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Small-cell comb does not control Varroa mites in
colonies of honeybees of European origin
Thomas Seeley, Sean Griffin
To cite this version:
Thomas Seeley, Sean Griffin. Small-cell comb does not control Varroa mites in colonies of
honeybees of European origin. Apidologie, Springer Verlag (Germany), 2011, 42 (4), pp.526-
532. <10.1007/s13592-011-0054-4>. <hal-01003589>
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Small-cell comb does not control Varroa mites in colonies
of honeybees of European origin
Thomas D. SEELEY, Sean R. GRIFFIN
Department of Neurobiology and Behavior, Cornell University, Ithaca, NY 14853, USA
Received 9 July 2010 – Revised 26 September 2010 – Accepted 4 October 2010
Abstract – We tested the idea that Varroa destructor can be controlled in colonies of the European subspecies
of Apis mellifera by providing them with combs built of small cells, in which immature mites might have
difficulty developing for lack of space. We established seven pairs of equal-size colonies that started out equally
infested with mites. In each pair, one hive contained only standard-cell (5.4 mm) comb, and the other contained
only small-cell (4.8 mm) comb. We measured the colonies' mite loads at monthly intervals across a summer. No
differences arose between the two treatment groups in their mean mite loads (mites per 100 worker bees or mite
drop per 48 h). We suggest that providing small-cell combs did not inhibit mite reproduction because the fill
factor (thorax width/cell width) was only slightly higher in the small cells than in the standard cells (79% and
73%, respectively).
Apis mellifera / Varroa destructor / small cell / mite control / cell size
1. INTRODUCTION
The mite Varroa destructor is a new parasite of
European honeybees living in North America,
having been introduced to these bees only in the
mid-1980s (Wenner and Bushing 1996;Sanford
2001). This mite and its associated viruses are a
major cause of colony mortality worldwide. As a
rule, if a colony of European honeybees does not
receive mite control treatments, the mite popula-
tion will grow from just a few mites to several
thousand mites in 3 to 4 years, ultimately killing
the colony (Ritter 1988; Korpela et al. 1992;
Wenner and Thorp 2002). One possible non-
chemical method of mite control that has been
much discussed and debated (e.g., Erickson et al.
1990;Johnsen2005) is to reduce worker cell
width from the current size of 5.2–5.5 to 4.9 mm.
It has been suggested (e.g., Medina and Martin
1999) that smaller cells may cause higher
mortality of immature mites, because the mites
develop directly beside immature bees in cells
(Donzé and Guerin 1997) so a smaller space
between the developing bee and the cell wall
might hamper the movements of the immature
mites, reducing their ability to feed and thereby
raising their mortality.
One study that has provided support for the
concept of small-cell combs for Varroa mite
control was conducted in South Africa with
African honeybees (Martin and Kryger 2002).
Colonies were established in which larger bees
(Apis mellifera capensis, similar in size to
European bees) and smaller bees (A. m. scutel-
lata, smaller than European bees) were reared
together in small-cell (4.6 mm) combs, and it
was found that both mother mite mortality and
male offspring mite mortality were higher in the
cells with larger bees. The authors report that
mother mites and male protonymphs appeared
to get trapped in the upper part of the cells that
contained the larger bees, thereby preventing the
Corresponding author: T.D. Seeley,
tds5@cornell.edu
Manuscript editor: Marla Spivak
Apidologie (2011) 42:526–532
Original article
* INRA, DIB-AGIB and Springer Science+Business Media B.V., 2011
DOI: 10.1007/s13592-011-0054-4
mites from reaching the feeding site on the
abdomen of the developing bee pupa. This is,
however, a special case in which some of the
largest worker bees found in A. mellifera were
reared in some of the smallest cells produced by
A. mellifera, and it may be that the elevated
mite mortality that was observed requires such
extreme conditions.
A second relevant study is that of McMullan
and Brown (2006). Working with A. m. melli-
fera in Ireland, they gave colonies small-cell
(5.04 mm) combs and standard-cell (5.48 mm)
combs and found that the worker bees reared in
the small-cell combs were less than 1% smaller
(in head width and thorax width) than those
reared in the standard-cell combs, even though
the small cells were 8% less wide than the
standard cells. Evidently, when bees are reared
in small cells, the reduction in bee size is not
proportional to the reduction in cell size, so the
cell “fill factor” (thorax width/cell width,
expressed as a percentage) is higher and there
is less room for the mites. McMullan and
Brown (2006), for example, found that the fill
factor was 73% in colonies with standard-cell
combs but 79% in ones with small-cell combs.
