The Nosema Problem: Part 7b – The Causes of Dysentery in Honey Bees: Part 2
Contents
Water and the winter cluster. 1
Water homeostasis and buffering in the winter cluster. 3
Atmosphere and Humidity within the winter cluster. 3
Evaporation via respiration. 5
The Nosema Problem: Part 7b
The Causes of Dysentery in Honey Bees: Part 2
Randy Oliver
ScientificBeekeeping.com
First Published in ABJ in January 2020
Finally ― it’s time to get to what actually does cause dysentery in a hive, and (in my next article) what the colony (or the beekeeper) can do to minimize its occurrence. In investigating this subject, I was surprised by how much hard-earned knowledge, published years ago by astute researchers, seems to have been forgotten.
The bees’ need for water
During a strong nectar flow, the colony is awash in the excess moisture that needs to be evaporated to ripen that nectar into honey for storage. But for a bee to later use the honey as an energy source, it needs to add back water to dilute it for uptake through its proboscis [[1]], as well as for digestion. The preferred sugar concentration for consumption appears to be in the 40-60% range [[2]]. For much of the year, workers forage for the water necessary to dilute the honey ― if necessary, even flying at temperatures as low as 40°F (4°C) [[3]]. But the focus of this article is about what happens when it gets too cold to fly, and the bees are stuck in a winter cluster with no outside source of water.
Water and the winter cluster
In the winter cluster, bees are in a tricky situation ― if there’s not enough liquid water available, they will desiccate and die, due to the unavoidable water loss from breathing. But on the flip side, too much moisture in the cluster can lead to the growth of mold, fermentation of honey, and dysentery (Fig. 1).
Figure 1. This hive was inadvertently tipped slightly backward, preventing rainwater from draining out. In my experience, water pooled on the bottom board is extremely stressful to a colony during our California winter ― often leading to dwindling or death.
Water Balance
The “winter bees” in the cluster have little need for protein, due to their well-developed fat bodies, and can survive for a long time on a diet of honey (sugar) alone. The weight loss of a hive in winter cluster (not rearing brood) is in the ballpark of around a pound a week ― presumably mainly due to the consumption of its honey. That honey consists of roughly 83% sugars (mostly glucose and fructose) and 17% water. The bees metabolize only the sugars, according to the following equation:
C6H12O6 + 6 O2 → 6 CO2 + 6 H2O
When you do the math, the metabolism of the sugar in that pound of honey produces 6/10 of a pound of water. Add to that the 17% of liquid water already present in the honey, and you wind up with that pound of honey turning into 2/3 of a pound (1¼ cups) of water (initially held within the bees’ bodies). The bees in the cluster cannot allow that 1¼ cup of water to accumulate, and must deal with it in some manner.
Practical application: The bees recycle roughly ¾ cup of that excess water into saliva to dilute the next pound of honey for consumption, but that doesn’t affect the net 1¼ cups of water gained each week that still needs to be dealt with in some way.
Water homeostasis and buffering in the winter cluster
Bees in the winter cluster have only a few options for what to do with the water in their bodies resulting from the consumption of honey; they can either:
- Hold it in their hind gut (up to a point), or
- Defecate it (not a desirable option in the cluster), or
- Exhale it through their spiracles via “respiratory transpiration,” or
- Feed it out through their proboscis (generally to another bee).
In this article, we’ll look at Options 1 and 2 (The Problem). I’ll cover Options 3 and 4 (The Solutions) in the final installment of this series.
Water in the gut
As explained in an excellent paper on the winter cluster by Johansson [[4]]: Water not absorbed in the midgut is retained in the hindgut, where it forms a reserve that is utilized by osmotic diffusion when the water content of the haemolymph becomes too low.
