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The removal of capped drone brood: An effective means of reducing the infestation of varroa in honey bee colonies

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117 Why does removal of drone brood influence varroa populations? The preference of the parasite Varroa destructor for the drone larvae in Apis mellif-era rather than worker larvae, has already been described in 1977 by Grobov 7 and in 1980 by Ritter. 15 This preference (ratio of varroa in drone cells versus varroa in work-er cells) is calculated to be 8.6 by Schulz, 20 8.3 by Fuchs 6 and 6 by Rosenkranz. 16 Ruttner and his colleagues 18 proposed in 1980 to use this preference of varroa for cells occupied by drone brood as a vehicle for their own end. Other writers have shown that in their respective locations par-tial removal of drone brood allowed them to significantly reduce the population of par-asites in colonies. Purposes of the trial The trial presented in this paper had two objectives: ● To evaluate under central European con-ditions the impact of removal of drone brood on populations of varroa. ● To determine whether removal of drone brood is valuable in a control scheme based on autumn treatment with formic acid. Some acaricides used as alternative controls against Varroa destructor, for example formic acid or essential oils, are not always sufficiently effective. We propose as complimentary measures the removal of drone brood or the division of young colonies in spring. These interventions serve to retard the development of varroa populations, and thus reduce the pressure of infestation. They have the advantage of being able to be carried out at the height of the beekeeping season when recourse to chemotherapy would present serious risks of contamination of the honey harvest.
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Original Article 117
Why does removal of
drone brood influence
varroa populations?
The preference of the parasite Varroa
destructor for the drone larvae in Apis mellif-
era rather than worker larvae, has already
been described in 1977 by Grobov7and in
1980 by Ritter.15This preference (ratio of
varroa in drone cells versus varroa in work-
er cells) is calculated to be 8.6 by Schulz,20
8.3 by Fuchs6and 6 by Rosenkranz.16
Ruttner and his colleagues18proposed in
1980 to use this preference of varroa for
cells occupied by drone brood as a vehicle
for their own end. Other writers have
shown that in their respective locations par-
tial removal of drone brood allowed them
to significantly reduce the population of par-
asites in colonies.21,14,17,5 11,16
Purposes of the trial
The trial presented in this paper had two
objectives:
To e valuate under central European con-
ditions the impact of removal of drone
brood on populations of varroa.
To determine whether removal of drone
brood is valuable in a control scheme
based on autumn treatment with formic
acid.
Design of the
experiment
This trial was carried out in a production
apiary of about 20 colonies of A. mellifera
established in Dadant Blatt hives. Formic
acid was the only acaricide previously used
in this apiary located near Berne, Switzer-
land. All hives were equipped with a mesh-
Bee World 84(3): 117–124 (2003) © IBRA
The removal of capped drone
brood: an effective means of
reducing the infestation of
varroa in honey bee colonies
J
EAN
-D
ANIEL
C
HARRIÈRE
,A
NTON
I
MDORF
,B
ORIS
B
ACHOFEN
,
AND
A
NNA
T
SCHAN
Some acaricides used as alternative controls against Varroa
destructor, for example formic acid or essential oils, are not always
sufficiently effective. We propose as complimentary measures the
removal of drone brood or the division of young colonies in spring.
These interventions serve to retard the development of varroa
populations, and thus reduce the pressure of infestation. They have
the advantage of being able to be carried out at the height of the
beekeeping season when recourse to chemotherapy would present
serious risks of contamination of the honey harvest.
118
FIG. 1. A brood frame from which we have removed the lower part of the comb acts as a drone frame.
The frame is placed in the brood nest so that it is quickly build and laid in.
119
protected floorboard over the whole bot-
tom of the hive. We divided the hives into
two homogeneous groups on the basis of
the natural fall of varroa in October of the
preceding year, which gives a reliable indica-
tion of the number of overwintering
mites10,12and on the strength of the colonies
in spring.
The drone frame
One frame of brood, from which we had
removed the lower half of the comb,
became the drone frame. One such frame
was introduced to the side of the brood
nest of each hive in the test group at the end
of March. During the whole period of brood
rearing we regularly removed the capped
drone brood from this frame by cutting out
the capped cells, whenever it exceeded a
minimum of 1dm2(fig. 1). Drone brood
around the edges of other frames was not
removed. Normally, the drone combs are
rapidly constructed as the amount of drone
brood built in a nest is governed by negative
feedback from drone comb already con-
structed13and availability of sucrose sources
(e.g. good nectar flow or honey stores in
the hive).
