The Lives of Bees
Page 24
ony living in the wild. We will focus here on the nectar and pollen sectors
of a colony’s economy, because the resin sector has been examined already
in chapter 5, and the water sector will be explored in chapter 9, when we
look at nest thermoregulation.
Most studies of nectar and pollen consumption by honey bee colonies
are based on colonies managed for honey production, but because this
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book focuses on the lives of wild colonies, we will focus on what I have
learned about these matters by studying unmanaged colonies, or simulated
wild colonies. These were colonies that I housed in hives the size of natural
nest cavities and then monitored—mainly by weighing them once a
week—for three years. As I explained in chapter 6, I did this work in the
early 1980s, when I was living in the city of New Haven, Connecticut,
which is about 400 kilometers (250 miles) east of Ithaca. The populations
of my simulated wild colonies ranged from a minimum of about 8,000
adult bees in late winter (March) to a maximum of some 30,000 adult bees
in spring (May–June), just before they produced swarms. These popula-
tion numbers represent biomasses of roughly 1 and 4 kilograms (2.2 to
8.8 pounds), respectively. Because honey bee colonies have considerable
mass and because they consume large quantities of food, I could track the
food consumption of my study colonies during most of the year by moni-
toring changes in the total weight of a colony, its food reserves, and its
nest. (Hereafter, I will refer to the sum of these three weights as “hive
weight.”) Between late September and late April, these colonies collected
little or no food, so their hive weights dropped steadily as they consumed
their honey and pollen stores. As was discussed in chapter 6, on average,
the total mass of food eaten over a winter by each of these colonies was
approximately 25 kilograms (55 pounds), of which approximately 1 kilo-
gram (ca. 2 pounds) was pollen and the rest was honey.
Determining a colony’s total consumption of honey and pollen for the
summer—late April to late September, in New York and New England—is
more complicated than for the winter, because resources now flow into
the colony, counterbalancing losses in colony weight due to food con-
sumption. Fortunately, extended periods of cool, rainy weather occurred
in the summer, during which the bees did not forage. Losses in hive weight
at these times indicated the summertime rate of resource use. (Note: these
weight losses must underestimate the rate of food consumption. This is
because honey and pollen resources that are consumed but are then con-
verted into brood are not represented in the losses of hive weight.) The
drops in hive weight during rainy weather ranged from 1.0 to 4.0 kilo-
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grams (2.2 to 8.8 pounds) per week and averaged about 2.5 kilograms
(5.5 pounds) per week. Given a summer season of 22 weeks (late April to
late September), I estimate the total mass of resources consumed over the
summer by a wild colony in New York or New England is about 2.5 kilo-
grams/week × 22 weeks = 55 kilograms (120 pounds). The pollen por-
tion of this total can be estimated by noting that it requires about 130
milligrams (0.004 ounces) of pollen to produce a bee, and that the average
colony population across the summer is about 30,000 bees. Given that a
worker bee lives about one month in summer, we can estimate that a wild
colony in New York or New England rears about 150,000 bees each year
over the five- month summer season. Consuming about 130 milligrams of
pollen per bee reared, a colony needs about 150,000 × 130 milli-
grams = 20 kilograms (44 pounds) of pollen each summer to support its
brood rearing.
To summarize, I estimate that the yearly food consumption of a wild
colony where I live is approximately 20 kilograms (44 pounds) of pollen
and 60 kilograms (132 pounds) of honey (25 kilograms/55 pounds in
winter plus 35 kilograms/77 pounds in summer). These are, of course,
only rough estimates; the precise values will vary depending on colony
size, local climate, and forage abundance. The comparable figures for colo-
nies managed for honey production in Europe and North America are all
considerably higher. These colonies have been estimated to rear 150,000–
250,000 bees annually and to consume 20–35 kilograms (44–77 pounds)
of pollen, and 60–80 kilograms (132–176 pounds) of honey each year.
The number of foraging trips required to procure the materials con-
sumed by a wild colony and the efficiency of this foraging work are both
rather easily calculated. With respect to pollen, a typical load weighs about
15 milligrams (0.0005 ounce), so the collection of 20 kilograms (44
pounds) of pollen requires approximately 1.3 million foraging trips. Given
an average total flight distance—out and back—of 4.5 kilometers (2.8
miles), a flight cost of 6.5 joules per kilometer, and an energy value for
pollen of 14,250 joules per gram, the total cost of flying to collect this
pollen is about 3.8 × 107 joules (1.3 × 106 trips × 4.5 kilometers per
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194 Chapter 8
Fig. 8.4. A forager collecting pollen from New England aster ( Symphyotrichum
novae- angliae) flowers.
trip × 6.5 joules per kilometer), and the pollen energy value is nearly
2.9 × 108 joules. These numbers show that worker honey bees achieve an
approximately 8:1 ratio of energy return in collecting pollen (Fig. 8.4).
