The Lives of Bees
Page 17
5 percent of the comb builders. This surprising result shows that future
work on the control of comb building needs to focus on the behavior of
unemployed comb builders, to find out how these bees acquire information
about their colony’s comb fullness and nectar intake.
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132 Chapter 5
CONTROL OF COMB TYPE
Besides deciding when to build comb, colonies must decide what type to
build, worker comb or drone comb. Both types of comb are used for honey
storage, but only the large cells in drone comb can be used for rearing the
large, heavy- bodied drones. A colony living in the wild devotes a surpris-
ingly high percentage of the total comb area in its nest to drone comb: 17
percent, on average (Fig. 5.13). It does so because having plenty of drone
comb enables a colony to invest heavily in producing drones, which are
critical to its reproductive (genetic) success. Figure 5.11, from Michael L.
Smith’s study, shows, however, that during the first several weeks after
moving into their new homesites, all three colonies in this study built only
worker cells and reared only worker bees. This makes sense, for it is
worker bees that will build the combs and collect the honey and pollen
that are essential for the survival of a fledgling colony.
Eventually, however, a healthy colony will grow strong enough to invest
in building both drone comb and worker comb. How does it regulate the
proportion of each comb type in its nest? In principle, each comb- builder
bee—or perhaps the queen—must be informed about the current relative
amount of each kind of comb in the nest, but how is this possible given the
large size of the whole nest relative to that of a worker bee? The queen is
uniquely suited to evaluate this feature because she spends much of her
time—when she is not resting or being groomed—walking around on the
comb and measuring the sizes of empty cells to decide which kind of egg
to lay: fertilized eggs in worker cells and unfertilized ones in drone cells.
If she keeps a running count of the two cell types, then she might be able
to perceive any imbalance between the two types of comb and might com-
municate this to the workers so they can correct it by adjusting the type
of new comb they build.
Stephen C. Pratt investigated the information pathways that the bees use
to guide their decisions about which type of comb to build. He did so by
setting up queenright, full- size colonies housed in two 10- frame hive bod-
ies and then adjusting the number and type of combs in each hive and the
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4
2
Number of nests 0
0
5
10
15
20
25
Drone comb (%)
Fig. 5.13. Top: The cell diameters in natural honey comb have a bimodal distribu-
tion: the smaller size on the left (approximately 5.2 millimeters/0.20 inches wall
to wall) is used to rear worker brood, and the larger size on the right (approxi-
mately 6.5 millimeters/0.26 inches wall to wall) is used to rear drone brood.
Bottom: The percentage of drone comb by area in the nests of 8 wild colonies
collected in upstate New York, and 22 simulated wild colonies. Values are clus-
tered around the mean value of 17 percent.
access by the workers to the combs in their hive, to create different experi-
mental treatments (Fig. 5.14). Stephen’s experiments revealed three things.
First, the bees in a colony require contact with the drone comb for its
presence to inhibit their building of additional drone comb. Second, con-
tact with the drone comb by the queen plays no part in this inhibition. Third,
the inhibition of building more drone comb can come from the drone
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134 Chapter 5
Drone comb
No drone comb
Empty frame
Worker comb
Drone comb
Fig. 5.14. Design of the experiment testing the need of the bees to have direct
contact with drone comb for proper regulation of drone- comb building. Bees
(workers and queen) were separated from two drone combs by a wire- mesh
screen in the Drone Comb treatment. As a control, two worker combs were
similarly screened off in the No Drone Comb treatment. In both treatments, the
hive had two empty frames where the bees built comb. A feeder jar filled with
sugar syrup (to encourage comb building) sits atop each hive.
comb itself, though the inhibition is even stronger if the drone comb is
filled with brood. These findings show that the workers are not responding
to a volatile chemical signal built into the drone comb, since they require
direct contact to respond to its presence. These findings also show that the
queen is not acting as a central regulator of drone- comb building, even
though she is well positioned to collect information on drone comb need.
So how do workers decide which type of comb to build? It may be that each
comb- builder bee crawls over the existing combs and slowly accumulates
information on their cell- size composition by measuring directly some
number of cells. And it may be that worker bees use the same mechanism
of cell- size measurement as is used by queen bees: inserting the head and
forelegs into a cell and sensing how they contact the cell’s walls.
