> Such as "cascade" was clearly observed after the invasion of varroa into
> most countries.  As colonies began to collapse, the remainder were
> overwhelmed by drifted mites.  I've seen this occur myself.
>
> I was referring to a seasonal cascade her in the US .  If the effect was
> due to mite crashes,  they would start earlier in the season in the south
> as they are ahead of the northern states in brood/mite development.
>

Wouldn't it be the other way around? Start earlier in the North, and later
in the South?

The "mite bomb" if we are to assume it would happen, would occur when the %
of mites exceeds the % of brood. The infestation rate causes the collapse,
which causes migration (I'm using migration, not immigration, but you are
free to use the term you like).

Brood rearing continues longer in the south than it does in the North.
While the South has been rearing brood longer, so might have more mites in
number, they are also continuing to rear brood later, meaning the
percentage of mites to brood should be less than a colony in the North that
stopped (or decreased) brood rearing earlier than the southern colonies.

If you combine that concept with the idea that the southern hives have
higher mite numbers, you might get a situation where the migration occurs
at the same time, due to the higher number of mites causing a collapse in
the south and a higher percentage of mites to brood in the north that
causes the collapse. Both of which leads to migration.

Of course, I'm not willing to concede August is the *only *time this
occurs. I've seen hives collapse from sudden increases in mite numbers in
the months of September and October here in NC, in addition to August. But
it's tough to tell if it's a migration issue or an efficacy issue. If I
check mite counts and I have a 2% infestation rate in August after a
treatment, and then an 8% infestation rate in September, is that because
the 6% were hiding under the caps and I didn't count them (leading to a low
efficacy rate of the treatment), or was it because the 6% migrated into the
hive from a collapsed hive, or was it because the 2% went on a breeding
spree and increased 4 fold in 30 days? I don't know the answer. I just know
I've lost hives to it, and it's frustrating.

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