Using Drone
Brood as a Varroa Control
Current scientific data
indicates that the removal of drone brood, when performed correctly can
slow mite population
growth, in a bee colony, by about 50%. Though its use alone will not
prevent
a bee colony being
overwhelmed, it is an important tool within an integrated approach to varroa
control. With the advent of varroa resistant to pyrethroid treatments, such as Bayvarol
and Apistan,
the removal of sacrificial drone brood can enable other
less effective controls to reduce infestation
to a safe level.
How does it work?
Varroa mites reproduce
within sealed bee brood. A hormone in bee larval food, which is more prevalent
in drone brood,
triggers this. As a result mites have a preference to enter drone brood in
order to reproduce.
One mature mite entering a drone cell of Apis mellifera will
emerge together with the imago drone and
five daughters having an average survival rate of 83%.
Mites entering worker brood cells will emerge with
three daughters
having a survival rate of 46%. Female mites have the ability to reproduce up to
three times
so the potential for
large-scale increase of mites breeding in drone cells is clear.
When infesting its natural
host, Apis cerana,
reproduction is only possible in drone brood, which coupled
with other factors, enables a symbiotic relationship with
that host.
Varroa mites have a
preference to be near the centre of the brood nest. If an area can be given to
drone
brood it can be a
bait trap for varroa. As bee larvae mature mites enter the cells to reproduce
just prior
to the cells being
capped. When the comb is sealed it can be cut out or removed taking with it any
mites
that are present.
However if these combs are left to hatch then an increase in varroa populations
levels
will result.
it can only be used in colonies rearing drones. Small
colonies will not be suitable, as they have no interest
in developing drones.
In the
populations are at a level where control will not be required,
see the handout ‘1PM and Varroa’, then drone
brood removal should
be delayed.
How effective is it?
Research indicates that when
performed correctly, i.e. combs have not been left to hatch, then
it can reduce
mite population growth by 50%. Swiss beekeepers report
finding it very effective when colony mite populations
are below 700. With
higher mite levels it will have little or no impact on the mite population.
Why should it be considered for use in conjunction with
other controls?
To be effective as a ‘once a
year’ control an efficacy in excess of about 96% is required. Many controls
such
as formic or lactic
acid will give a maximum efficacy of about 80%. This means that at some point
in the current,
or a future season,
another control will have to be used. To illustrate the point, consider an
average colony having
a mite population of
800 in early August. At this level the colony is not at immediate risk but some
control is
required. If we remove 80% of the mites this leaves 160 mites
which may increase to a level to put the colony into
an ‘at risk’
situation early in the following season. If drone brood removal had been
carried out in the colony then
the population would have been halved to about 400. If
we remove 80% this leaves a mite population of 80 which,
unless there is invasion from another source, will leave
the colony safe for the following season.
If it is intended to use ‘Apiguard’,
after removal of the honey crop, drone brood removal can enable this product to
be a very effective
control measure.
Please see ‘Fact Sheet 13 —Apiguard’.
How is it performed?
There are three simple ways
of performing this procedure.
a) Full Brood Comb.
A brood comb is fitted with
drone brood foundation and placed adjacent to the brood nest. The bees will
draw out
the foundation, the queen will lay eggs into the comb
and when it has been capped over the frame can be removed.
The comb can then be cut out and destroyed. If
a narrow strip of the comb midrib is left at the top of the frame,
similar to a foundation starter strip, the frame can be
returned to the colony for re-use.
b)Shallow Comb.
If a shallow ‘super’ frame
of worker comb is placed between two fully drawn brood comb frames, the bees
will draw
out comb below the bottom bars. During the drone season
this will invariably be drone comb. When sealed over it is
simple to cut off using the hive tool or a knife. The frame
can then be re inserted for another cycle.
c)Cartridge.
This is a refinement of b).
A small frame is made up which clips or is held by two pins to the base of the
shallow frame.
When the time comes for removal it is removed
and a replacement cartridge fitted.
Systems b) & c) are
probably more effective as these combs can be placed near the centre of the
brood nest where most mites will congregate. if a full
frame is placed in the centre it will create an undesirable break in the brood
nest.
The
equivalent of three to four full brood frames of sealed drone brood needs to be
removed for maximum effect.
What else do I need to be aware of?
Do not leave bait drone
combs in the hive longer than 23 days. Drones may start hatching on day 24,
which could lead
to an increased mite
population. If you carry out swarm management on a nine-day rota
then removal on day 18 fits in
well with five
‘safety’ days to spare. With system b) & c) it can be effective if two
frames are used on an alternate
removal period of 9
days.
At the end of the season,
sometimes earlier, bees will build worker comb rather than drone in the traps.
You will not wish
to destroy this brood
so move the frame to one side of the brood nest. When hatched and as the brood
nest reduces in
size the frame can be
moved to the end of the chamber
for subsequent replacement