To summarize, what we now know about how bees control the type of
cells they build is that a comb- builder bee may be informed about the need
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The Nest 135
for each type of cell by noting the relative encounter rates that she has had
with completed worker and drone cells. We also know that a comb- builder
bee may be able to coordinate her work—that is, whether to build worker
comb or drone comb—with that of her nest mates by noting the relative
encounter rates that she has had with cells under construction. Each of these
hypotheses, however, awaits a rigorous experimental test.
THE PROPOLIS ENVELOPE
Perhaps the most eye- catching difference between the inside of a hollow-
tree home of a wild colony and the inside of a lumber- built hive of a
managed colony is the look of their walls. The walls inside a bee tree are
coated with tree resins that make them shiny and waterproof (Fig. 5.4),
whereas the walls inside a bee hive—even one used for years and years—
have no such coating, so they usually look as dull and porous as freshly
planed boards. In short, only wild colonies live surrounded by a propolis
envelope. Most of this envelope is a thin (less than 1 millimeter/0.04
inch) coating of resin on the nest cavity’s walls, but if there are cracks in
the walls the bees will fill them with dark seams of resin. Also, if the en-
trance opening is oversize—hence too drafty or too inviting to invad-
ers—the bees will reduce it by building a sturdy wall of dried resin (Fig.
5.7). Constricting the entrance opening is the most conspicuous use of
&nb
sp; tree resins by the bees, and it is the origin of the word beekeepers use for
the resins bees apply in their hives, propolis ( pro: in front of; polis: city or community).
What are the bees’ sources of tree resins, and how do the bees collect
this sticky stuff? The only tree on which I have observed bees collecting
resin (from sticky leaf buds) is an eastern cottonwood tree ( Populus deltoi-
des) that grows near my laboratory. However, bees have been observed by
others collecting resin from the buds and wounds of many other tree spe-
cies in North America and Europe, especially horse chestnut ( Aesculus hip-
pocastanum), white poplar ( Populus alba), quaking aspens ( Populus tremula
and P. tremuloides), and various birches ( Betula spp.). In these species, the
leaf buds shine with protective coatings of resin. Bees further collect resin
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136 Chapter 5
Fig. 5.15. Honey bee with a load of shiny resin in the corbicula (pollen basket)
of her left hind leg.
from conifers—including spruces ( Picea spp.), pines ( Pinus spp.), and
larches ( Larix spp.)—that have injury sites oozing the trees’ protective
resins. A resin- collector bee loads herself with this gluey material by chew-
ing off a bit of resin with the mandibles, then grasping it with the forelegs,
passing it to one of her midlegs, and finally shifting it from the midleg to
the corbicula (pollen basket) on the same side. This action is repeated until
a load of glistening resin fills each corbicula (Fig. 5.15). When a resin col-
lector returns to her nest, she crawls across the combs to one of the places
where resin is being used and then she stands there for 5–20 minutes while
other bees, resin users, bite off pieces of resin from the two loads in her
corbiculae.
The resin collectors and resin users of a honey bee colony are fascinat-
ing bees. But who are they? How do they handle resin inside their nests?
And how do they control their collection of this building material? Until
recently, we had no solid answers to these questions. I was delighted,
therefore, when Professor Jun Nakamura, from the Honeybee Science
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The Nest 137
Research Center at Tamagawa University in Japan, came to my laboratory,
in 2002, to try to solve these mysteries. He conducted his investigations
using a colony of approximately 3,000 bees that lived in a glass- walled
observation hive that was housed in a heated room. To study individual
resin users and resin collectors, he added eight cohorts of zero- day- old bees
to the colony, one cohort every three to four days, during the month of
May. He quickly learned how rare the resin users can be, for he observed
only 10 of his 800 labeled bees engaged in resin- use behaviors: caulking
(forcing resin into cracks and crevices) and resin taking (biting resin from
the corbiculae of a resin collector). He also learned that all 10 resin users
were middle- aged bees. They were doing their resin work when they were
14–24 days old, which was after they had quit working as nurse bees (e.g.,
feeding larvae and eating pollen) and before they would begin working as
foragers. The resin collectors in Jun’s colony were all elderly bees (age
range 25–38 days old), as were the nectar, water, and pollen collectors in
this colony.
Jun also learned that with resin—unlike with pollen, nectar, or water—
there is not a strict partitioning of work between collectors working out-
side the hive and users working inside the hive. This became clear when he
video- recorded 35 resin collectors, starting when each one entered the
observation hive. He found that upon entering, each resin collector walked
quickly and directly to a place in the top or side of the hive where resin
users were getting resin from collectors and using it to fill the cracks be-
tween the hive’s glass walls and its wooden frame (Fig. 5.16). Five of these
resin collectors had carried home a chunk of resin in their mouthparts—
along with full loads in their corbiculae—and they walked immediately to
a work site and began to do some caulking themselves, using the resin they
carried in their mandibles. In contrast, the resin collectors whose loads
were entirely in their corbiculae did not engage in caulking. They simply
stood around for 5–18 minutes, waiting for bees (resin users) to bite off
chunks of their loads and carry them off to sites being caulked.