Three recent studies conducted either in the
southeastern USA (Ellis et al. 2009; Berry et al.
2010) or in Ireland (Coffey et al. 2010) have
directly tested the idea that giving combs of
small cells to colonies of European honeybees
will reduce their susceptibility to Varroa. All
three studies found no evidence that providing
colonies with small-cell combs rather than
standard-cell combs impedes reproduction by
Varroa. A fourth recent study on this subject,
conducted in New Zealand (Taylor et al. 2008)
also found no effect of cell size on mite
infestation, but this fourth study was confound-
ed by difficulty in getting the bees to build
combs consisting entirely of small cells, result-
ing in small differences in cell size between the
two treatment groups. In the present study, we
extended this work by conducting a test in the
northeastern USA in which we established pairs
of colonies, with each pair consisting of one
colony living in a hive containing standard-cell
combs and one colony living in a hive contain-
ing small-cell combs. To be certain that the
small-cell combs in our study consisted entirely
of small cells, we used small-cell combs built of
plastic. The two colonies in each pair were
started as artificial swarms made from the same
source colony and with closely matched mite
loads. We then measured the mite loads of the
bees in the paired colonies at monthly intervals
across a summer to see if the colonies living in
hives equipped with small-cell combs showed
signs of reduced reproduction by the mites.
2. MATERIALS AND METHODS
The study was conducted over the summer of
2009 in Ithaca, NY, USA. On June 2, we prepared 14
1.0-kg (=approximately 7,700 bees) packages of
honeybees, 2 from each of 7 strong colonies that
hadscoredhighlyinaVa r r o a mite drop test
conducted 6 weeks earlier, on April, 18–20; 77, 26,
34, 66, 53, 55, and 37 mites per colony collected on a
sticky board in 48 h. We did not treat the seven
source colonies for mites before we shook the bees
from them for our packages, so we could be confident
that the bees in our packages were well infested with
mites. We could also be confident that both packages
in a pair had similar numbers of mites (this was also
checked, as described below) because the bees in
each pair of packages came from the same source
colony. Each package was given a new Minnesota
Hygienic queen (Olivarez Honey Bees, Inc., Chico,
CA, USA).
Immediately after making the packages, we began
feeding them with a 50/50 (v/v) sucrose solution
brushed onto the wire screen of one side of each
package cage. We continued this feeding for the next
3 days, June 3–5. On the evening of June 5, we
installed each package in a single-story deep Lang-
stroth hive containing ten frames of comb. One
package in each pair was installed in a hive
containing frames of standard-cell comb built of
beeswax, and the other package in each pair was
installed in a hive containing frames of small-cell
comb built of plastic (Honey Super Cell, Westmor-
land, CA,USA). (Note: We used plastic combs to be
certain that the colonies that received the small-cell
treatment had only small cells in their combs. We
Small-cell comb does not control Varroa mites 527
failed in two attempts in 2007 and 2008 to get our
bees to build beeswax combs consisting entirely of
small cells by giving them frames containing small-
cell foundation (4.9 mm, Dadant and Sons, Hamilton,
IL, USA).) There were no drone cells in any of the
frames of comb used in this study. We measured the
mean width of the cells in each hive by measuring the
width of ten cells in a straight line (inclusive of wall
widths) in the center of one side of each frame of
comb. For the seven hives with standard-cell combs,
the mean cell widths were 5.38, 5.40, 5.40, 5.36,
5.39, 5.38, and 5.38 mm, hence 5.38 mm on average.
For the seven hives with small-cell combs, the mean
cell widths were all the same: 4.82 mm. Thus, there
was a mean reduction in cell width in the small-cell
combs of 0.56 mm, or 10.4% (0.56/5.38=0.104).
Both groups of hives (those with standard-cell
combs or small-cell combs) were located at the
Liddell Field Station, but were arranged in separate
apiaries spaced 120 m apart. Within each apiary,
adjacent hives were spaced 5 m apart. We put all the
colonies in the same general area to minimize
location effects on any differences we might find
between the two group, but in different apiaries to
minimize transmission of Varroa mites between the
two groups. We managed the colonies for honey
production, giving each one a second deep hive body
containing frames of standard-cell or small-cell
combs in early July to prevent swarming and to
provide more space for brood rearing and honey
storage.
