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The digestive index as a benchmark to quantify the digestive capabilities of honey bees (Apis Mellifera)

  • Gruenwald Laboratories GmbH

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An easy method to obtain the digestive index as a benchmark number for measuring the digestion capabilities of Apis mellifera is developed and presented in this paper. The aim of this method is to provide an entity that allows to quantifying digestion efficiency of the western honey bee. This will enable a fundamental understanding of digestive illnesses in bee hives as well as a general insight into the biological mechanisms into the digestive apparatus. Although the methodology presented in this work is based on experimental data of honey bees it can, in principle, be applied to other insects as well, provided that the necessary experimental data can be made available. A small number of seven bees is used to provide an example of how to calculate the digestive index. The bowels were extracted from these seven dead honey bees and their mass in fresh and dried condition was used for the determination of the digestive index. The mathematical model for the calculation of the digestive index is described. Thus, it is demonstrated that the digestion capability of honey bees can be quantified by the simple method outlined in this paper.
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Indian Journal of Entomology, 80(2): 149-153 (2018) DOI No. : 10.5958/0974-8172.2018.00101.3
Karl-Franzens-University, Department of Zoology,
Universitaetsplatz 2, 8010 Graz, Austria
*Gruenwald Laboratories, Taxberg 50, 5660 Taxenbach, Austria
An easy method to obtain the digestive index as a benchmark number for measuring the digestion
capabilities of Apis mellifera is developed and presented in this paper. The aim of this method is to
provide an entity that allows to quantifying digestion efficiency of the western honey bee. This will
enable a fundamental understanding of digestive illnesses in bee hives as well as a general insight
into the biological mechanisms into the digestive apparatus. Although the methodology presented in
this work is based on experimental data of honey bees it can, in principle, be applied to other insects
as well, provided that the necessary experimental data can be made available. A small number of seven
bees is used to provide an example of how to calculate the digestive index. The bowels were extracted
from these seven dead honey bees and their mass in fresh and dried condition was used for the
determination of the digestive index. The mathematical model for the calculation of the digestive
index is described. Thus, it is demonstrated that the digestion capability of honey bees can be quantified
by the simple method outlined in this paper.
Key words: European honey bee, digestive index, characteristic number, quantification, digestive capability,
bowel extraction, mathematical model
The western honey bee (A. mellifera) is of
immense importance for ecologic and economic
systems due to its pollination activities. Human food
security and biodiversity are heavily dependent on these
insects (Brotschneider and Crailsheim, 2011; Tautz and
Heilmann, 2007).The health of a bee hive along with
its capability to survive is a priority for bee keepers. A
key factor concerning the health of bee hives is an
adequate nutrition of the bees, which has a strong
connection to the bees’ capability of digesting their
fodder (Brotschneider and Crailsheim, 2010).
Additionally, irregularities in the bees’ digestive patterns
can be an indicator of diseases of the digestive system.
Such illnesses can be caused by insufficient or
otherwise flawed nutrition of the honey bees
(Brotschneider and Crailsheim, 2013).
The experimental investigations in this manuscript
are carried out with conventional bee fodder (i. e. a
saccharose solution in water). This kind of feedstock
is rich in carbohydrates that are essential for the well
being of the bees (Brotschneider and Crailsheim., 2010;
Haydak, 1970). Another widely used food source, viz.
honey, is not included in this study as it has numerous
disadvantages over saccharose solution (e.g. higher
financial costs, a stronger tendency to crystallise and
possibly pathogens that can be contained in the honey)
(Papachristoforou et al., 2013).
To aim upon a reliable indicator of the functionality
of the digestive system of A. mellifera, a novel
characteristic number, the digestion index, is developed
along with a simple method of how to obtain it. The
computation of this index is based on statistically
evaluated experimental data on the mortality of caged
honey bees. Thus, the main focus is on the
mathematical method rather than getting the exact value
of the digestive index. Such are refinement would
require a more extensive data pool, which is beyond
the scope of this work. Nevertheless, it will be shown
that the idea of the digestive index is based on
experimental techniques that allow to obtain a
reproducible, comparable number determining the
functionality of the digestive system in a quantitative
manner. This quantification of the digestive capability
could lead to a better understanding and an early
recognition of illnesses of the digestive tract of A.
mellifera, such as the Nosema disease (Li et al., 2016;
Holt and Grozinger, 2016; Maes et al., 2016; Snow,
2016) and others.