Parallel calculations can be made about the number of collecting trips
required to produce the 60 kilograms (132 pounds) of honey that a colony
consumes in a year. Knowing that nectar is on average a 40 percent sugar
solution, while honey is an 80- plus percent sugar solution, and that a typi-
cal nectar load weighs about 40 milligrams, we can calculate that the pro-
duction of 60 kilograms of honey requires approximately 3 million forag-
ing trips. More number crunching indicates that worker honey bees
achieve an approximately 10:1 ratio of energy return when they collect
nectar.
These numbers make it clear that the collection of food each year by a
wild honey bee colony living in a cold climate region is an enormous un-
dertaking. Each such colony can be thought of as an organism that weighs
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Food Collection 195
1–5 kilograms (ca. 2–10 pounds), rears 150,000 bees, and consumes
some 20 kilograms (44 pounds) of pollen and 60 kilograms (132 pounds)
of honey each year. To collect this food, which comes as tiny, widely scat-
tered packets inside flowers, a colony must dispatch its workers on some
4 million foraging trips, with these foragers flying some 20 million kilo-
meters (12 million miles) overall. Given these facts, we can expect that
honey bee colonies have been under strong natural selection for great skill
> in the acquisition and use of their food.
A VAST SCOPE OF OPERATION
Among the more mind- boggling attributes of a honey bee colony is its
ability to conduct its food- collection operation over an immense area
around its nest extending more than 100 square kilometers (40 square
miles). A colony can exploit food sources over such a broad area because
each of its foragers can fly to and from flower patches located 6 or more
kilometers (3.6 miles) from home. A flying bee cruises along at about 30
kilometers per hour (18 miles per hour), so a 6- kilometer trip takes only
about 12 minutes, which does not sound so special, but if you consider the
small size of a bee, you will realize that a foraging range of this scale is truly
impressive. A 6- kilo meter flight performed by a 15- milli meter- (0.6- inch- ) long bee is a voyage of 400,000 body lengths. A comparable performance
by a 1.5- meter- (5- foot- ) tall human would be a flight of some 600 kilo-
meters (360 miles), such as from Boston to Washington, D.C., or Zurich
to Berlin, or Los Angeles to San Francisco.
Among my fondest memories from when I was a novice beekeeper—
some 50 years ago—is lying in the grass among my hives in late summer
and gazing up at the foragers crisscrossing the blue sky like shooting stars.
Naturally, I wondered how far my colonies’ foragers were flying to reach
their work sites. Several years later, when I was a college student, I read
scientific papers that reported studies of the scope of a colony’s foraging
operation. In some of these studies, the investigators had used the familiar
mark- and- recapture method: foragers were labeled in their hives—with
paint, fluorescent powders, sugar syrup containing radioactive isotopes,
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196 Chapter 8
or a genetic color marker—and then the fields around their hives were
searched for the labeled bees. In another study, one that I found much
more exciting than those just mentioned, the scientists moved colonies to
the semidesert badlands of northwestern Wyoming, where there were no
sources of nectar except in two irrigated areas, separated by 28 kilometers
(17.4 miles), in which alfalfa ( Medicago sativa) and yellow sweet clover
( Melilotus officinalis) were being grown. The researcher, John E. Eckert,
placed colonies along “the old winding stagecoach road” that connected
the two irrigated areas, and then he determined which of his colonies
discovered and exploited the two agricultural oases. In still other studies,
Norman E. Gary and colleagues devised a clever advancement on the usual
mark- and- recapture method: install a row of magnets over the entrance
of a study colony’s hive; capture bees from flowers in crop fields all around;
weakly glue a steel identification disk on each captured bee’s abdomen;
and carefully record where each ID disk was deployed. When the foragers
from the study colony return to their hive, the magnets over the entrance
automatically recapture the ID disks. By referencing the records of which
ID disks were deployed in which fields, researchers can determine quite
precisely where the bees retuning home to this colony with tags were
foraging.
