the bees into foragers, Leoncini et al. acquired a number of beehives containing bees of known age, and then fed them “candy” (honey and powdered sugar mixed together) laced with either ethyl oleate or nothing. In hives that were fed candy with ethyl oleate, young bee maturation to foragers was delayed by approximately 2 days compared to hives fed candy that did not contain ethyl oleate (from ~18 days to ~20 days).

This is a good example of how bees are able to control a colony-level activity through completely decentralized means: there is no central authority telling which bee when to mature. Instead, individual bees sense their environmental conditions (including ethyl oleate concentration, which is in turn controlled by other individual bees), and respond to them via genetically controlled pathways (e.g. maturing into foragers or not).

Reference:

Leoncini, I., Y. Le Conte, G. Costagliola, E. Plettner, A.L. Toth, M. Wang, Z. Huang, J. B├ęcard, D. Crauser, K.N. Slessor, and G.E. Robinson 2004. Regulation of behavioral maturation by a primer pheromone produced by adult worker honey bees. Proceedings of the National Academy of Sciences 101: 17559-17564. (abstract)

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