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150 Indian Journal of Entomology, 80(2), 2018
All experiments were conducted between early July
and the end of August with worker bees (A. mellifera),
<24 hr of age, obtained from the hives of the biological
institute of the University of Graz. All bees were kept
in single use cages and fed ad libitum with the
saccharose solution. The saccharose solution consisted
of 50% by weight saccharose, 50% by weight water
and a pH value of 6.84. Generally, a saccharose based
feed is usually prepared either as a 1:1 or as a 3:2
solution in water (Barker and Lehner., 1973; Herbert,
1992; Hüsing and Nitschmann, 1987). The bees were
continuously fed in 24 hr intervals and the cages
checked on a daily basis for dead specimen. Each cage
was initially populated by 100 bees and the experiment
continued until only 20 bees were still alive.
By weighing the fodder containers with a high
precision scale before feeding and after 24 hr, the
amount of fodder, incorporated by the bees, was
determined (the partially empty fodder containers were
replaced with fresh ones, containing 2 ml of saccharose
solution daily). At no instance a completely emptied
food container was to be found during any of the
experiments (i.e. the bees did not use all the feed during
the course of a day).Taking into account the fact that
caged honey bees do not empty their bowels, it was
concluded that the entire content of the end bowel
represents indeed all the undigested part of the fodder.
Following the procedures given by Carreck et al.
(2013), the head of the dead bee was removed with
dissecting scissors. After that the abdomen and the
intestine channel was entirely pulled out with suitable
tweezers (Fig. 1).
After removing the ventriculus, proventriculus and
the honey bladder, the mass of the fresh rectum was
weighed with a high precision scale (Mettler Toledo®;
electronic balance - accuracy 1%). Afterwards, the
end bowels were incubated for five days at 70°C in a
hot cabinet (Heraeus Instruments®) and weighed again
to obtain the dry mass of the end bowel. The results
of the average fodder intake /bee /day and the weight
intestines are depicted in Figs. 2 and 3, respectively.
It seems to be surprising that the maximum value
of the wet bowel mass at the end of day 10 (~ 34 mg)
seems to be too low for a considered average fodder
intake of 20 mg saccharose solution/bee /day. However,
it has to be taken into account that 50% of the solution
is water, which is generally not digested and ends up
only partially in the end bowel. Hence, roughly half of
the mass of the fodder (~ 100 mg in ten days) will not
Fig. 1. Extracted end bowel (rectum), middle bowel (ventriculus), proventriculus with valvula cardiaca and honey bladder.
The mass of the end bowel was used in this work
Fig. 2. Number of living bees (black) and average daily fodder intake (50% saccharose solution) / bee (red) with 1%
error bars (defined by the accuracy of the high precision scale) as a function of time. The number of bees decreased
from 40 to 33 over a course of 15 days. The arithmetic mean value of fodder consumption was 20.05mg
Number of bees
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The digestive index as a benchmark to quantify the digestive capabilities of Apis mellifera 151
K. Gruenwald and J. Gruenwald
be stored in the bowels. Furthermore, a considerable
quantity of saccharose will either be metabolised and
incorporated in the bees’ body mass (and, thus, not
reaching the bowel), used for building honey combs
or burnt in the cells and exhaled as CO2. With the data
gained so far, it is possible to quantify the digestion
capability of A. mellifera with the simple model that is
outlined in the results and discussion below.
The mathematical model to determine the digestive
index proposed herein is based the weight of the rectum
of caged A. mellifera specimen after their natural
death. The rectum was chosen because of following:
1. The caged honey bees did not empty their intestines
while being held in the cages, and thus, total amount
of fodder administered ends up in the rectum and
remains there as the digestion activity stops after the
death of the bee. 2. The content of the rectum has the
highest mass compared to the mass of the rectum itself
and compared to some minor undigested fodder
remaining in the other parts of the intestines (Fig. 1).
This ensures that the measurement error is minimised
(determined by the accuracy of the electronic balance-
For the calculation of the digestive index, several
parameters are required: the amount of fodder on the
i-th day (fi), and the number of days from the start of
the experiment until the natural death of the honey bee
(t). From these first two entities the total amount of
ingested fodder (F) can be calculated as:
Equation (1) indicates that the fodder consumption
of each day has to be summed up properly after the
bee’s death for each of the i days of the bee’s life in
the experiment. If the fodder intake has only marginal
variations from day to day, it can be considered as
constant over time and (1) reduces to the product of
average fodder intake/day, multiplied with the number
of days the specimen was alive.