It is still not known how the resin collectors sensed their colony’s need
for resin. They could have done this directly (while engaged in using resin,
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138 Chapter 5
Resin
Nectar
ggle-dancing
Wa
g
Unloadin
Fig. 5.16. Spatial distributions of waggle dancing and unloading for resin collec-
tors and nectar foragers of a colony living in a two- frame observation hive. Arrow
at bottom of each plot denotes the hive entrance.
if they did so) or indirectly (by noting their unloading delays) or both. It
seems clear that the resin collectors in the study colony sensed an ongoing
need for resin, because of the 102 resin collectors that Jun labeled and
watched come and go from his observation hive, 68 (67%) continued col-
lecting resin for the rest of their lives. (The other 34 bees eventually
switched to collecting nectar or pollen.) It may be that these resin collec-
tors sensed an ongoing need for resin, because every week or so Jun re-
placed the dirty glass walls on his observation hive with clean ones, and in
doing so he probably stimulated the resin users by reopening the crevices
where the glass walls contacted the wooden frame of the hive. We know
that the resin collectors are stimulated by finding gaps, crevices, and rough
surfaces—all places that are hard to keep clean—in their nest cavities.
(Indeed, many of the bee- supply companies in the United States now sell
propolis traps: plastic sheets that are perforated with hundreds of narrow
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The Nest 139
slots about the width of a pencil’s lead. These sheets are laid across the
combs in the topmost hive body, to stimulate the bees to collect and de-
posit propolis, which will be harvested for use in making antibiotic tinc-
tures and lotions.) Recently, Michael Simone- Finstrom and colleagues at
the University of Minnesota have discovered that resin collectors are bet-
ter than pollen collectors at learning to associate the stimulus of their
antennae touching a surface with a 1- millimeter (0.04- inch) wide slot
with the reward of a droplet of sugar syrup. The same is true when resin
collectors and pollen foragers were tested for their ability to learn to as-
sociate touching a rough surface (a small piece of sandpaper) with a sugar-
syrup reward. It is not known whether this difference between resin col-
lectors and pollen foragers in learning tactile stimuli is based on their
recent experiences or their genetic
s, but either way, it is likely that this
ability helps resin collectors to learn about the presence of gaps, crevices,
and rough surfaces in their colony’s nest.
Most beekeepers find propolis a messy hindrance to their work, since
it can make it hard to crack open a hive and manipulate the frames of comb
inside. For the bees, though, propolis must be extremely valuable, for
otherwise they would not expend the effort of chewing off bits of sticky
plant resins, packing them onto their hind legs, hauling the gluey blobs
back to their hive, and then using this sticky stuff to fill the cracks and
coat the walls inside their nest cavities. Recent work has revealed that the
propolis envelope functions mainly as an antimicrobial surface that lowers
a colony’s costs of defense against disease. We will examine this subject
closely in chapter 10, when we explore the multifaceted matter of colony
defense. But before we dive deeply into this and the three other major
topics of colony functioning—reproduction, food collection, and thermo-
regulation—we will look broadly at the life of a wild colony by examining
its remarkable annual cycle.
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6
ANNUAL CYCLE
Oh, give us pleasure in the orchard white,
Like nothing else by day, like ghosts by night;
And make us happy in the happy bees,
The swarm dilating round the perfect trees.
—Robert Frost, “A Prayer in Spring,” 1915
One key to understanding the natural lives of honey bee colonies living in
cold- climate regions is their unique annual cycle. In winter, when colonies
of all the other social bees—bumble bees and social sweat bees—have
dwindled away, leaving only a residue of mated queens living alone deep
in hibernation, honey bee colonies continue to function as full social
groups, each one consisting of some 15,000 worker bees and one queen
bee. Moreover, rather than becoming dormant, as is the rule for insects
living in cold climates, honey bee colonies remain active and fight the cold.
Each one keeps the perimeter temperature of its winter cluster above
about 10°C (50°F), even in ambient temperatures of −30°C (−22°F) or
colder. To achieve such strong temperature control, honey bees nest inside