I have long believed that photoperiod had a lot to do with brood cessation but observation makes me think that it is more complicated then that alone. Here at 47 north we lose close to 10 minutes a day right around the equinox.  Adds up fast. 

I think that temperature also plays a large role. Once the bees start clustering during the nights, brood rearing falls off quickly. In support of this hypothesis I have yards here that range in altitude between 900' (75 meters) along the Snake River to just over 2000' (275 meters) here at home in the Blue Mt. foothills.  First frost here is usually around mid Sept. with consistent nighttime lows in the 30s (0c) and nearly no brood. The lower yards will go about 2 weeks longer where the nighttime lows are easily10 (5 to 8c) degrees warmer. Daytime highs vary little between the elevations, 60s and 70s. (15 to 20c). Also I have kept bees on the Olympic Peninsula of WA at a slightly higher latitude but with a much milder maritime climate and the brood cycle will  continue well into Oct or even Nov.

To reiterate, once nighttime temps. are low enough to cause clustering, even with still available pollen my bees will shut down brood rearing but continue to hoard pollen for the following spring as long as they have good flying conditions and natural pollen available.  

Paul Hosticka
Dayton WA

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