The Lives of Bees
Page 18
protective cavities, press tightly together to form a well- insulated cluster,
and pool the metabolic heat generated by isometric contractions of their
powerful flight muscles. The fuel for this impressive, winter- long heat pro-
duction is the 20 or so kilograms (some 45 pounds) of honey that each
colony has stockpiled in its nest over the previous summer.
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Annual Cycle 141
Fig. 6.1. Worker honey bee collecting pollen from eastern skunk cabbage ( Symp-
locarpus foetidus), which flowers early in the spring. Only the flowers are visible
above the muddy soil; the stems remain buried below the surface of the soil, with
the leaves emerging later.
The honey bee’s annual cycle is special in other ways besides the process
of overwintering as a warm and active colony. Shortly after the winter
solstice, when the days begin to grow longer but snow still blankets the
countryside in Ithaca, each colony raises the core temperature of its winter
cluster to about 35°C (95°F) and starts to rear brood. Initially, there are
only a hundred or so cells of brood in a colony’s nest, but by early spring,
when the red maple trees, pussy willow bushes, and skunk cabbage plants
( Symplocarpus foetidus) have come into bloom and are providing the bees
with plentiful nectar and pollen (Fig. 6.1), more than 1,000 cells hold
developing bees, and the pace of a colony’s growth is quickening daily.
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142 Chapter 6
Come late spring, when the bumble bee queens and sweat bee queens are
just rearing their first daughter- workers to adulthood, honey bee colonies
have already grown to full size—30,000 or so individuals—and are start-
ing to reproduce. Colony reproduction by honey bees involves not only
the simple process of rearing males, which fly from their nest to find and
mate with virgin queens from neighboring colonies, but also the intricate
process of swarming (colony fissioning), in which a labor force of some
10,000 to 15,000 worker bees, together with the colony’s mother queen,
suddenly departs in a swirling mass to establish a new colony.
The main aim of this chapter is to present an overview of the life of a
wild honey bee colony living in the woods around Ithaca. It does so by
describing the pattern of events that unfold in these colonies across a year.
This view reveals several fundamental themes about how honey bee colo-
nies live in the wild, themes that will unify the subsequent chapters. A
second goal of this chapter is to explain why colonies of honey bees living
in the cold- temperate regions of Europe and North America lack a period
of winter dormancy and therefore possess an annual cycle that is unique
among all the insects living in these places. As we shall see, the distinctive
annual cycle of Apis mellifera as it lives in temperate regions is a blending of
its current ecology and its ancient history. Novel adaptations for living in
a seasonally cold climate have been superimposed on the physiological and
social characteristics that this bee inherited from its tropical ancestors.
ANNUAL CYCLE OF ENERGY INTAKE AND EXPENDITURE
Surviving winter by fighting the cold is energetically expensive. In the
heart of winter, a honey bee colony weighs approximately 2 kilograms (4.4
pounds) and consumes energy—mostly for heat production—at a rate of
20–40 watts, roughly the same rate as a small incandescent lamp. Natu-
rally, a colony’s long- term survival requires that its energy expenditure
over winter be balanced by its energy storage over summer, when energy-
rich nectar can be gathered to rebuild its honey stores. A strong flow of
energy back and forth between colony and environment is, therefore, a
key feature of the natural history of honey bees, and one that provides both
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Annual Cycle 143
biologists and beekeepers with a valuable window on the lives of these
bees. By monitoring this energy flow throughout a year, one acquires both
a synoptic view of a colony’s annual cycle and a detailed picture of one of
the major challenges that it faces every year: the need to amass within a
short summer season an ample supply of winter heating fuel.
A simple but effective way to monitor the net flow of energy into or out
of a honey bee colony is to record changes in its total weight: the weight of
the bees, the nest, and the food stored in the nest. Weight is gained when
food is brought into a colony but is lost when stored food is consumed,
when a colony divides for reproduction, and when a colony’s members die.
Detailed records of the weight changes of honey bee colonies across a sum-
mer or throughout a year have been published for many temperate regions
of the world, including the United States, Canada, Germany, and the United
Kingdom. Almost without exception, however, these records were col-
lected for apicultural purposes, so they describe patterns of net weight gain
and loss for beekeepers’ colonies that were managed for honey production
and were living in agricultural landscapes. Therefore, the discussion that
follows is based primarily on the findings of a study that I conducted with
the aim of shedding light on the energy budgets of wild colonies.