The bees need a reservoir of water, since with every breath they exhale, they lose some water from their body. This is a serious issue for insects that can’t access liquid water, so they control how often they open their spiracles to breathe. Thus, it’s advantageous for a wintering bee to hold some water in reserve in its hindgut to replace that lost by evaporation during respiration. But that evaporation due to breathing is entirely dependent upon the relative humidity of the intake air, and if the humidity around the bee is too high, it won’t be able to evaporate the water that it gains from consumption of honey. So we need to understand the humidity within the winter cluster.
Atmosphere and Humidity within the winter cluster
The bees in a winter cluster can be quite tightly packed, taking up only a fraction of the volume that they occupy when it’s warm [[5]]. The cluster is surrounded by a “mantle” of bees facing inward, with their bodies squeezed so closely together that they can control any air movement in or out of the cluster.
A fascinating study by van Nerum and Buelens [[6]] found that unlike humans, whose urge to breathe is determined by the CO2 content of their blood, it appears that bees will tolerate a very high CO2 concentration before initiating ventilation, and instead control their respiration and metabolism in response to the oxygen level. The authors found that in the center of the cluster, the oxygen content is allowed to drop to as little as 15% (down from 21% in the air that we breathe), and carbon dioxide is allowed to rise to up to 5-6% (up from 0.04%) (Fig. 2). The researchers found that the lowered oxygen content (hypoxia) in the winter cluster results in the bees in the core decreasing their resting metabolic rate, thus reducing food consumption and minimizing heat loss from needed ventilation. It also conserves water within the cluster.
Figure 2. Relative humidity (blue line) is high in the mantle of the cluster, but low in the core. Carbon dioxide (red line) inversely increases greatly in the core. These means of 10 measurements also indicate that there is nearly no air circulation in the cluster when the bees go into this “hypoxic” (low oxygen, low metabolic) state. Chart after van Nerum & Buelens [[7]]
The winter cluster of bees is akin to a warm blooded mammal with a cool skin temperature. But unlike a mammal, the cluster lacks a common bloodstream to transport O2, CO2, water, or heat ― so the bees instead depend upon air flow to do so. A number of researchers have measured humidity in the cluster, finding that it oscillates up and down in the range of 45-70%, but without a concurrent change in temperature, suggesting that the bees are able to ventilate out moisture while at the same time conserving heat. Ellis [], noting that much of the cluster is typically on drawn comb, points out that dark comb full of silk cocoons can absorb up to 11% of its own mass in water when exposed at high humidity. Thus, such combs can exert a substantial buffering effect on humidity within the hive.
Practical application: Dark drawn combs may help to buffer humidity in the winter cluster. Some experienced beekeepers suggest that colonies winter better on old comb containing cocoons, but there is a paucity of research on the subject ― another question calling for investigation.
The control of humidity within the cluster appears to be of utmost importance in the winter cluster, since the relative humidity determines the rate at which bees lose water during respiration.
Evaporation via respiration
As explained in the Johanssons’ “The Honeybee Colony in Winter” [[9]]:
… Since honeybee faeces are liquid, the diffusion of water through the wall of the hindgut into the haemolymph, and subsequently through the tracheal wall where it evaporates into the atmospheric air within the cluster, prevents an excessive accumulation of water in the hindgut.
But as I mentioned previously, that evaporation of water from the tracheal wall is dependent upon the relative humidity of the air that the bee breathes in. A wonderful “old-school” study by Woodrow in 1935 [[10]] demonstrated how humidity affects a bee’s ability to purge itself of excess water via respiratory transpiration. Woodrow placed bees in small cages, all at around 71°F, but held at different humidities, feeding them 50:50 sugar syrup, and carefully observed their bodies and behaviors over time, as well as their longevity (Fig. 3).
