STRESS
ERIC H. ERICKSON
Few beekeepers
question whether their systems of bee breeding and colony management adversely
affect the normal biological processes of honey bees. And even fewer consider
assuring that the evnironment within the hive is "natural"as close
as possible to that which is optimal for honey bee survival. It seems that
we have come to expect that honey bee colonies are generic and are only found
in nearly square white boxes just as children believe that milk comes from
paper or plastic containers. The fact is, of course, that before the intervention
of beekeepers, feral (wild) honey bees were (and still are) highly adapted
to native habitats and utilize as domiciles naturally occurring cavities in
living trees, rock crevices, ground holes and other similar spaces. As beekeepers,
we assume that the white boxes we provide as hives are somehow adequate if
not better than natural cavities. We find it difficult to understand why,
given our breeding and management strategies, our bees are often unable to
withstand the onslaughts of weather, diseases, mites and perhaps even the
incursion of Africanized bees. The fact is that from the very moment we place
bees in artificial wooden hives, we impose upon them a large measure of stress.
Natural ...
When a honey
bee swarm exits a natural cavity in search of a new domicile it is, under
normal circumstances, guided entirely by its own instincts. It exercises these
instincts in the selection of a well insulated, properly sized cavity. Herein
it builds comb using inherent skills. The attachment of these combs to the
interior ceiling and walls of the cavity as well as a small upper entrance
establishes relatively stable thermal and humidity zones for brood rearing
and communication. This construction greatly restricts the movement of bees,
air and hive odors. The newly founded colony will rear its young, establish
its own defenses against enemies (diseases, parasites and predators) and gather
and store provisions in a manner that ensures a nutritionally adequate and
well balanced diet. If it fails in these endeavors, for whatever reason, the population declines
and with continued failure, dies. Shortly thereafter, wax moths move in to
clean (by destroying the combs) and restore the cavity to its nearly original
state.
This housekeeping force readies the cavity for the next swarm. In this
natural scenario, three biological facts are evident:
1) environmental stress imposed on this
colony is minimal; 2) colonies with genetic composition that reduces fitness
for
Artificial...
But then mankind,
as a not‑so‑benevolent keeper of bees, enters the scene and, usually
unknowingly, imposes stress. The beekeeper captures that swarm, hives it in
an uninsulated, free standing, oversized box and forces it to build comb.
The cells may be highly variable in size depending on the foundation used.
The flow of interior air and hive odors is greatly increased by the additional
spare around remove‑able frames and between hive bodies and by an enlarged
entrance placed at the bottom. This results in increased energy expenditure
by the bees for temperature
and humidity control.
The bees are
subjected to management schemes that result in exorbitantly large populations
and the removal of excessive quantities of pollen and/or honey. The beekeeper
alters the diet of the colony first by stimulating increased levels of foraging
and brood rearing and then by selectively removing pollen and honey (e.g.
early season honey and/or pollen and leaving only late season stores which
often have
Old, dark
combs, usually contaminated by continuous
exposure to naturally‑occuring microbes, plant toxins and man‑
made pesticides, may be kept for thirty years or more. The bees are bred for
behavioral traits foreign to their survival (but in harmony with their current
environment) and are subjected, often unprotected (except for an uninsulated
box), to the environmental extremes of winter cold, desert heat and/or more
pesticides.
In this man‑made
scenario three facts, entirely different from the natural scenario above,
are evident: 1) domestic honey bee colonies in box hives are subjected to
stresses seldom encountered in nature; 2) domestic colonies whose genetic
fitness may be reduced are nursed along, often unknowingly, so that undesirable
genes may be perpetuated; and 3) housekeeping chores normally carried out
by wax moths are added to the responsibilities of worker bees or remain undone.
Thus, toxins disease organisms and other undesirable elements of the environment
often accumultate in the hive for many years, further reducing the ability
of the colony to function normally. The wonder then is not that so many domestic
colonies dwindle or succumb for whatever reason, but rather that so many survive
in spite of beekeepers!
It is upon
this latter point that we must learn to focus if we are to understand honey
bee stress and then reduce it. Remember that bees function like other insects,
mammals and even human beings. They learn and remember via both short‑ and long‑term memory but can be confused by exceptional or adverse
elements. For example, they sense and respond to their environment, but they
can adapt only within certain limits and thus may become chilled or overheated.
They function less efficiently under nutritional stress and their immune system
can be compromised by toxic elements in the environment. The task is to recognize
stress when it occurs in domestic colonies and then to assist our bees through
difficult times.
STRESS…
Stress as
it occurs in honey bees is still poorly defined. To evaluate fully the effects
of honey bee stress inducers, we need to know much more than we presently
do about the natural biology of honey bees. Having said this, however, let
us examine in detail what we know and can presume about several probable sources
of honey bee stress.
CLIMATE, WEATHER AND THE BOX HIVE.