The heart of my study was monitoring the weekly weight changes of
two unmanaged colonies. Each one occupied a hive that consisted of two
deep Langstroth hive bodies (total volume, 84 liters/22.2 gallons), so the
nest cavities of my two study colonies were larger than the typical nest
cavity of a wild colony (Fig. 5.3). I mounted each colony’s hive on platform
scales and weighed both hives every Sunday evening from early November
1980 until late June 1983. Except for twice- monthly measurements of
brood rearing in late spring, summer, and early autumn, neither colony
was disturbed or manipulated once the study began. The least natural as-
pect of the ecology of these colonies was their location: the Othniel C.
Marsh Botanical Garden, which is in a leafy residential neighborhood in
the small city of New Haven, Connecticut. (At the time, my wife and I
lived in the caretaker’s house in this garden, and having the hives nearby
made it easy to take readings of their weights every Sunday night.) The
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144 Chapter 6
forage available to honey bee colonies living in this city is perhaps more
plentiful than that available to colonies living deep in forests, so the results
of this study probably under represent the difficulty of food collection by
honey bees living in the wild.
The weight records, presented in Figure 6.2, show that winter was a
time of dramatic weight loss for these colonies. On average, the two colo-
nies studied lost 23.6 kilograms (52 pounds) each year between September
and April. Except for approximately 1 kilogram (2.2 pounds) attributable
to the removal of dead workers from the nest, these weight losses repre-
sent consumption of stored food—honey
and pollen. I suspect that most
of the 20- plus- kilogram (44- plus- pound) cost of overwintering comes
from the high cost of rearing brood in winter. It is telling that the two colo-
nies lost weight twice as rapidly in March, when brood rearing was intense
(0.84 kilograms/1.85 pounds per week), as in December (0.42 kilo-
grams/0.93 pounds per week), when the colonies were broodless.
Further evidence of the high energetic cost of midwinter brood rearing
comes from experiments conducted in the 1930s by Clayton L. Farrar, an
entomologist at the University of Wisconsin, who compared the winter
weight- loss records of colonies with and without stored pollen, hence
with and without winter brood rearing. The weights of colonies with pol-
len fell 22.7 kilograms (50 pounds), on average, between October and
May, whereas those of colonies lacking pollen dropped only 11.8 kilo-
grams (26 pounds) over the same period. Presumably, this 10.9- kilogram
(24- pound) difference in average weight loss arose primarily from the
higher energetic cost of thermoregulation that a colony incurs when it
starts rearing brood in the middle of winter (see chapter 9). Whereas a
colony without brood needs only to maintain the surface temperature of
its winter cluster at about 10°C (50°F), a colony with brood must maintain
its core temperature at approximately 35°C (95°F), the optimal brood-
nest temperature for a honey bee colony.
Figure 6.2 also illustrates a second important fact about the annual
cycle of honey bee colonies: the brevity of the period each year when colo-
nies experience a net energy gain. On average, the two colonies that I
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Annual Cycle 145
) 70
swarm swarm
60
swarm
swarm swarm
50
Colony weight (kg 40
)
97531–1eekly change in J M M J
W colony weight (kg
S
N
J
M
M
J
S
N
J
M
M
1981
1982
1983
Fig. 6.2. Weekly changes in the weights of two honey bee colonies (hive plus bees
and stored food) living in New Haven, Connecticut. The data shown here are
representative of the weight- change patterns recorded for both colonies that
were studied.
studied gained weight for only 14 weeks each year. What is even more
striking, 86 percent of the annual weight gain by these colonies occurred
between April 16 and June 30—a period of just 75 days.
The rather grim picture that emerges from this look at the energetics
of colonies living in a cold climate is one of an ever- looming energy crisis.
A colony consumes 20 or more kilograms (44 or more pounds) of honey
each winter, yet it has little time each summer in which to rebuild its food
reserves. As we shall see in the next section, this energy problem is espe-
cially acute for newly founded colonies, which, unlike colonies that are
already established, cannot fall back on food reserves from previous years.
Moreover, newly founded colonies must also cover the high costs of build-
ing their nests of beeswax combs. It may be that not all honey bee colonies
experience such severe energy problems. Colonies living in milder cli-
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146 Chapter 6
mates, or in habitats richer in forage than those just discussed, may well
find other problems—such as predators or shortages of nest sites—far
more challenging. Nevertheless, for colonies of Apis mellifera living on their
own in the northern parts of the species’ range in Europe and North
America, I think the primary obstacle to their survival is balancing the
winter losses and the summer gains in their annual energy budgets.
