Drone brood culling
I hope we shall see more discussion of this topic, has there been any
serious evaluation of the
technique as a means of limiting the growth of varroa populations?
Apologies for the length of this post, I would be glad to receive comments.
For the last two years I have made up some UK National brood frames with
horizontal bars about two
thirds of the way down; the upper part was fitted with normal worker
foundation and the lower part
left empty. Early in the 1996 season one or two of these frames were placed
just outside the brood nest
in two colonies and the bees duly filled the empty space with drone comb.
Between the end of May
and the end of July, during the usual inspections, if this drone comb was
largely capped I cut it out
and the bees obliged by refilling it so that the process could be repeated.
I was then able to open a sample of the drone cells and count the mites
which were found. By
measuring the total area of capped cells I then made an estimate of the
total number of mites which I
had removed from the colony. The results were disappointing, in three cases
I found no mites the
other samples gave estimated totals removed of 26, 38 and 115 varroa.
At the end of September the hives were treated with Bayvarol and the two
test colonies produced 702
and 1282 mites, as counted on the floor below a 3mm wire screen. Curiously
the control colony which
had no drone culling ended up with only 663 mites although it had a rather
higher infestation at the
start of the season, as measured by natural mite mortality. All three
colonies were kept in the same
apiary for most of the season.
The sample is small and I do not think that the actual numbers are
significant. But I feel safe in
deducing that in my situation this method of controlling varroa, as
described above, is not effective.
At the end of the season I need to treat anyway and little is gained by
taking out a small number of
mites earlier. This may be because I seem to be in a situation now where
there is very little
reinfestation from untreated colonies in the area. It would be an advantage
if I could slow the growth
to such an extent that I needed to treat only every other year, but I am
not there yet.
I suggest that there are also some disadvantages to drone culling. I
believe there is a loss of worker
brood area, although it has been suggested that giving a drone comb area
reduces the amount of drone
comb drawn elsewhere. I have not noticed this and I find it difficult to
believe that workers on one
comb remember that there is a big area of drone comb several frames away.
It is possible that they
draw drone comb if there are not many drones in the hive, in which case
culling drone larvae will
encourage them to build more drone comb. Does anyone know what stimulates
workers in an ordinary
colony to take down worker cells and build drone cells?
Culling drone brood may also give a selection pressure for varroa which
prefer worker cells; this
would be a change for the worse, though the results above suggest that
relatively few varroa actually
find their way into drone cells anyway.
Maybe in a situation where the number of varroa build up to harmful levels
more quickly drone
culling is a useful method for limiting the damage, especially the damage
due to the spread of viruses.
So back to my initial question - does anyone know? Have any large scale
trials been done?
Brian Gant
Buckfast, UK