Criteria evaluated
The number of capped drone cells removed
from the colonies was determined, and the
number of varroa in this comb was counted.
All colonies were managed following the
same apicultural practice. The strength of
the colonies was estimated from mid-March
until September using the Liebefeld method8
in order to evaluate any impact of the
removal of drone brood on population
development. Honey production was meas-
ured. During the whole period of the trial
the natural fall of varroa was measured once
a week, giving an indication of the progress
of infestation of the colonies. During August
and September we made two series of three
short-term treatments with formic acid,
then we checked the efficiency of these
treatments by the natural fall in October.9
The trial was carried out in 1993, and
repeated in 1994.
Effect on varroa
populations in 1993
The year 1993 was marked by an early
spring and a good nectar flow which
0
10
20
30
40
23.03. 12.04. 02.05. 22.05. 11.06. 01.07. 21.07.
Date
Natural drop Varroa/day
1993 Without removal n=10
With removal n=8
1994 Without removal n=9
With removal n=9 9
FIG. 2. Effect of the removal of drone brood on the natural drop fall of varroa in 1993 and 1994 (average).
120
encouraged the raising of drones, and thus
permitted the frequent removal of capped
drone cells. It was thus possible to take an
average of 4.1cuttings of drone brood per
colony (minimum 1, maximum 6) between
15 April and 15 July.
We removed an average of 3374 capped
drone cells per colony carrying 788 varroa
(table 1). For these two figures there are
important variations per hive.
The average natural falls of the test and con-
trol groups (fig. 2) differed progressively
from the month of May. While the fall of
mites remained low in the hives where we
had cut out drone brood, it rose very rap-
idly in the hives without removal. This
increase is an indication that the progress of
varroa populations is to a large extent
retarded by the elimination of mites found
in the drone brood.
The formic acid treatments in August and
September confirmed the effect of the
biotechnical measures: the populations of
mites in the test hives at the end of the sea-
son were 3.5 times less than in the control
hives. In this latter group five hives out of
eight showed an infestation greater than
5000 mites with a maximum of 12 928. Bees
with deformed wings were seen in some of
the control hives because of the excessive
load of parasites.
Effects on the bees in
1993
The honey harvest and colony development
were not significantly affected by the
removal of cells of drone brood (fig. 3).
Also, there was no significant difference
between the two groups in the total quan-
tities of worker brood raised during the
year: test, 140 551±22 675 cells; control,
142 852 ± 16 853 cells (average ± s.d.).
Effects on varroa
populations in 1994
The spring of 1994 was cold and rainy, char-
acterized also by a weak nectar flow, which
TABLE 1. Results of th
Year Number of cuts Drone cells removed
Variable
11993
with removal mean (n= 10) 4.13374
s.d. 1.4 1681
without removal mean (n= 8) - -
s.d. - -
11994
with removal mean (n= 9) 2.3 3588
s.d. 1.11657
without removal mean (n= 9) - -
s.d. - -
1Natural drop of varroa measured in the week before treatment with formic acid
*The means of the groups with and without removal of drone brood in the same year are statistically different (ttest; P0.05)
121
permitted an average of only 2.3 cuttings of
drone brood per colony (minimum 1, max-
imum 5) between 3 May and 28 June. We
were able to remove 3588 capped drone
cells per colony with 434 varroa (table 1).
As in 1993 the natural fall of mites in the
control group hives rose rapidly from mid-
May, while the rise in the test hives did not
happen until six weeks later, and in a more
gradual manner (fig. 2).
The controlled treatments with formic acid
showed that in spite of the reduced number
of cuttings, this biotechnical method had
restricted the consequent development of
varroa populations. During the formic acid
treatments we counted more than double
the parasites in the hives without drone
brood removal.
Effects on the bees in
1994
The unfavourable nectar flow in 1994 did
not allow any harvest of honey, and thus
made a comparison between the two
groups impossible. The colony strength and
total number of worker cells raised was not
significantly influenced by the removal of
drone brood.
The removal of drone
brood removes the
pressure of infestation
without hindering the
colony
These results show that under central
Europe conditions the removal of drone
brood is an efficient means of slowing the
development of varroa populations, even
when the number of cuttings is reduced.
