Excerpts:

Varroa destructor is an ectoparasitic mite of the adult honeybee,
which parasitizes the bee brood. This mite has killed millions of
honeybee Apis mellifera colonies, worldwide, eliminating wild
populations throughout Europe and North America, and resulting in the
loss of billions of dollars in agricultural production. Varroa kills
host colonies indirectly by providing a new transmission route for a
few naturally occurring honeybee viruses such as the deformed wing
virus.

In Apis cerana (the original host of Varroa), mite reproduction occurs
only in the small number of sealed male (drone) honeybee brood cells.
Consequently, mite populations within an A. cerana colony are low (<
800) and no adverse effects are seen. In A. mellifera EHB colonies, V.
destructor also reproduces in the much more numerous worker brood
cells, enabling mite populations to increase up to 2000-fold annually,
causing colony death within one year.

However, mite populations in similar-sized AHB colonies stabilize at
1000–3000 mites per colony, allowing colonies to survive indefinitely,
although the resistance mechanism, until now, has remained elusive.
The Africanized honeybee (AHB) has a unique tolerance to V. destructor
that is not present in the A. mellifera European honeybee.

The short adult longevity of AHB (21 days versus 25–180 days for as a
result of the tropical or sub-tropical climate indicates that >12,000
mites are needed to kill an AHB colony. Therefore, although Deformed
Wing Virus is present in AHB and A. cerana colonies, mite populations
stabilize at levels well below that required to kill the colony.

It is unlikely that AHB evolved Varroa tolerance after the AHB hybrid
was created as a result of increased hygienic behaviour or brood
attractiveness because such factors are unlikely to lead to a
stabilized mite population. Instead, tolerance has probably resulted
from pre-existing resistance characteristics fortuitously coming
together in the hybrid. That is a high level of mite offspring
mortality in worker brood and a short life span in the adult honeybee.

from

Africanized honeybees have unique tolerance to Varroa mites
by Stephen J. Martin and Luis M. Medina
TRENDS in Parasitology Vol.20 No.3 March 2004

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