The studies using one or another of these three approaches reported
that most of the foragers from the colonies studied were traveling to flow-
ers within 2 kilometers (1.2 miles) of their hives, but that the bees would
fly as far as 14 kilometers (8.7 miles) to reach flowers if none were closer
(the situation of some colonies in Wyoming). None of these studies, how-
ever, provided a picture of the spatial distribution of the foraging efforts
of a colony living in nature. This is partly because these studies were con-
ducted in artificial settings—the study colonies were placed either along-
side crop fields or in a barren, semidesert habitat—and partly because
their findings reflected not only where the colonies’ foragers went to find
flowers but also where the researchers went to find bees. Inevitably, the
researchers did not go everywhere the bees went. Furthermore, most of
the studies made with mark- and- recapture methods were conducted in
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settings where the forage was unusually plentiful, such as alfalfa fields and
almond orchards in full bloom.
In the spring of 1979, I started a study with a friend, Kirk Visscher, the
aim of which was to get an accurate, bird’s- eye view of the foraging activi-
ties of a colony living under natural conditions. Our approach was to map
out, day by day, the places being visited by the foragers of a full- size colony
that we would establish near the center of the Arnot Forest. We did so
using a technique that had been pioneered by one of Karl von Frisch’s
students—Dr. Herta Knaffl—in 1948–1950: spy on the waggle dances
performed by the foragers in a colony living in an observation hive. The
attraction of this approach is that it reveals where a colony’s foragers are
going even if they are exploiting multiple flower patches and each patch is
many kilometers (or miles) away. This technique, however, does not reveal
all the places where a colony’s foragers are working each day, because on
any given day it is only the bees returning home from the most profitable
sites that advertise their work sites with recruitment dances. Bees that are
working flower patches that merit continued exploitation, but not addi-
tional foragers, do not advertise these sites by performing waggle dances,
so these sites are invisible to anybody monitoring the dances performed
within a colony. Even so, this dance- surveillance method produces an ac-
curate picture of the spatial scale of a colony’s foraging operation, because
every major flower patch that a colony exploits is advertised by waggle
dances produced during the initial, buildup stage of its use by the colony.
The first step in our investigation was to build an observation hive that
was roomy enough to house a full- size colony of bees (Fig. 8.5). Ours had
a volume of 40 liters (10.6 gallons), the median volume of the nest cavities
of the wild colonies in the area (Fig. 5.3). This hive held four large combs,
whose total area (1.35 square meters/14.5 square feet) matched what is
found in the nests of the colonies living in hollow trees around Ithaca.
Because we wanted to be able to see all the waggle dances performed in
our hive, we guided all the foragers in our study colony to enter the hive
on the front side of its huge wall of comb. We also discouraged these just-
returned foragers from crawling to the back side of this comb by closing
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198 Chapter 8
0 10 20 cm
0 1 2 3 4 5 6 7 8 9
9876
Dance floor
54
and
3
sampling grid
21
Wedge directing incoming
Entrance tunnel
bees to one comb side
Fig. 8.5. The large observation hive used f
or sampling and reading the dances of
the foragers in a full- size colony to determine where the colony’s foragers were
gathering their food.
off with beeswax all the passageways between the two sides of the comb
that were near the hive’s entrance. Most foragers perform their waggle
dances soon after entering their nest (or hive), so by directing the traffic
of incoming foragers to one side of the comb, we created a well- defined
“dance floor” near the entrance on the front side of our hive’s wall of comb.
We drew a sampling grid on the glass over this dance- floor area, so that
we could sample the dancing bees at random when we began our data col-
lection. At this point, we installed a colony of approximately 20,000
worker bees and a queen (and some drones) in the hive, and a few days
later we moved it to the Arnot Forest, where we installed it in a special
hut that we had positioned in the forest’s center. An important feature of
this hut was its roof, built of translucent fiberglass panels, which provided
diffuse sunlight for our observations of the bees’ dances.
A few days later, we began collecting data from the bees performing
waggle dances in our large observation hive. This work involved sitting
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Food Collection 199
beside the hive from 8:00 a.m. to 5:00 p.m. and manually recording data
from one (randomly chosen) dancing bee at a time. For each bee, we re-
corded four items: 1) the angle (relative to vertical) of her waggle runs,
2) the duration of her waggle runs, 3) the color of her pollen loads (if any),
and 4) the time of day of her dance. Using these four pieces of information,
we could estimate where she was working, and we could determine
whether pollen was available at her work site or only nectar. Finally, we
plotted each dancer’s recruitment target on a map. This gave us a synoptic
picture of the colony’s richest foraging opportunities—those being adver-
tised by its waggle- dancing foragers—on each day.
Figure 8.6 shows the distribution of distances to the food sources that
our study colony exploited in the Arnot Forest. It is based on observing
1,871 dancing bees during four nine- day periods spread over the summer
of 1980. We see that this colony’s foragers conducted some of their work