Not to criticize Zach, but I disagree, as would
every legitimate IPM education and extension
professional one could find.

> The most commonly used

...and therefore, the most commonly in error...

> # is x30, so in your case it is 136*30 =~4000 mites.

More accurately, it might be something like that under
certain highly specific conditions that any randomly-selected
colony is much more likely to NOT experience than experience.

> This can be way off depending on the amount of brood
> in the colony and whether the colony is hygienic or not.

And can also be way off depending upon a host of other
factors, which, if listed in full, would likely crash the
reader's e-mail client due to the sheer length of the list.

> The commonly quoted

...and therefore, commonly wrong...

> # is about 3000-4000 mites for economic threshould (worked
> out by Keith Delaplane in the Georgia/TN/SC area.

Hard to ignore, but worth the effort.  Its not even a threshold.

Perhaps this sort of estimate has value at U Georgia, but
off campus, in the cold, cruel real world, where Mean is normal,
and the Deviation is standard, we are forced to realize, if not
openly admit that an "economic threshold" has to be more than a
single simple pest count standing in isolation.  It has to be a
number that has some point of reference in terms of either:

        a) the hosts for the pests

        b) a prior measurement of pest population

...so that one can speak of "pests per acre", or "pests per plant",
or pest/bee ratio, or "pest population trends".

With colonies of bees, one has a difficult time doing more than
estimating bee populations (to learn the knack, see
http://www.dave-cushman.net/bee/beesest.html , but be aware
that the frames shown in Dave's illustrations are British
"BS National" sized).

If one wishes to avoid the extra work of estimating bee populations,
then to track mite populations, one must track mite populations over
time, with a series of measurements using a repeatable method, such
as a 24-hour natural drop under weather conditions that are "nominal"
(no rain, decent flying weather...)

The key point about "IPM" is that one must know pest population
in terms of how much damage the pests will do.  The dead-giveaway
with varroa is that their population growth starts to "go exponential"
or "spike", and the resulting increased population of mites is what
"overpowers" a colony.

A decent overview of the whole "Economic Threshold" issue is here:
http://www.cals.ncsu.edu/course/ent425/tutorial/economics.html

Note the use of the term "pest density" in the last paragraph on
the page.  Density is inherently a ratio.  A ratio is something
COMPARED to something.  Single numbers in isolation are useless,
misleading, and lack any sense of perspective.

> So in your case, you should treat.

But the rationale given is not a basis for making a treatment
decision.  What might be a valid basis for a treatment decision
is the much lower mite drops from colonies in the same yard,
but only because the colony in question has a mite drop 45 times
the other colonies.

             jim (I Fought the Lawn and the Lawn Won)

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