ANNUAL CYCLES OF COLONY
GROWTH AND REPRODUCTION
Correct timing of colony growth and reproduction is essential to honey
bee survival in cold climates. As we have seen, a colony of honey bees
survives the cold, flowerless days of winter through intensive thermoregu-
lation fueled by a strategic reserve of some 20 kilograms (44 pounds) of
honey that it has managed to amass over the previous summer. We will now
see that this impressive feat of building a food reserve requires a colony to
correctly time its growth and reproduction, for unless it does so, it will
not have a sufficiently strong workforce at the right time of year to meet
this life- or- death challenge.
The patterns of colony growth and reproduction across a year are easily
studied. Colony growth patterns have been described in two ways: by
periodically counting the number of brood- filled cells in a colony’s nest or
by making repeated censuses of a colony’s population of adult bees. De-
scribing the annual cycle of colony reproduction is a bit more difficult
because it has distinct processes for females (queens) and males (drones).
The seasonal pattern of colony reproduction by means of males is easily
described by counting the cells containing developing drones. The seasonal
pattern of colony reproduction through females could likewise be deter-
mined by monitoring the appearance of queen cells, the large and eye-
catching cells that cradle developing queens (Fig. 6.3). However, because
colonies frequently make false starts in their queen rearing—building
queen cells but then destroying them before the developing queens ma-
Fig. 6.3. One of the large, peanut- shaped cells in which queen bees are reared.
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Annual Cycle 147
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148 Chapter 6
ture—a more reliable method is to record the appearance of swarms,
which are, after all, the true units of female reproduction for honey bees.
Figure 6.4 shows the growth pattern of the two honey bee colonies that
I monitored in New Haven, Connecticut, from 1980 to 1983, based on
bimonthly censuses of their brood. Following several months without
brood in late autumn and early winter, the colonies begin rearing bees in
January or February, evidently in response to increases in day length. At
first, fewer than 1,000 cells containing brood are found in a bee hive or a
bee tree, but in late March or April this number soars, climbing to a peak
of some 30,000 or more developing bees per colony in May or June.
Shortly thereafter there appears a gap in brood rearing when, because of
the turnover in queens associated with swarming, the colony lacks an egg-
laying queen for 10–20 days. Once the new queen eliminates her rivals
and completes her mating, brood rearing resumes at nearly full tilt for a
few weeks but then starts a gradual decline over the remainder of the sum-
mer, finally ceasing altogether in October. This annual pattern holds for
honey bee colonies in temperate climates, although the precise timing of
the rapid springtime expansion varies markedly with latitude
. For exam-
ple, colonies located in Somerset, Maryland, studied by Willis J. Nolan,
entered the intensive growth phase three weeks before the colonies that I
studied in New Haven, Connecticut, 250 kilometers (150 miles) to the
north. Such geographic differences in annual cycle of brood rearing can be
partly under genetic control, reflecting adaptation to the local climate and
flora. This has been demonstrated by experiments conducted in France in
which colonies were exchanged between Paris (northern France) and the
Landes region (southwestern France). Beekeepers in the two locations
knew that the principal peaks of brood rearing in their colonies came at
different times: early summer in Paris and late summer in Landes. The
remarkable finding of this colony- transplant study was that the colonies
from each place kept their original (native) annual rhythms of brood rear-
ing in their new locations.
Reproduction by honey bee colonies commences in late spring, shortly
after the surge in worker brood production, and is largely completed by
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Swarm, 1981
Swarms, 1982
30
20
1981
1982
10
1983
Cells of sealed brood (x1000)
40
30
20
Number of swarms
10
d )
20
10
0
Monthly avg. maximum an minimum temperature (˚C
Jan
Mar
May
Jul
Sep
Nov
Winter
Spring
Summer
Autumn
Fig. 6.4. Annual cycles of brood rearing and air temperature (in New Haven,
Connecticut) and of swarming (in Ithaca, New York). Top: Mean number of cells
of sealed brood (pupae) in a colony throughout the year for two years, and during
the start of a third year. Middle: Seasonal occurrence of 301 swarms collected
over the 10- year period of 1971– 1981. Bottom: Annual pattern of air temperature
for New Haven, Connecticut.
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150 Chapter 6
Pollen
Brood
Honey
Empty
12
Worker