Once a month—from mid-June to mid-October—
we took measurements of three variables: (1) the
strength of each colony, (2) the infestation level of
Varroa mites in each colony, and (3) the size of the
workers in each colony. To measure colony strength,
we counted the number of frames of adult bees and
brood in each colony, doing so by visually examining
each side of each comb and estimating what fraction
(1/4, 2/4, 3/4, or 4/4) was covered with adult bees and
what fraction was filled with brood. To measure the
level of the Varroa mite infestation, we used two
methods. First, 2–6 days before we opened the hives
for the detailed visual inspections, we installed in
each hive a sticky board (Dadant and Sons, Hamilton,
IL, USA) for 48 h to get a 48-h mite drop count.
Second, when we opened the hives to inspect the combs
of each colony, we collected from the brood region of
each colony's nest a sample of 250 mL of bees
(approximately 300 bees) and used the powdered
sugar method to determine the number of mites on
these bees. (The bees and mites from each colony were
returned to it after we made our measurement.) To
measure the size of the workers in each colony, each
time we made a visual inspection of a colony, we
collected from its brood nest region a sample of ten
worker bees and measured each bee's head width
(±0.02 mm) using a dissecting scope equipped with an
ocular micrometer. In the summer of 2010, we
measured the thorax widths of bees collected from
the five surviving colonies with standard-cell combs
and from two new colonies with small-cell combs. All
the original colonies with small-cell combs died over
the winter of 2009–2010, so we set up two colonies on
small-cell combs in spring 2010, to get bees reared in
small-cell combs for thorax width measurements.
When we took our monthly measurements of the
colonies, we cut out any drone comb that the colonies
had built, usually along the bottoms of the frames. At
most, this involved removing 25 drone cells per
colony per inspection; none of the drone comb
contained drone brood. In this way, we prevented
drone rearing in our colonies and this meant that all
the mite reproduction in our study colonies occurred
in cells of worker brood. This ensured that our
measurements of the number of Varroa mites in our
colonies would reflect a difference, if any, in mite
reproduction between hives equipped with just
standard-size worker cells and just small-size worker
cells.
All descriptive statistics are reported as the mean±
SE. To test for differences between the means of
variables measured in both the standard-cell and the
small-cell colonies, we used paired comparisons t
tests (Sokal and Rohlf 1981) to take advantage of the
initial pairing of the study colonies.
3. RESULTS
3.1. The two treatment groups started
with matching distributions of mites
per colony
To see if the two treatment groups started out
well matched in terms of numbers of mites per
colony, we checked whether the initial counts of
528 T.D. Seeley and S.R. Griffin
mites per 100 adult bees were similar for the
two colonies produced from each of the seven
source colonies. Figure 1 shows that there were
differences in mean mite load among the seven
pairs of colonies, but that the two colonies in
each pair were closely matched in their mite
loads (correlation coefficient=0.88, paired com-
parisons t test, t
6
=1.24, P>0.25). It is clear,
therefore, that the two treatment groups started
out with virtually identical distributions of mites
per colony.
3.2. The two treatment groups ended
the summer with similar
mite infestations
Table I summarizes our findings regarding
the mite loads in the colonies of the two
treatment groups, measured in terms of mites
per 100 adult bees. It shows that for both
treatment groups, the mite loads started at a
relatively high level in June, then dropped to
lower levels in July and August, and eventually
increased in September and October. At no
point did we detect a difference in number of
mites per 100 adult bees between the colonies
living on standard-cell combs and those living
on small-cell combs.
Table II summarizes our findings regarding
the mite loads in the colonies of the two
treatment groups, measured in terms of mites
caught on sticky boards over a 48-h period
(=“mite drop count”). It shows that for both
treatment groups, the mean mite drop count rose
gradually over the summer and that the mean
mite drop count never differed significantly
between the two groups. The mean mite drop
counts were slightly lower (though not signifi-
cantly so) for the small-cell colonies in July,
August, September, and October, but we suspect
that this trend was a result of the small-cell
colonies being smaller than the standard-cell
colonies. The colonies in the hives with the
plastic, small-cell combs grew noticeably less
rapidly than those in the h ives with the
beeswax, standard-cell combs. For example, by
the middle of August, the mean numbers of
frames of adult bees and frames of brood in the
small-cell colonies were only 4.2±0.7 and 3.0±
0.4, whereas in the standard-cell colonies they
were 7.7±0.8 and 5.5±0.5.
