>
> > "Mites prefer drone larvae over worker larvae, but they will infest
> worker larvae and eventually kill the colony if preventive measures are not
> taken." Are you saying this is all wrong?
>
Not all wrong, but has been misinterpreted. Fuchs (1992) published a paper
in which he found that drone brood was more likely to be infested than
worker brood, by a factor of about 8 (I found that I've misplaced my
copy--will confirm as soon as I get a new copy).
But this did not mean that most of the mites were in the drone brood. Two
subsequent and excellent studies (Boot 1993 and Beetsma 1999) demonstrated
that the apparent "preference" for drone brood was due to the greater
chance of a nurse bee sticking her head into a drone cell than a worker
cell (due to the increased demand for food by a drone larva, the larger
cell size, as well as the longer invasion window). Other researchers
(LeConte, Teal) found that the pheromone released by drone propupae
initiated the "hop off the nurse bee" response to a greater extent than did
the pheromone released by worker propupae (a pheromone "read" by another
species is referred to as a "kairomone").
The end result is that the success of any mite in invading a cell of either
worker or drone brood is dependent upon the odds of the bee that the mite
is hitchhiking on, sticking her head into a cell containing a propupae of
the right age, that is emitting an odor (a releaser kairomone) that elicits
the invasion response of the mite--thus causing it to hop off that nurse.
Mites hitchhiking on older bees will never get the chance, since older bees
don't stick their heads into brood cells, thus varroa generally
preferentially climb onto nurse bees.
The greater the brood to adult bee ratio, the greater the chance of a mite
quickly being given the opportunity to invade a cell--thus the mite
reproductive rate is greater when the brood:bee ratio is greater.
Similarly, the greater the drone brood:worker brood ratio, the more likely
a mite is to invade a drone cell.
But this comes at a cost to the mite--if multiple mites invade a drone
cell, their reproductive success rate drops off. So it is not to the
mite's advantage to multiply invade drone cells.
The data set that I have at hand (roughly 800 worker cells inspected, 450
drone cells, in 9 different colonies in the Midwest during the summer,
colonies ranging 1-7% drone brood), indicate that the actual percentages of
mites in the brood was typically around 50%, but could go as low as 15%.
On average, 7% of the mites in a colony were in the drone brood, 40% in the
worker brood.
The above data, along with the the studies previously cited, indicate that
unlike in *Apis cerana *colonies, varroa in *Apis mellifera* colonies
reproduces mainly in worker brood, rather than in drone brood. Indeed, if
you use a cappings fork to inspect drone brood for mites, you often won't
find any, despite the fact that you can confirm (via alcohol wash) that the
colony is infested with varroa.
So in answer to Dan's question, drone brood removal would be most
efficacious early in the season, when the proportion of drone brood to
worker brood is the greatest.
Bottom line, yes, mites "prefer" drone brood if given the choice, but a
hitchhiking mite may not get that choice, should its ride not stick her
head into a drone cell of exactly the right age. Far greater chance of
being exposed to a worker larva of the right age.
By all means read:
Boot (1993) Invasion of Varroa jacobsoni into honey bee brood cells: a
matter of chance or choice?
Beetsma (1999) Invasion behaviour of Varroa jacobsoni Oud.: from bees into
brood cells
Nazzi (2015) Ecology of Varroa destructor, The Major Ectoparasite of the
Western Honey Bee, Apis mellifera
--
Randy Oliver
Grass Valley, CA
www.ScientificBeekeeping.com
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