Pete brought up mite immigration, which, as I've modeled varroa buildup,
stands out as a major factor (the other most important factors are mean
number of days of phoresy before invading a cell, reproductive success once
in a cell, and the mite mortality rate during phoresy).
If all colonies contained the same degree of mite infestation, there would
be no expectation of any effect from mite immigration, since it would
theoretically balance out. Immigration is akin to osmosis, in which there
would be a flow of mites between hives with high mite levels to hives with
low mite levels. The net effect would tend towards equalization of the
mite levels. If all colonies were infested to the same degree, the math
does not work for invasion contributing to mite buildup.
But this is not normally the case, my own data, and that of many others,
indicates that there is often huge colony-to-colony variation in mite
levels within flight range of any individual colony. And it is well
documented the there is often considerable drift of bees between colonies.
Add to that the finding that when mite levels get high in a colony, the
mites tend to lose their preference for nurse bees, and ride to a greater
extent on foragers. There is also considerable pick up of mites by
foragers when they are in the field--perhaps from bee-to-bee contact in
flowers? And then there is the huge mite transfer during the robbing out
of collapsing hives.
All the above add up to an overall flow of varroa from mite-infested hives
to those less infested.
Rates of such infestation have been measured by several researchers. A
late-season influx of 300-400 mites into a colony is not unusual (although
that number would be somewhat offset by mites drifting out on foragers).
And immigration of as many as 3000 mites has been documented.
So in answer to Charlie's question about Mel Disselkoen's claims, when I
insert a 3-week brood break in April into my current model, at a fairly
high r value (daily r = 0.022; roughly a doubling each month), with a
starting mite level in February of 1 mite in an alcohol wash of 300 bees
(77 total starting mites in the hive), and no mite immigration from
outside, the ending mite level of the colony in October would be 1177
mites--a level at which the colony could well survive the winter. Thus, my
own calculations support Mel's claims.
But as Pete notes, that would go out the window should Mel's apiaries be
surrounded by a bunch of untreated hives in which mite levels were allowed
to rise to high levels. An influx of 400 mites into his hives would then
tip them over the threshold, and they would be more likely to crash.
Pete is entirely correct in stating that the immigration of mites from
outside sources can make your efforts at mite management moot, especially
if robbing takes place. Let's say that you've got your September mite
count down to 3 mites in an alcohol wash (1 mite/100 bees). Say that there
are 12 frames of bees in your hive (at 4000 bees per frame), and that half
the mites are in the brood. That would give you a total of 960 mites in
the hive.
But then add a realistic 400-mite net immigration, and your alcohol wash
count would go up to 4.5 mites. In actuality, if the immigration took
place over the late summer, some of those mites would have reproduced, the
count would be higher.
But if the colony had received 2000 mites via immigration, then that
immigration would result in a mite wash easily over 10.
So as Charlie points out, it's all about your neighbors. If you're lucky
enough to keep your hives where there is not much immigration, then
splitting (with a long brood break) could well manage your mite levels (as
Charlie has found). But if you're surrounded by beekeepers who are
suffering 40% losses from varroa, their mites are going to wind up in your
hives.
--
Randy Oliver
Grass Valley, CA
www.ScientificBeekeeping.com
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