Worker division of labor emerges as an association between age and behavior: young bees are most often nurses while older workers forage. However, ontogeny is flexible. Foragers can revert to within-nest tasks while nurses can accelerate behavioral development and forage precociously. Once foragers, workers usually die within 7–10 days, but foraging efforts decline if no brood is present requiring care such as feeding. 

During periods without brood rearing, workers develop into stress- resistant ‘diutinus bees’ with lifespans of 280 days or more. This phenotypic plas- ticity produces different life-histories with worker lifespans ranging from a few weeks to nearly a year

Natural selection exploited the reproductive genetics of female bees to facilitate advanced division of labour between workers with different foraging biases. The frailty of foragers compared to nurse worker bees, at least in part resulting from declining vitellogenin levels at foraging onset, could be advantageous at a colony-level. 

By reducing vitellogenin, workers attain a physiology of high JH, reduced cell-based immunity, and increased oxidative stress susceptibility. Foragers can be exposed to a heavy load of pathogens in the field, and will return to the same source of food over the course of days. By being susceptible to substances that could harm the colony, foragers die before bringing much contamination to the nest.

Likewise, stress by handling, injury, and disease causes JH to increase and bees to initiate foraging. The regulatory connection between vitellogenin, JH, and stress thereby ensures that bees in poor condition transition to a state where frailty and extrinsic mortality pretty much guarantee rapid death at a safe distance from the colony.

This regulatory feedback loop has spurred hypotheses on how vitellogenin and JH together have become key life-history regulators in honey bees.

Peter Borst
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peterloringborst.com

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