Among the elements stressing honey bees,
few ravage domestic colonies more than the weather. While feral colonies are
for the most part comfortable within their natural cavities, domestic colonies
in uninsulated hives must struggle to survive the seasonal extremes of cold
winters and hot summers If they survive, and many do not, their productivity
is significantly reduced This is not to say that feral colonies are not affected
by seasonal and climatic change ‑ they probably are, but undoubtedly
to a far lesser degree due to the factors discussed below. They survive, in
part, because they still have the tools, acquired through millennia of evolution,
to cope with hardship.
In a living
tree, the honey bee colony is surrounded (usually) by several inches of heart
and sap wood plus a layer of living tissue (the cambium) and bark. The R value
(1 divided by the thermal conductivity of the material) for the cavity walls
in a living tree likely falls between 5 and 15 and perhaps higher, although
precise measurements are unavailable. The R factor for a one‑inch pine
board (which is actually about 0.75 inch) is 1, essentially zero insulation
(Wheast, 1980). Hence, the R factor for a box hive is far different from that
of the typical feral colony. (Note: the R value for walls in new homes in
many areas of the United is R=19). The living tissue surrounding
the tree cavity generates some beat from metabolic processes. Moreover,
the cells of the cambium carry cool water from the soil to the tree top, a
function that likely thermally stabilizes or cools the cavity slightly in
summer. In the winter, the cambium is supercooled but not frozen, thus contributing
insulation in addition to that of the cavity wall. In those areas of the world
where there are few if any trees large enough to accommodate a colony, honey
bees utilize rock crevices and ground holes.
The surrounding rock soil mass is a virtually unlimited thermal (and
humidity) buffer for these small caves, a point more easily understood when
one feels the air expelled at the entrance to a large underground cavern in
mid‑summer or mid‑winter.
Studies have shown that the temperature
inside an uninsulated box hive differs little from ambient temperature (Owens,
1971). Thus, depending on locality, internal hive temperatures outside of
the cluster may range from ‑30 degrees Ferenheit (‑34 degrees
Celius) to 115 degrees Ferenheit (46 degrees Celcius). This potential 145
degrees Ferenheit (70 degrees Celcius) temperature range is undoubtedly far
different than that of natural cavities which probably vary by no more than
plus or minus 30 degrees Ferenheit (17 degrees Celcius) This concept is strengthened
by the work of Severson and Erickson (1985) who showed that in Wisconsin the
colonies' consumption of honey for the production of heat does not vary
with the severity of ambient winter temperatures.
Thus, one must assume that once the heating mechanism reaches maximum output,
all the bees can do to survive increasingly cold conditions is to tighten
the cluster. Other work (Erickson, unpublished) indicates that during brood
rearing worker bees maintain
absolute humidity at near saturation within the brood nest. During the winter
months, the humidity within the cluster is only slightly lower. Since both
heat and moisture production are accomplished via the metabolism of honey,
it must be assumed that both honey stores and the physiological strength of
bees are unnecessarily reduced during winter in the uninsulated cavities of
box hives.
Several studies have shown that honey bees can compensate for and survive temperature extremes. However, what such studies have not considered is the drain on the physiological resources of the colony. The effects of this stress may well be significant in terms of reduced brood rearing or foraging and shortened worker bee life span.
COLONY SIZE.
The number of honey bees in a normal feral colony varies from about 14,000 to 25,000 (Seeley and Morse, 1976). Beekeepers, using a variety of strategies, are able to increase managed populations to approximately 60,000 (Farrar, 1968). These strategies include increasing available brood nest space (e.g. cavity size), reversing the brood nest, stimulative feeding and breeding honeybee stocks for increased brood production.
The basic
design of the Langstroth hive may also contribute to the increased size of
managed populations. For example, the spaces created by the development of
the moveable frame greatly alters air flow patterns within the hive. This
increase in the potential for air movement is further enhanced by beekeeper
efforts to ventilate hives and provide a greatly enlarged entrance relocated
at the bottom of the cavity. Conversely, the natural cavity that the bees
choose has combs that are attached to the ceiling and walls. Air exchange
is restricted between the large, undulating, pendulous combs. Ventilation
is greatly reduced by an upper (usually) entrance, generally a tiny knothole,
crack or crevice (Avitabile et al, 1978).
We know that
colony integrity is maintained, at least in part, by pheromones ‑ those
chemicals produced externally by bees (Gary, 1975). Gaseous products of in‑hive
metabolism, such as water, ethylene and carbon dioxide may also regulate bee
activity and behavior. Therefore, it is reasonable to assume that excessive
air circulation within the box hive and ventilation at the entrance significantly
alter concentrations of these bioregulators.
It is argued
that in cold climates, colonies must be ventilated to prevent the build‑up
of moisture and ice in the colony. But, this excess water is the product of
condensation on the uninsulated walls of box hives (Detroy et. al., 1982).