Figure 3. Caged worker bee consumption of 1:1 syrup, and maximum and average longevity at various humidities. Woodrow’s data suggests that at above around 70% RH, a bee is unable to maintain water balance via respiration (compare that to the humidity figures of the mantle in the previous figure). Chart after data from [[11]]
Woodrow found that bees would apparently sometimes die rather than to defecate excess water in their hindgut:
The first evidence of feces accumulation at the lowest relative humidity was noticed about the twentieth day and this condition slowly became more pronounced as the experiment progressed. As the bees in the different cages became more sluggish due to fecal accumulations, it is probable that some of them actually starved to death. The progressively smaller food consumption per bee per day no doubt is due partly to the fact that some of the bees were unable to feed. It appears that accumulation of feces is one cause of the death of the bees exposed to the higher ranges of relative humidity
Practical application: In the high relative humidity present in the cluster mantle, bees there will tend to accumulate moisture in their hindguts, to the point that they can no longer feed.
Free & Spencer-Booth [[12]] on the other hand varied the temperature, but did not measure humidity, offering caged bees both 67% sugar syrup and pure water separately ― measuring how much they consumed of each. They found that:
Very little water was drunk at environmental temperatures of 25°C. or lower but, at 35°C. and above, relatively enormous quantities were taken.
Note that they weren’t sure whether the increased water consumption was solely to replace moisture lost through respiration, or whether at the higher temperatures some transpiration occurred through the cuticle covering their bodies [[13]].
Practical application: At broodnest temperature, bees need additional water to prevent dessication.
Möbus [[14]] put this all together to explain that the bees in the center of the winter cluster will experience desiccation, whereas the bees in the outer shell will accumulate more metabolic water than they can evaporate.
Practical application: The bees in the core of the cluster suffer from thirst, whereas the bees in the mantle are experiencing moisture abundance.
To rectify this situation, it appears that the bees in a winter cluster from time to time exchange positions — those on the inside move outward to consume honey diluted by its absorption of condensing water, while those bees in the outer mantle move to the center to “dry out” (perhaps by sharing dilute saliva, as well as to allow their enzymes to warm up for optimal functionality).
Omholt [[15]] also points out that the bees from the periphery (that were clustered over cool, moisture-rich honey) would likely bring a crop full of that diluted honey to the bees within the cluster. This mechanism would answer a question that I’ve long had about how the bees on empty drawn comb in the core of the cluster get fed).
Practical application: I’ve yet to find any research on the actual movement of honey or water from the bees in contact with honey at the periphery of the winter cluster, to workers elsewhere in the cluster (that are not in contact with honey). Another avenue of research calling for investigation!
Defecation/Dysentery
The easiest way for a bee to get rid of excess moisture in its gut is to simply defecate, and during warm weather, they readily fly out to do so. But that option is off the table when it’s cold, although they will take advantage of any warm days to take cleansing flights. The last thing that a bee “wants” to do is to defecate within the hive. So the key question is, “Then what causes them to do so?”
Over the course of two winters, Erwin Alfonsus at the University of Wisconsin addressed “The Cause of Dysentery in Honeybees,” published in 1935 [[16]]. Alfonsus was a keen observer of bees, and performed the sort of “old-school” detailed and meticulous experiment that I so enjoy reading. His introduction read:
Dysentery, a winter disease of honeybees, has been known since Aristotle’s time. Normal defecation in the honeybee takes place on the wing during the flight season. The wintering bee, confined to the hive, is deprived of this opportunity, and the fecal material accumulates in the rectum. An over-accumulation of feces may lead to a forcible discharge in the hive or on the alighting board; this occurrence is called dysentery.
Over two winters, Alfonsus tested various feeds in order to see whether they would induce dysentery. He fed shed-wintered colonies either natural stores of honey (including honeydew), 1:1 sugar syrup, dilute sugar syrup, fall honey, syrup inoculated and fermenting with yeast, syrup boiled until brown, autoclaved syrup, syrup with dextrin (hydrolyzed starch), crystallized honey, aged honey, crystallized sugar syrup, sugar candy, or candy with honey and pollen.
His conclusions:
The only factor showing any relation to the amount of accumulated feces was the moisture content. The dry matter in feces did not increase fast enough to be considered a causative factor of dysentery. The dilution of feces, however, is very conspicuous and increased as the season advanced and as dysentery became more apparent. The first bees left the cluster to discharge their feces around the entrance when the mean fecal accumulations amounted to 33% of the total body weight of the bees and when the feces contained approximately 80% moisture…The occurrence of dysentery is similar in protected colonies interrupted during the honey flow by rain. The bees are shut in and forced to eat unripe honey. Within a short time, the rectum expands and defecation may take place.
