> Pete, I don't follow you.  Why do you think that there must be some other signal?

Because brood rearing commences before pollen starts to come in, "anticipating" spring and "diutinus" (long lasting) bees are raised in fall during abundant pollen and nectar flow, again "anticipating" the coming dearth. Along these lines:

> We found that seasonality per se exerted strong effects on colony and individual level traits. Colonies collected in the summer maintained total worker mass despite high mortality. In contrast, colonies collected in the fall lived longer, and accumulated lipids, including when reared on protein-biased diets. Food macronutrient content had mainly transient effects on physiological responses. Extremes in food carbohydrate content however, elicited a compensatory response in summer worker ants, which processed more protein-biased foods and contained elevated lipid levels. Our study, combined with our previously published work, strongly suggests that underlying physiological phenotypes driving behaviors of summer and fall ants are likely fixed seasonally, and change circannually.

Interestingly, despite having much higher worker mortality,
summer lab colonies on average maintained their initial biomass,
which after five weeks was not significantly different than that of
fall colonies. In other words, summer colonies had a high rate of
worker turnover, with the queen’s reproductive output matching
losses to mortality in many cases. Brood biomass was significantly
greater in summer colonies, corresponding with bursts in colony
somatic growth and production of sexual alates.

As fall approaches it makes evolutionary sense that long-lived
social insect colonies, similar to solitary animals, halt reproduction
and slow growth in favor of sequestration of resources, making
them available when the environment is lacking.

Cook, Steven C., et al. "Summer and fall ants have different physiological responses to food macronutrient content." Journal of insect physiology 87 (2016): 35-44.

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