Thus, both condensation and ventilation draw moisture from the cluster, stressing
the bees by causing them to step up the metabolism of honey to maintain both
temperature and humidity in their "comfort zone"
COMB CELL
SIZE.
Unbeknownst to most beekeepers, the issue of the relative size of the cells of honeycomb (and foundation) has been the subject of controversy since the late 1800s and perhaps earlier (Erickson et al., 1990) when, in Europe, the diameter of the raised imprint of the cell on manufactured foundation was 5.0 mm. However, Baudoux, beginning in the late 1800s, conducted a series of experiments which demonstrated that this smaller than natural size induced developmental abnormalities in bees and reduced colony productivity.
In further
experiments, be demonstrated that larger bees with longer tongues could be
produced in abnormally large (6.0 mm, diameter) cells. Finally, be purported
to show that this increased size would result in greater
colony productivity and that the size of bees in subsequent generations
would be inherited. Baudoux's latter two views have since been debunked. More
recent studies (Grout, 1937) failed to provide scientific evidence for increased
honey production by colonies with bees produced in larger cells.
What has emerged
from all of this is the concept that bigger is better ‑ but is it? The
current industry standard for cell size on manufactured foundation is 5.4
mm or larger. But the diameter of cells instinctively built by honey bees
is slightly less than 5.2 mm (see Erickson et. al., 1990). The difference
in cell size means that more bees can be produced per unit area in a brood
nest of small cells. This translates into more rapid spring buildup an d probably
less metabolic energy expended in the production of each bee. It might also
result in a shortened time for larval/pupal development.
Here, the
issue of stress must again be raised. Do enlarged cells stress bees just as
Baudoux demonstrated for abnormally small cells (see Erickson et al., 1990)?
Could nutritional, wintering, disease and mite problems be reduced by returning
to natural cell size at least in brood nest combs?
COMB AGE.
Beekeepers usually prefer to retain the old combs in their hives for many (20 to 40) years as opposed to replacing them. They believe that the process of comb building, the conversion of honey into wax, significantly reduces net colony honey production. However, I am unaware of any scientific data to support this contention.
The honey produced in old, dark comb
is usually darker in color, and the bees may be smaller due to residue buildup
within the cells. Many organic molecules and most pesticides are lipophillic
(fat and wax ‑loving"). This, of course, includes beeswax which
is one of the most efficient waxes in this regard. Because of their high lipid
affinity, many toxic and potentially hazardous substances from the environment
are bound up in beeswax combs. Thus, it can be argued that the wax produced
by bees serves as the ‑liver" of the colony by providing a natural
cleaning mechanism in the hive. Such a mechanism would ensure a clean environment
for brood rearing and supply of healthy, palatable food, but the ability of
wax combs to absorb toxicants is not unlimited. Hence, the struggle to keep colonies vigorous on old combs seems much like trying to keep a patient alive
with a cirrhotic liver. It is likely that the perceived savings from the retention
of old combs would be more than offset when new combs increase the productivity
of healthy colonies and reduce reliance on medications and supplemental feeding.
This has long been the contention of Mr. Glen Stanley, Des Moines, Iowa (pers.
comm.).
Like all worlds, that of the honey bee is filled with hazards not the least of which are naturally occurring toxins. There are, for example, toxins in the nectar and pollen of some plants as well as toxins produced externally by fungi that may develop on these floral products. Still, bees gather these materials, usually without harmful side effects. Perhaps, if we understand all of the natural mechanisms like beeswax that protect bees from such toxins, we may be able to utilize these to protect our colonies from pesticides and other manmade chemical hazards.
PROPOLIS.
Propolis is an admixture of plant resins, beeswax and hive debris. Worker bees use some kind of solvent, probably glandular in origin, to mix these materials into the familiar brown, sticky substance that many beekeepers find objectionable. Strains of bees that produce very little propolis have been developed.
Propolis
is likely highly beneficial to bees because it contains antimicrobial chemicals
called terpenes. Terpenes such as pinene, limonene and geraniol, just to name
a few, are well known bacteriocides, fungicides and miticides. Such terpenes
have been shown to be of great importance in the biologies of other insects.
Thus, one can readily speculate that the reduction of propolis in domestic
beehives may have rendered our colonies more susceptible to diseases and mite
infestations.
BIGGER BEES.
Space does
not permit discussion of the many potential shortfalls that may emanate from
past and current breeding programs. However, it is timely to address one,
perhaps misguided, selection effort ‑ breeding larger bees. As previously
noted, beekeeper preoccupation with large bees is longstanding; and while
size is, in part, a function of relative comb cell size, there is also a large
heritable component. Thus, today we have larger queens producing larger workers
in larger cells.
