To add to the "what triggers winter bees"  discussion. 
A recently published paper looked at a gene methylation process that changes the transition of bee life span.   

The discussion in this paper is interesting -- it presents the gene methylation time clock as the ancient conserved process, and the juvenile hormone// vitellogenin   interaction as a overlying, but independent,  component adapted to social insects (when the sexual maturity 'timeclock' became irrelevant to sterile workers).

Read the paper: 
DNA methylation affects the lifespan of honey bee (Apis mellifera L.)
workers e Evidence for a regulatory module that involves vitellogenin
expression but is independent of juvenile hormone function
Carlos A.M. Cardoso-Júnior*, Karina R. Guidugli-Lazzarini, Klaus Hartfelder

Insect Biochemistry and Molecular Biology 92 (2018) 21e29

https://doi.org/10.1016/j.ibmb.2017.11.005

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