Under our climatic conditions, and in the
context of an alternative control pro-
gramme using only short-term formic acid
treatments in autumn, these biotechnical
measures are shown to be indispensable in
preventing colonies from perishing as early
as July. The results are probably the same as
for long-term treatment with formic acid.
he 1993 and 1994 trials.
Varroa in removed Natural drop Mites killed Honey harvest
drone brood before treatment1by treatment (kg)
mites/day with formic acid
788 3.50 15316.6
677 2.18696 3.4
-40.20* 5693* 7.7
-34.49 3853 4.3
434 11.54 2093
352 11.42 1104
-28.02 4437*
-26.27 2948
122
The removal of drone brood as we have
described is only one measure of a system,
and does not in any case allow the aban-
donment of other treatments, as has been
confirmed by the observations of
Rosenkranz,17,16Schulz,21Marletto11 and
Wilkinson.24 Some authors have suggested
the introduction of uncapped drone brood
into colonies with no other brood with the
aim of trapping the mites.4,19,3,2 This method
is comparatively labour intensive, and even
though an efficiency of up 90% can be
attained, it does not relieve the beekeeper
of using some acaricide treatment.
In our trial the removal of drone brood had
no negative effect on the development of
the colonies and on honey production. See-
ley,22 by providing colonies with added drone
combs, measured a significant reduction of
honey yields in comparison with colonies
without addition. But he concluded that
providing colonies with drone combs might
still be desirable since eliminating mites may
compensate for the negative effect of drone
comb addition on honey yields.
Allen1and Seeley22 claimed that colonies
given a frame of drone comb had less drone
cells on the edges of the other worker
brood frames. An additional advantage is a
significant harvest of wax. The number of
drones in our colonies is sufficient to guar-
antee the fertilisation of queens.
Examination of drone
brood? Not viable for
diagnosing varroosis
Our results showed that it is not possible
to calculate the size of the varroa popula-
tion parasitising a colony simply by examin-
ing the infestation rate of drone brood. This
is probably influenced in part by the cycles
of drone brood production in each colony
and in part by the cyclical nature of the
infestation of cells by varroa. The parasite
load of drone cells was seen to vary from
one- to six- times in the space of a week,
without any relation to the actual varroa
population. This confirms the observations
of Ritter & Ruttner15who also observed the
weakness of the infestation of drone brood
0
5,000
10,000
15,000
20,000
25,000
01.03. 01.04. 02.05. 02.06. 03.07. 03.08. 03.09. 04.10.
Date
Number of bees
1993 without removal
with removal
1994 without removal
with removal
FIG. 3. Colony development for the groups with and without drone brood removal in 1993 and 1994
(average).
123
as a measure of colony infestation. Wilkin-
son & Smith,24 on the other hand, concluded
using a theoretical model, that sampling nat-
urally produced drone brood is valuable for
estimating the level of mite infestation in a
colony.
Will varroa adapt itself
to this biotechnical
control?
The often expressed fear that removal of
drone brood will select for a population of
varroa that prefer worker brood does not
seem to be justified. We should remember
that the removal of drone brood occurs
only during a short period, and for the rest
of the year the mites are obliged to breed
in worker cells. Even during the drone rais-
ing season there will always be more varroa
in worker cells simply because there is usu-
ally 10-times more worker brood in a nor-
mal productive colony as the area of drone
combs in feral colonies is only around 17%
of the total23.
Implications for
beekeepers
This trial has shown the efficacy of removal
of drone brood in retarding the develop-
ment of varroa populations. This biotechni-
cal control allows the deferral of acaricide
treatments until the end of summer with-
out damaging infestation of the colony. This
method is important for the success of
some strategies of alternative control, as for
example that which relies exclusively on
autumn treatments with formic acid. On its
own however the removal of drone brood
is insufficient to keep the parasite under
control.
Properly planned, the removal of drone
brood can be integrated without much
increase of work into the normal manage-
ment of modern apiaries.
What to do in practice?
Three points to note:
Introduce the drone frame into the
colonies sufficiently early (end of March-
beginning of April).
The drone frame should be introduced
into the brood nest so that it can be
quickly built-up and have eggs laid in it. In
this position the drone brood will also
capture many more parasites16.
