The Consequential Weeks of Late Summer

There’s an argument to be made that some of the most consequential weeks in a colony’s year happen as summer begins to wind down.

The signs of this period are subtle. Foragers may be coming and going like they were in mid-summer. Bees may be bearding outside the hive on hot afternoons. Depending on where you live, summer or fall forage may still be ahead. But inside the colony, a quieter seasonal transition is beginning.

The workers that are raised in late summer and fall can be very different from the bees that carried the colony through June and July. These are often called winter bees, and their job is to live long enough to carry the colony through winter and help raise the first generation of bees the following spring.

Summer Bees and Winter Bees

Summer and winter bees aren't different species or castes. You can't reliably tell them apart just by looking at them. The difference is physiological.

During the active season, worker bees move through a demanding progression of jobs, eventually becoming foragers. Those weeks of flight and activity take a toll, and summer workers commonly live for only a matter of weeks.

Winter bees develop differently. They have more developed fat bodies and greater nutritional reserves, including higher levels of vitellogenin, a protein associated with longevity and other important functions.

A 2024 PLOS ONE study examined fat body lipid content, hypopharyngeal glands, vitellogenin, and other molecular markers to better understand the physiological state of overwintering bees.

The result is remarkable: instead of living for weeks, winter bees can survive for months, maintaining the colony through winter and helping care for the first brood of spring.

When Does a Colony Start Raising Winter Bees?

There isn't a date you can circle on the calendar, nor a moment when a queen simply begins laying “winter bee eggs.” The physiology of the workers develops in response to conditions within and around the colony.

One particularly interesting part of that process is brood rearing. In a 2009 study appropriately titled The Nurse’s Load, researchers examined the relationship between brood rearing, vitellogenin, and worker longevity. They found that reduced exposure to brood-rearing tasks was associated with vitellogenin accumulation, one of the characteristics of long-lived workers.

So while you can't look at a capped cell and know there's a winter bee inside, a shrinking brood nest and the colony's broader seasonal contraction can offer clues that the transition is underway.

Why These Weeks Matter

A colony can have a large adult population and look strong while simultaneously raising the bees it will depend on months later. Those two populations aren't necessarily telling you the same story.

Varroa destructor makes this timing particularly important. In 2012, researchers in the Netherlands studied colonies during the transition from short-lived summer bees to long-lived winter bees. Colonies treated for Varroa earlier had lower mite infestation while their winter bees were developing. Those bees lived longer, and the colonies had higher winter survival.

Another 2012 study, conducted in Switzerland, followed colonies from summer into winter and found that both Varroa and deformed wing virus were associated with shorter winter-bee lifespans.

Together, the research helps explain something that isn't necessarily obvious from standing outside a hive in late summer: a colony can look strong and still be raising a compromised winter population.

The important question isn't only how many bees are in the hive today. It's what condition the next bees are in.

What Can You Actually See?

You can't identify a winter bee walking across the comb. What you can observe is the colony changing around them.

Brood production may begin contracting. Foraging patterns change with available bloom. The balance between brood, adult bees, and resources begins shifting away from summer expansion and toward winter survival.

Meanwhile, traffic can still be heavy, and bees may be bearding on hot afternoons. The colony may look every bit like summer behavior as usual.

Think in Seasonal Blocks, Not Dates

There’s no universal date for this transition. Temperature, day length, forage, brood rearing, colony condition, and other factors all play a role.

A colony in northern Europe isn't following the same calendar as one in Southern California. Neither is a colony experiencing drought following the same trajectory as one surrounded by abundant late-season forage.

The useful thing isn't memorizing a date. It's recognizing the transition. Late summer may still look like summer to us, but inside the hive, the colony is already beginning the work of making it to spring.