De Jong writes in 2003:

Although the mite Varroa destructor is widely recognized as the most
important problem for apiculture throughout the world (De Jong, 1997),
little is known about why it is lethal for colonies throughout most of the
world, requiring treatment with chemicals, while in other regions, such as
Brazil and some other parts of tropical America, the bees are maintained
without the need for treatment (Rosenkranz, 1999).

This tolerance to mite infestation appears to be influenced by climate and
bee race as honey bees in tropical America are normally little affected, and
Africanized bees are more tolerant than European races of bees.

A key factor, which appears to be correlated with the infestation levels in
the colonies, is the fertility of the mites.

Unfortunately, it is virtually impossible to objectively compare all of the
various studies made on varroa reproduction, due to a lack of uniformity in
the items analyzed and often because the methods are not fully explained.

http://www.funpecrp.com.br/gmr/year2003/vol1-2/gmr0032_full_text.htm

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