Avoid at all cost the emergence of drones
from the drone frame, as this will
increase the varroa population. If the fol-
lowing visit cannot be planned to occur
before the emergence of drones, then
the frame should be removed and
replaced with a full frame of worker cells.
To avoid an increase in work, it is neces-
sary to integrate the removal of drone
comb into the normal apiary manage-
ment for this time of year. Given the
normal growth of colonies, swarm con-
trol, placing and checking of honey boxes,
the removal of drone brood should result
in little increase in work.
Acknowledgement
Translation by Peter Kerr, Auckland, New Zealand
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J
EAN
-D
ANIEL
C
HARRIÈRE
*,A
NTON
I
MDORF
,B
ORIS
B
ACHOFEN
,
A
NNA
T
SCHAN
Swiss Bee Research Centre, Dairy Research Station, Liebefeld, CH-3003, Bern,
Switzerland
*E.mail: jean-daniel.charriere@fam.admin.ch
... Good results can be achieved when 4 to 5 fully capped trap frames are removed per season (Charrière et al., 2003). It is worth noting that DBR is mainly used by small-scale beekeepers in Europe and is considered labour-intensive or not effective enough as a single treatment elsewhere (Evans et al., 2016;Whitehead, 2017). ...
... When done properly, the effectiveness of DBR is demonstrated by the fact that the number of mites during colony development in spring and early summer was significantly lower than in untreated colonies (Wantuch & Tarpy, 2009). Final infestation rates of colonies after late summer treatments were also substantially lower than in colonies where DBR was not performed (Calderone, 2005;Charrière et al., 2003). However, to date, there are few data on how many mites a single drone frame can actually carry. ...
... It is known that drone brood attracts varroa mites on average eight times more than worker brood and is, therefore, an effective means of controlling this pest when removed (Charrière et al., 2003). Due to limited data, it is currently unclear how many mites are removed by a single frame and at what status drone cells were cut. ...
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... Cette méthode est répétée 3 fois d'avril à juin pour une meilleure efficacité. Elle permet de réduire l'infestation à court terme (Cahier technique, 2019; Charrière et al., 2003). En effet, les colonies devront être traitées en fin de saison. ...
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Background Varroa destructor is a parasite of honeybees. It causes biological damage leading to the colony collapse in the absence of treatment. In recent years, acaricide resistance has emerged in Varroa mites, leading to a decrease in treatment efficacy. We modelled the action of Apivar® (amitraz) treatment, using three input parameters: treatment duration, treatment period, and daily mortality due to the treatment. The output parameters were cumulative mite mortality during treatment, the residual number of Varroa mites, and treatment efficacy, expressed as a percentage. Results The model was validated by monitoring efficacy in the field, in 36 treated hives. According to the model, treatment in the absence of brood is optimal. For a long period without egg laying during the winter, an initial infestation of 100 mites and a start date for treatment of August 7th, a minimal treatment efficacy of 98.8% is required for stabilisation of the mite population for year to year. More effective treatment is associated lower cumulative numbers of dead Varroa mites over the entire treatment period. Thus, the total number of dead mites observed during the monitoring of field efficacy provides information about more than just the initial level of colony infestation. The proportion of resistant mites can be modelized by a decrease of daily mortality rate influencing treatment efficacy. Management of the initial Varroa mite infestation of the colony by the beekeeper can compensate for the decrease in treatment efficacy for resistance thresholds of up to 40% of resistant mites. Conclusion Treatment efficacy depends on several parameters, including initial level of infestation, treatment period and the presence of acaricide resistance. Amitraz resistance may lead to treatment failure, even if the beekeeper is able to keep initial infestation rates low. This article is protected by copyright. All rights reserved.
... So, efforts should be made to reduce the population of drones in the colony when these drones are not required for mating of the gynes, as a drone bee consumes 8-10 times the honey consumed by a worker bee in its daily routine. Partial removal of drone brood in term of the removal of 3-4 completely capped drone combs at the beginning of the season also reduces the fi nal mite population about 50-70 % (Charrière et al. 2003 ). ...
... Sustainable Varroa control is a labor-intensive process requiring a combination of different measures, e.g. monitoring of mite fall, drone brood removal trapping (Calderone, 2005;Charriére et al., 2003), and application of miticides in rotation. Such "integrated pest management" needs to consider the population dynamics of Varroa as well as the honey bee colony so that measures can be applied at appropriate times of the year (Rosenkranz et al., 2010). ...
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