The question that must be asked is "do larger bees require a longer
developmental period?" If so, what then is the impact on colony vitality, particularly population
size, worker bee replacement rates, efficiency of brood food utilization,
susceptibility to diseases and mites, as well as efficient heating of the
brood nest? The issue of bee size deserves extensive study, particularly in
regard to alteration of the impact of stress‑inducing environmental
hazards on populations of honey bees! For example, are larger bees more or
less susceptible to temperature or humidity extremes, and pesticides?
FORAGE RESOURCES.
The single most important factor limiting the growth, development and productivity of an otherwise normal honey bee colony is the availability of pollen and nectar. The plant ecosystem (not the beekeeper) drives all aspects of colony development and performance. Some plant species or cultivated varieties naturally produce greater quantities of nectar and pollen. Even so, plants stressed by water, light or nutritional deficiencies may limit or cease production of nectar and pollen, thus stressing nearby colonies.
Honey bees require a balanced diet.
Since few, if any, single species of pollen are nutritionally complete bee
diets, plant species diversity is essential
for development of healthy, vigorous
colonies. Frequently, this diversity is lost in areas suffering from drought
and where monoculture is practiced on weed‑free farms. Plant stress
may also lower the nutritional value of the floral reward, either nectar of
pollen. A.,; a result of any of these conditions, colonies may dwindle. The
best adapted and otherwise unstressed
color‑Lies will survive longest on the resources of environmentally‑stressed
plants.
Finally,
the ease with which bees can forage successfully within a patch of flowers
is well recognized. Flower accessibility is important, but all too often,
beekeepers fail to recognize that the nearer their colonies are to floral
resources, the more efficiently those resources will be harvested. The issue
is simple ‑ the farther a colony must fly to gather nectar, the more
honey it will use as fuel. Equivalent flight miles per gallon of honey (km
per liter) can be easily calculated for a bee or a colony (ca. seven million
MPG). Wear and tear on the bee is also important. Bees only fly an average of about 140 miles (ca. 240 km) up to a maximum
of 500 miles (800 km) in a lifetime before wearing out (Neukirch, 1982). Thus,
while bees may forage at distances of up to five miles from the hive, such
distances reduce foraging efficiency and the working life of the bee.
THE ENEMY.
The
honey bee colony has a number of natural stress inducers and enemies including weather, natural disasters, predators, parasites
and disease. The latter are well described in the book edited by Morse (1990).
However, none of these inflict as much stress on the domestic colony as the
beekeeper.
My
purpose in writing this article is simply to emphasize the fact, as stated
in the opening paragraph, that beekeepers all too often unnecessarily stress
their bees. Hopefully, by drawing attention to some of the little recognized but
significant sources of honey bee stress, beekeepers around the world will
be able to improve their colony management strategies and hence their profits.
References:
Avitabile,
A., D. P. Strafstrom and K. J. Donovan. 1978. Natural nest sites of hone) bee colonies in trees in Connecticut,
USA. J . Apic. Rea. 17:222‑226.
Detroy,
B. F., E. H. Erickson and K. Diehnelt. 1982. Plastic hive covers for outdoor
wintering of honey bees. Am. Bee J.
122:583‑587.
Erickson,
E. H., D. A. Lusby, G. D. Hoffman and E. W. Lusby. 1990. On the Size of Cells: Speculations on foundation as a colony management tool. Glean. Bee Cult. 118:89‑101.
Farrar,
C. L. 1968. Productive
management of honey bee
colonies. Amer.
Bee J. 108:1 ‑ 20.
Gary,
N. 1975. Activities and behavior of
honey bees.
IN: The Hive and the Honey Bee, Dadant & Sons (eds.), Hamilton, IL, p.
185‑264.
Grout,
R. A. 1937. The influence of size of brood cell
upon the size
and variability of the honey bee
(Apis mellifera L.). Iowa Agr. Exp. Stn. Res. Bull. No. 218, p. 260‑279.
Morse,
R. A., ed. 1990. Honey Bee Pests, Predators, and
Diseases, (second edition). Ithaca, Cornell Univ. Press.
Neukirch,
A. 1982. Dependence of the life span of
the honey bee (Apis mellifica) upon flight
performance and energy consumption. J. Comp. Physiol. 146B:35‑40.
Owens,
C. D. 1971. The thermology of wintering honey
bee colonies. USDA Tech. Bull. No. 1429, 32 p.
Seeley,
T. D. and R. A. Morse. 1976. The nest of the honey bee
(Apis mellifera L.). Insectes Sociaux 23:495‑512.
Severson,
D. W. and E. H. Erickson. 1985. Honey consumption by honey bee colonies in
relation to winter degree‑day aocumulation. Amer. Bee J. 125:643‑644.
Weast,
R. C. (Ed.). 1980. CRC Handbook of Chemistry
and Physics, 60th Edition, CRC Press, Boca Raton, Florida, 370p.