After the death the rectum has to be extracted as
soon as possible and its mass has to be measured in
the fresh condition (Mf) as well as after drying the
rectum and its content (Md). With these values, two
varieties of the digestive index (DIf and DId) can be
Definitions (2) and (3) guarantee a normalisation
of the digestive index by mapping DIf,d onto an open
interval (0,1), so that 0 <DIf,d< 1 always holds. This is
necessary to specifically exclude the limiting cases (i.e.
DIf,d 0 and DIf,d 1), which are not likely to be observed
in nature. The former case indicates that Md = 0, which
would mean that either the total amount of fodder was
metabolized or it didn’t enter the rectum in the first
place (this could be theoretically possible if the insect
dies immediately after the forage but in such a case
the specimen should not be included into the analysis
at all). The latter case would hold if the metabolic
activity of the bee was completely disabled, which, in
turn, would not allow the evaluation of the digestive
ability. It shall also be emphasised here that the
determination of two variations of DI is not futile, as it
allows drawing some conclusions about the H2O
household of the bees.
In order to do that, the following ratio (W) is
Fig. 3. Fresh (black) and dried (red) bowel mass as determined from the seven dead specimens
obtained in the experiment. Arithmetic mean values: Fresh: 26.7 mg; Dry: 17.16 mg.- Error
bars 1%. The numbers in brackets denote the life time of each specimen in days. After 10 days
no more bees died and the experiment was terminated after 15 days
Bowel number
Bowel mass, fresh (mg)
ܦܫ݂=: ܯ݂
σ݂݅݅ ܯ݂
=: ܯ
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152 Indian Journal of Entomology, 80(2), 2018
It describes how much water is still present in the
content of the rectum. By comparison with the water
content of the fodder (i.e. mass- % H2O in the sugar
solution) the water intake of A. mellifera can also be
studied in detail. The results of these formulae are
depicted in Figs. 4 and 5 (the numbers in brackets,
again, denote the lifetime of each specimen in days).
These observations reveal that maximum
discrepancy between the exact calculation and the
calculation with average fodder intake is only 6%,
which is considered to be already quite good, taking
into the small amount of samples. It shall be pointed
out that this paper presents a new method, which is
made more clear with an exemplary calculation based
on the aforementioned seven end bowel samples,
rather than aiming at minimising the statistical error of
the procedure. Thus, the small sample number is not
regarded as an inaccuracy.
This simple concept allows to accurately quantify
the digestion capabilities and the water intake of
European honey bees for the first time, provided a
sufficiently large number of samples is investigated,
so enough that the standard statistical methods could
A simple method to quantify the digestion capabilities
of A. mellifera, in the form of the digestive index, has
been presented herein. This is an important step forward
in the diagnosis and understanding of diseases of the
digestive system of European honey bees. This method
might in principle be applied similarly on other insects,
and the statistical inaccuracy of this technique is only
about 6% even for a sample number as low as 7.
However, more data is needed to gain insight into which
interval of the digestive index can be considered be a
benchmark for a healthy digestive system of A.
mellifera and which values of the DI are a signal for
diseases of the intestinal tract. In order to get a
comprehensive database for such diagnosis, it is
necessary to gain regional and seasonal measurements
Fig. 4. Digestive Index (DI) computed for each of the seven end bowels, according
to equations (2) and (3). The indices “exact” and “average” indicate that the
calculation was performed with the summation over the fodder intake for each
day and the average (20.05 mg) taken from Fig. 2.
Digestive Index
Bowel number
Fig. 5. Water content of the extracted end bowels, calculated with equation (4). It can be
seen that the water content reaches nearly 50% in the samples.
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The digestive index as a benchmark to quantify the digestive capabilities of Apis mellifera 153
K. Gruenwald and J. Gruenwald
from different varieties of honey bees, which is far
beyond the scope of this paper. Nevertheless, as the
presented method is a very robust and easy one, the
collection of relevant data should be feasible in the
near future. However, the exemplary calculations reveal
that the digestive index of a healthy honey bee in Austria
might be around 0.23 with a water content of 37.7%
on an average. Values >0.3 or < 0.15 could in this case
be considered to be a sign of a dysfunctional digestive
capability of A. mellifera.
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(Manuscript Received: October, 2017; Revised: January, 2018;
Accepted: January, 2018; Online Published: January, 2018)
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