>
> > whether brood break is an effective treatment against varroa.
>
We can use simple biology and arithmetic to estimate the effect of an
induced brood break on the varroa population of a hive. Let's say that you
are inducing the brood break early in the season--say on the 1st of April.
And let's say that we duplicate Wagnitz's treatment:
"The queens in the requeen treatment groups were caged 5 days prior to
inserting queen cells, and were killed when the queen cells were inserted.
Queen cells normally take 10-14 days from queen
emergence to the start of egg laying, so this technique provided 18-21 days
without egg laying"
I'll use a 20-day brood break (during which the reproduction of varroa
would be zero), and assume that the colony started with a realistic total
population of 200 mites. For a 10-frame colony, assuming that half the
mites were in the brood, an alcohol wash would give a count of 0.5
mites/100 bees--a fairly typical wash at that time of season for a
well-managed colony.
The typical daily intrinsic rate of increase (r value) for varroa in my
hives is roughly 0.015 (typical range from other studies 0.010-0.030). So
without the brood break, those 200 mites would increase over 20 days to P x
e\rt (P=200, r=0.015, t=20), thus to 270 mites--a 35% increase.
During that brood break, there would have been an attrition of mites, at an
r value of approx -0.005, thus resulting in a reduction of the original 200
mites to 181 remaining (we'd expect an even greater reduction if the colony
was a good mite groomer).
Thus after the brood break, instead of containing 270 mites, the colony
would contain only 181 mites.
Now let's then plug those numbers into the expected growth of the varroa
population (ignoring any mite immigration) though mid August (that would be
another 116 days). The 270 mites would be expected to increase in the
untreated hive to 1538 mites, compared to 1031 mites in the untreated.
That's half again as many mites, a proportional difference which would
continue to grow exponentially (although at a lower rate) until
broodrearing ceased in late fall. Summary:
start pop 20 day 116 additional days
no break 200 270 1538
break 200 181 1031
Although an induced brood break would not be expected to reduce the mite
population as much as would treatment with an effective miticide, the
degree of reduction of the mite population could hardly be considered as
trivial. Note that the 1031 mites in the colony with the brood break
barely exceeds the widely-accepted threshold for successful wintering
(~1000 mites), whereas the control colony is well over that number.
Of course, immigration and any number of other factors can take effect over
those 116 days of summer, but the proportional difference in mite
infestation rates between the colonies would be expected to stay roughly
the same.
--
Randy Oliver
Grass Valley, CA
www.ScientificBeekeeping.com
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