A recent study suggests that high temperature and low humidity
probably account for success of some honey bees against varroa:

> Varroa destructor is currently the most important pest of the honeybee Apis mellifera L., causing the death of millions of colonies during the past 50 years. The only genetic line of A. mellifera which has consistently been found to be resistant to V. destructor is the Africanized honeybee. Mite resistance was first observed in Brazil and the low level of mite fertility was considered to be the reason. More recently it was suggested that the less virulent Japanese mite genotype found in Brazil was the main reason for mite resistance of AHB.

> Three independent studies carried out in different tropical regions of Mexico showed that AHB colonies were able to survive without the application of any mite control measures, while the mite population cycled between 1000–8000 mites. In sharp contrast, European honeybee (EHB) colonies kept in the same region, died within a year because of the rapidly increasing mite population. Because the mite genotype (Korean) and mite fertility levels were similar between EHB colonies in the UK and AHB colonies in Mexico, this suggests that other factors, different from those found in Brazil, underlie the resistance of AHB to V. destructor.

> Though we are currently unable to explain the link between any factor and offspring mortality, there are three possibilities:

(1) Offspring mortality is linked to honeybee genetics; but as this
factor is similar in all the colonies studied, it would then be due to
a dominant trait of AHB. In previous works on EHB in tropical climate,
there were strong differences between AHB and EHB colonies, which
strengthens the hypothesis of a dominant trait of AHB.

(2) Offspring mortality is linked to mite genetics; this is unlikely
as mortality rates change in the various studies of the same mite
genotype (Korean).

(3) Offspring mortality is linked to environmental factors, e.g. the
combined effect of high temperature and low humidity. This hypothesis
is the most likely, since the offspring mortality cycles mirror
changes in the external environmental conditions.

-- 
Luis MONDRAGÓN, Stephen MARTIN, Rémy VANDAME (2006) "Mortality of mite
offspring: a major component of Varroa
destructor resistance in a population of Africanized bees"

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