It shouldn't surprise anyone that selection of bees for tracheal mite
resistance, in its detail, is not as simple as indicated by a summary.
However, it doesn't take much detailed monitoring of ( 20 to 50) hives
of a variety of origins, to be convinced that a range of susceptibility
to tracheal mites exists (and this, including it's inheiritability, has
been amply demonstrated with, among others, Page and Gary's
publications in 1989).
The ranking of test lines, by introducing newly emerged bees of known
origin into an infested hive, measures one aspect of resistance which
can be thought of as "attractiveness" of a bee to mites (its actual
mechanism may be something else).
Here's some possible mechanism's to think about (maybe someone will
test them). Young bees of a particular line may not become infested
because they "smell" more like old bees, or they start to smell like old
bees, at a younger age than other lines. 2. Newly emerged bees of a
"susceptible" line may move from the brood nest (surounded by young
bees, with no migrating mites) out to the "forager" area of the hive, at
an earlier age, 3. Resistant bees may groom their nestmates more
frequently and thereby reduce the mite's reproductive success (this
trait would be a characteristic of the host colony, rather than the
introduced test bees, so might not be selected in the test we used.
However, if resistant bees also groom themselves more, it could be
reflected in what we might think of as "reduced attractiveness").
I've worked in cooperation with the British Columbia Bee Breeders'
Association since 1989, in a project to select from within existing bees
in Canada, lines with resistance to tracheal mites which also have
superior beekeeping qualities.
We did several generations of selection (using the new bee mark and
retrieve method) and crossing in isolated yards, then tested the results
on a whole colony basis. (The simultaneous introduction controls to a
great extent, for Jerry Bromenshenk's concern about variation in time of
year: the host colony is what it is, for all lines. Ranking indicates a
difference in the line's susceptibility). That was an important test.
The mites in the host colony had a great choice in bees to infest, a
real smorgasbord. If a particular line was slightly unattractive, it
might be ignored (suggesting high resistance). In an individual colony,
however, mites don't have such a choice. (You might never eat mutton and
brussels sprouts at a smorgasbord, but you wouldn't go hungry if they
were all you had.. OK you get the point. I lost my appetite too).
Our observations of the correlation of "resistant" ranking of marked
bees, and relatively low mite populations in colonies, indicated that it
was a good measure. (Medhat Nasr did a larger test in Ontario and found
the same thing).
We haven't had the problem of great variability of tracheal mite
populations mentioned by Dr. Bromenshenk. Generally mite counts in
specific colonies changed on a trend which could be measured (and
distinguished bewteen colonies) by 50 bee samples on a 2 month interval.
We first used 1 month intervals, but cut back to 2 (or sometimes longer)
to reduce cost. This was adequate to distinguish differences between
groups (lines) but may not have been suitable for precise distinction of
small differences between individual colonies, which Jerry might
require.
In 1993 and 1994, lines of the selected B.C. stock were compared to
lines of Buckfast stock imported directly from Europe by Guelph
University, and to unselected lines from Ontario and B.C. The comparison
was duplicated in B.C. and Ontario (2 projects with coordinated
protocol) and involved 144 hives. The hives were started at about 15 %
of bees infested, and monitored for 2 year. The analysis did become a
bag of snakes in some ways (variable conditions, small sample size after
2 years) but it was clear that in relatively unfavorable conditions
tracheal mites increased substantially more (beyond 50 % of bees
infested) in the unselected and Buckfast stock, than in the selected
B.C. stock (below 20 %). In good conditions (crops above 200 lb per
hive) the resistance differences appeared to be not biologically
significant (all less than 20 %).
The conclusion?
Tracheal mite resistance is available to be selected in the bees in
North America.
Natural selection will eventually cause the North American bee
population to become resistant. A selection program based on new bee
marking and retrieval can speed the process (artificial insemination
too, although we didn't use it). Too radical a selction might unduly
reduce the beneficial variation in stocks, so I wouldn't advocate
everyone getting or even aiming at, one best stock.
Of course there will be deceptive advertising, and also honest mistakes.
Kerry Clark, Apiculture Specialist
B.C. Ministry of Agriculture
1201 103 Ave
Dawson Creek B.C.
V1G 4J2 CANADA Tel (604) 784-2225 fax (604) 784-2299
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