To compensate for the difference in mean
colony size between the treatment groups, we
divided each colony's mite drop count by the
number of frames of adult bees. This yielded a
value of mites/sticky board/48 h/frame of bees.
The results are summarized in Table II.Wesee
Mites per 100 adult bees
(standard-cell comb colony)
84012
4
12
8
Mites per 100 adult bees
(small-cell comb colony)
Figure 1. Correlation plot that shows how the two
colonies in each pair of colonies used in the study
started out with similar loads of Varroa mites. The
two colonies in each pair were established using
1.0 kg of bees taken from the same mite-infested
source colony. In each pair, one colony was given
standard-cell combs and the other was given small-
cell combs.
Table I. Comparisons of the number of mites per 100
adult bees between colonies living on standard-cell
combs vs. small-cell combs.
Date Mites per 100 adult bees P value
Standard-cell Small-cell
June 16 6.48±1.00 7.14±1.12 >0.50
July 16 2.33± 0.58 3.33±0.79 >0.25
Aug 12 1.81±0.62 3.00±1.11 >0.50
Sept 19 4.71± 1.42 6.62±0.84 >0.15
Oct 17 6.45±0.83 8.43±1.00 >0.20
Small-cell comb does not control Varroa mites 529
that this correction for colony size differences
between the two treatment groups eliminates the
suggestion of fewer mites in the small-cell
colonies. Indeed, there is evidence that by the
end of the summer, in September and October,
the number of mites per adult bee was becoming
higher in the small-cell colonies than in the
standard-cell colonies.
Table III summarizes our findings regarding
worker bee head width across the summer for
bees in the standard-cell and the small-cell
colonies. We see that there was a slight
(approximately 0.08 mm, or 2.1%), but signif-
icant (P<0.01), reduction in head width in the
small-cell colonies, but that there was no change
in head width in the standard-cell colonies.
Table III also shows our findings regarding
worker bee thorax width for bees in the two
types of colony. We see that there was a slight
(approximately 0.14 mm or 3.5%), but signifi-
cant (P <0.01), difference in thorax width
between the two types of colonies.
4. DISCUSSION
Despite our hope that equipping colonies of
European honeybees with small-cell combs
would protect them from Varroa by inhibiting
the mite's reproduction, we found no evidence
of this in our study. We used manufactured
plastic combs for our small-cell treatment, and
we removed from all our hives any drone comb
that the bees built, so there can be no doubt that
the colonies in our small-cell treatment group
had only small cells (mean width 4.82 mm) in
their combs and that the colonies in our
standard-cell treatment group had only standard
cells (mean width 5.38 mm) in their combs.
Despite this unambiguous difference in cell size
between our two treatment groups, we found no
difference in number of mites per 100 worker
bees or in mite drop count between the two
treatment groups. If it were the case that Varroa
reproduction on European honeybees is imped-
ed when these mites parasitize bees living on
small-cell combs, then over the summer, we
should have seen a decline in the mite levels in
the small-cell colonies relative to the mite levels
in the standard-cell colonies. But we saw no
sign of such a decline. On the contrary, we
found that the mean mite counts started out the
same for the two groups of colonies and
remained indistin guishable over the next
4 months. We conclude that the small-cell
treatment did not hamper the growth of the
populations of Va r r o a mites in our study
colonies in the northeastern USA (NY state), a
conclusion that echoes those of two analogous
studies conducted in the southeastern USA
(Florida and Georgia) (Ellis et al. 2009; Berry
et al. 2010) and one study conducted in Ireland
that examined mite reproduction in small-cell
(4.91 mm) and standard-cell (5.38 mm) combs
(Coffey et al. 2010).
Likewise, our findings regarding the effect of
reducing cell size on bee size are consistent with
what McMullan and B rown (2006)have
reported. When they gave colonies either
small-cell (5.04 mm) combs or standard-cell
(5.48 mm) combs, they found that the bees
Table II. Comparisons of the counts of mites caught on a sticky board over 48 h between colonies living on
standard-cell combs vs. small-cell combs.
