Reduced honey bee longevity can be caused by these, to name a few:

Nosema ceranae
> The decrease in Vg expression after N. ceranae infection is consistent with the reduced lifespan reported for worker bees with suppressed Vg expression. 

Large colonies
>  Results clearly demonstrate that workers introduced to our large colony environments lived shorter than workers introduced into a smaller colony environment. Both principal life history stages were shortened concurrently: With other factors statistically accounted for, workers in a large colony environment initiated foraging earlier and died after a shorter foraging lifespan. 

SEE:

Immune suppression in the honey bee (Apis mellifera) infection by Nosema ceranae (Microsporidia)
Mariano Higes, et al
Environmental Microbiology (2009) 11(9), 2284–2290

Honey bee (Apis mellifera) workers live longer in small than in large colonies
Olav Rueppell, Osman Kaftanouglu, Robert E. Page Jr
Experimental Gerontology 44 (2009) 447–452

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