| STRESS AND HONEY BEES |
| 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 environment 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 survival are quickly eliminated, often before they can produce drones; and 3) wax moths are beneficial insects that eliminate old and perhaps contaminated food stores, toxins and disease organisms. This cavity restoration assures that new swarms construct new, uncontaminated combs for brood rearing and food storage. 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 space around removeable 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 lower nutritional value). Sometimes, these are replaced with artificial substitutes. Both natural food quantity as well as quality are thus altered. 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 heat 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. Honey bees have been selectively bred for centuries. Among the many traits that have been altered or enhanced are productivity, gentleness, color and size. Unfortunately, every artificial selection and breeding program has its inherent risks because enhancement of one trait can, and often does, lead to the loss of others -such as reproductive advantage and natural immunity. Frequently, such a loss goes unnoted for many generations, and usually it is not until calamity befalls the population that the loss is recognized. 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. As a result of any of these conditions, colonies may dwindle. The best adapted and otherwise unstressed colonies 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. |