By Lilian Shen
posted
Dec 27, 2012
As the landscape settles into winter, one of the things we
notice (and likely enjoy) is the virtual absence of insects. As
small, cold-blooded creatures, insects cannot stay active at low
temperatures; they quickly chill, their metabolism stops, and they
freeze to death.
To escape an icy demise, insects in northern latitudes employ many
tactics for winter survival, such as overwintering as
freeze-resistant eggs, or fortifying their bodies with natural
antifreezes and hiding in protected crevices.
Not so the honeybee, a familiar, non-native insect that made its
way to the Americas via settlers in 1622. Honeybees are native to
Africa, and adhering to their warm-latitude origins, remain active
all winter. Individually, they'd stand no chance against months of
subfreezing weather, but as a collective, they've developed several
extraordinary ways to survive in cold northern climes.
In the Northeast, a honeybee's preparation for winter begins
with the first flowers of spring, but this process accelerates as
summer wanes. The collection of fall nectar, most notably from
goldenrods and asters, can be essential to the colony's winter
survival. The average hive may need 60 or more pounds of honey to
get through a northern winter.
Another crucial late-summer process is the production of winter
bees, bees with a different blood protein profile and greater body
fat than their summer bee counterparts. The winter bees get the
colony through winter, while the summer bees, the ones that so
diligently foraged for nectar and nursed the developing winter
bees, die off in the fall. Beginning in September, the colony also
rids itself of drones, male bees whose only role is to mate with
new queen bees. All drones are forced out to starve. Lastly, the
colony stops producing new bees, as eggs and larvae require
temperatures of 90-96 degrees Fahrenheit to develop.
As flowers disappear and temperatures fall, bees stop foraging
and remain in the hive. When the mercury falls below 64 F, bees
begin a behavior known as clustering, where they gather together to
form a ball that extends through several honeycombs in a typical
hive. The cluster consists of an outer mantle of tightly-packed
bees which surrounds an inner core of bees that are more
loosely-packed and free to move about. The all-important queen bee
is sequestered at the center.
As the temperature drops, the cluster becomes tighter and
tighter, shrinking to one-fifth of its original size. At their most
tightly-packed, the mantle bees form a layer that approaches fur in
its insulating qualities. Bees become comatose if they chill below
43 F; thus, when their thorax temperature falls to about 54 F, the
mantle bees exchange places with bees from the core. Mantle bees
that get too chilled to return to the core drop off and die.
Once the temperature in the hive falls below 50 F, the cluster
can only maintain its life-supporting interior temperature of 64 F
(at the periphery) to 90 F (center) by actively producing heat.
Like mammals, the core bees begin to shiver by pumping their large
flight muscles. This is why honey stores are critical. The bees eat
honey and transform it into heat through metabolism. After they
empty one honeycomb, the cluster slowly shifts sideways and upwards
towards a full one.
This survival mode, however, has some inherent hazards. If there
is not enough honey, or if it gets so cold that the bees cannot
move to full honeycombs, the colony starves.
Eating honey, especially if it is high in indigestible material,
leads to a need to void feces. Usually, this is taken care of when
winter days warm above 50 F, allowing bees to leave the hive for
cleansing flights during which they dot the snow with tiny yellow
splashes. If prolonged cold stops bees from leaving the hive, they
eventually defecate inside, and if enough bees do this, it leads to
the death of the entire colony.
To make matters worse, metabolism produces water vapor as a
byproduct, and this must be allowed to escape from the hive. Savvy
beekeepers drill a hole near the top of the hive for this purpose.
Otherwise, the vapor condenses and ice-cold water drips onto the
bees, causing them to freeze to death. In the particularly bad and
long winter of 2000-2001, many beekeepers in the northeast reported
losses of 50 percent of their colonies. Some suffered losses of 90
percent.
One of nature's marvels is the temperature achieved within a
mere ball of bees in subzero weather. So take off your winter hat
and put your ear to the thin wood of a hive; you'll hear the
humming of bees by the thousand. The sound is enough to transport
you to summer meadows, even in the cold, dark depths of winter.
Li Shen is an adjunct professor at the Dartmouth Medical School
and the chair of the Thetford, Vt, Conservation Commission. The
illustration for this column was drawn by Adelaide Tyrol. The
Outside Story is assigned and edited by Northern Woodlands magazine
and sponsored by the Wellborn Ecology Fund of New Hampshire
Charitable Foundation: wellborn@nhcf.org
