Age of first foraging: How do workers know when to "graduate"?

Honey bee workers change jobs as they age. For the first 2-3 days after emerge from a cell, they spend most of their time cleaning cells and eating pollen. The protein in the pollen help the bees to development a pair of their special glands (hypopharyngeal glands) in the head, they are then ready to secret proteinaceous food to feed the larvae. So for the next 7-10 days workers spend the majority of their time feeding the larvae and taking care of the queen, and they are properly called "nurses". When workers are about 14 days old, they are now "mid-aged" and engage in activities that are near the nest periphery: receiving nectar collected by foragers, drying down the moisture in nectar to make it honey, building and fixing the wax cells throughout the hive. Some workers also have alternative career paths to become the "guards" and "undertakes". Guards stay near the entrance to keep foreign workers from entering a wrong hive -- they are usually in a highly alert position with their front 2-4 legs lifted from the substrate and their wings held tight together. They jump into other workers when they return to the hive to smell them if they are hivemates or not. Undertakers perform the job of removing dead bees out of the colony. Finally when workers are about 21-30 days old, they "graduate" from the home school to go out to forage. Foragers collect nectar as the source material for honey, pollen as their only protein source, water to dilute honey (in spring time) or to be evaporated to keep the hive cool (in summer), and propolis to seal cracks, Once a worker becomes a forager, she rarely does anything else inside the hive -- she becomes highly specialized and only does one job: foraging.

For many years it is not known what factors regulate the age of first foraging. Surely workers do not have calendars to keep track of everyone's age and say "worker number 251, you are now three weeks old and you should start foraging!" So how can on average, workers start foraging more or less around the same time (three weeks of age) in summer? Working with Gene Robinson, I have discovered that workers use a simple negative feedback system to regulate this process: as old foragers die off, the inhibition that keeps the other workers "at home"is reduced and other workers therefore graduate to become foragers (essentially replacing the dead foragers). Therefore, one can make bees to become foragers much earlier by making a single cohort colony that only has newly emerged bees (no foragers) -- in fact they can become foragers as early as 5-7 days of age compared to the usual 21-30 days. Conversely, keeping foragers at home would have the effect of delaying the onset of foraging on younger bees. This makes ecological sense: when there is a nectar dearth and foragers are not doing much outside, they would contact their hivemates at home more often and transmit the "inhibitor" more efficiently, thus delaying their age of first foraging. The alternative way of deciding when to become a forager: say using the abundance of freshly collected nectar or pollen, would make a "bad decision" -- younger bees would starting foraging earlier than usual due to the lack of newly collected food, but they would not find anything to forage, because there is currently a nectar dearth.

This research was conducted at University of Illinois and Prof. Gene Robinson is pursuing the checmical identity of the "inhibitor".

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