Date Mites/sticky board/48 h P value Mites/sticky board/48 h/frame of bees P value
Standard-cell Small-cell Standard-cell Small-cell
June 10 11.2±3.5 13.4±3.2 >0.50 5.12±1.80 4.65±1.27 >0.50
July 13 21.9±8.7 15.9±3.8 >0.50 7.33±3.12 6.19±1.49 >0.60
Aug 10 27.1±5.7 23.4±2.9 >0.40 3.61± 0.62 4.04±1.09 >0.60
Sept 17 46.1±10.6 39.0±12.4 >0.40 4.13±0.44 7.50±2.39 >0.50
Oct 16 55.6±10.1 52.1±11.4 >0.40 5.24±0.69 10.65±2.51 >0.15
530 T.D. Seeley and S.R. Griffin
reared in small-cell combs were only about 1%
smaller (in head width and thorax width) than
those reared in standard-cell combs, even
though the small cells were 8% less wide than
the standard cells. Similarly, we found that bees
reared in our small-cell combs (4.82 mm) were
only 2.1% smaller in head width and 3.5%
smaller in thorax width than those reared in our
standard-cell combs, even though our small
cells were 10.4% less wide than our standard
cells. It seems clear now that when bees are
reared in small cells, the reduction in bee size is
not proportional to the reduction in cell size.
The concept of small-cell combs as a non-
chemical means of Varroa mite control seemed
to be supported by the finding reported by
Martin and Kryger (2002) that when relatively
large worker bees (A. m. capensis) and relative-
ly small worker bees (A. m. scutellata) were
reared together in the small-cell (4.6 mm)
combs of A. m. scutellata, many fewer fertilized
females mites were produced in cells containing
the larger bees. They also reported that the low
reproductive success of mites in cells with the
larger bees was due to high levels of mother
mite and male protonymph mite mortality.
Martin and Kryger (2002) surmised that the
reason for the higher mite mortality in cells with
larger bees was a lack of space for the mites to
move aroun d inside the cells holding the big
bees. This may have occurred in the situation
studied by Martin and Kryger (2002), because
their investigation involved some of the largest
workers of A. mellifera (those of A. m. capensis)
being reared in some of the smallest cells of A.
mellifera (those of A. m. scutellata, approxi-
mately 4.6 mm wide).
It now seems highly doubtful, however, that
when workers of the European races of A.
mellifera are reared in small cells (4.8 mm, this
study; or 4.9 mm, Ellis et al. 2009, Berry et al.
2010, Coffey et al. 2010), the fill factor is high
enough to inhibit mite reproduction and so
achieve mit e control . We measured the fill
factor (the ratio of thorax width to cell width,
expressed as a percentage) for both the
standard-cell combs and the small-cell combs
in our study. The bees reared in our standard-
cell combs had a mean thorax width of 3.95 mm
and, thus, a fill factor of 3.95/5.38=73%,
whereas the bees reared in our small-cell combs
had a mean thorax width of 3.81 mm and, thus,
a fill factor of 3.81/4.82=79%. These values of
fill factor are identical to those reported by
McMullan and Brown (2006)forA. m. mellifera
in Ireland reared on standard-cell (5.48 mm) and
small-cell (5.04 mm) combs: 73% and 79%.
Because the fill factors were rather low for both
groups of bees, and only slightly higher for the
bees living on small-cell combs, it is perhaps not
surprising that we found no evidence in support
of the notion that small-cell combs hinder the
reproduction of mites on European honeybees.
ACKNOWLEDGMENTS
This research was supported by the US Department
of Agriculture (Hatch grant NYC-191522). We thank
Table III. Comparisons of the head widths and thorax widths of worker bees between colonies living on
standard-cell combs vs. small-cell combs.
Date Standard-cell Small-cell P value
Head width (mm)
16 June 2009 3.80±0.02 3.78±0.01 >0.50
16 July 2009 3.76±0.01 3.71±0.01 <0.01
12 Aug 2009 3.76±0.01 3.70±0.02 <0.01
19 Sept 2009 3.78±0.01 3.70±0.02 <0.01
17 Oct 2009 3.78±0.01 3.70±0.01 <0.01
Thorax width (mm)
7 Sept 2010 3.95±0.03 3.81±0.03 <0.01
Small-cell comb does not control Varroa mites 531
Professor Nicholas Calderone and two reviewers for
providing helpful comments on the manuscript.
La diminution de la taille des cellules des rayons
ne réduit pas l'infestation par Varroa destructor
dans les colonies d'abeilles d'origine européenne.
Apis mellifera / Varroa destructor / taille de la
cellule / lutte anti-acarien / cellule réduite
Waben mit kleinen Zellen sind nicht geeignet um
den Varroabefall in Völk ern der europäischen
Honigbienen zu kontrollieren.
Apis mellifera / Varroa destructor / kleine Zellen /
Milbenkontrolle / Zellgröße
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