SUMMARY AND CONCLUSIONS
- Dysentery of honeybees is caused by excess moisture in the feces.
- This excess moisture is due to the consumption of dilute food or water. It is generally produced by crystallization of the stores; this divides the honey or syrup into a solid crystalline portion and a liquid portion. The liquid portion contains an excess quantity of moisture.
- Pollen, dextrin, minerals, burned sugar, and fermenting syrup do not produce dysentery.
- Chilling and disturbing honeybees may cause defecation, but do not produce dysentery in a healthy colony.
- Long confinement of bees during the winter, as well as a short confinement on unripe honey, produce dysentery.
- Water alone or dilute syrups produce dysentery in bees if absorbed during confinement.
- Dysentery appears when the fecal accumulations reach 33% of the total body weight of the bees. General defecation does not take place until the accumulation reaches about 45%.
Practical application: Alfonsus’s paper did not mention nosema. Dysentery, rather than being a symptom of nosema infection, appears to be due to unmanageable moisture accumulation in the guts of wintering bees. I’ll wrap this series up in the next installment, in which I’ll cover the things that bees, and their keepers, can do to solve this problem
Literature cited
[1] Simpson, J (1964) Dilution by honeybees of solid and liquid food containing sugar. Journal of Apicultural Research 3(1): 37-40.
[2] Eyer, M, et al (2015) No spatial patterns for early nectar storage in honey bee colonies. Insect. Soc. DOI 10.1007/s00040-015-0432-4.
Kim, W, et al (2011) Optimal concentrations in nectar feeding. PNAS 108(40):16618-16621.
[3] Chilcott, A & T Seeley (2017) Cold flying foragers: Honey bees in Scotland seek water in winter. American Bee Journal 158(1):75-77.
[4] Johansson, T & M Johansson (1979) The honeybee colony in winter. Bee World (60:4): 155-170.
[5] Severson, DW & EH Erickson (1990) Quantification of cluster size and low ambient temperature relationships in the honey bee. Apidologie 21: 135-142.
[6] van Nerum, K, and H Buelens (1997) Hypoxia-controlled winter metabolism in honeybees (Apis mellifera). Comparative Biochemistry and Physiology 117(4):445-455
[7] Ibid.
[8] Ellis, M, et al (2010) Brood comb as a humidity buffer in honeybee nests. Naturwissenschaften 97:429–433. Also see:
Ellis, MB (2008) Homeostasis: Humidity and water relations in honeybee colonies (Apis mellifera). MS Thesis, University of Pretoria. Ellis has written a number of more recent papers, detailing water dynamics in various hive configurations.
[9] Johansson, T & M Johansson (1979), op cit.
[10] Woodrow, AW (1935). Some effects of relative humidity on the length of life and food consumption of honeybees. J. Econ. Ent. 38: 565-568.
[11] Ibid.
[12] Free, J. B. and Spencer-Booth, Y. (1958) Observations on the temperature regulation and food consumption of honeybees (Apis mellifera). J. Exp. Biol. 35: 930 -937.
[13] Wigglesworth, V (1945) Transpiration through the cuticle of insects. Journal of Experimental Biology 21: 97-114.
[14] Möbus, B (1998) Rethinking our ideas about the winter cluster; Part II. ABJ August 1998: 587-591. Eugene, can you fill in the volume number? It would be 138.
[15] Omholt, SW (1987) Why honeybees rear brood in winter. A theoretical study of the water conditions in the winter cluster of the honeybee, Apis mellifera J. Theor. Biol. 128: 329-337
[16] Alfonsus, E. C. (1935). The cause of dysentery in honeybees. Journal of Economic Entomology, 28(3): 568-576.