> To my surprise, I often find few or no mites in the drone brood of my trap frames.

Following that:

"Examination of drone brood? Not viable for diagnosing varroosis"

Our results showed that it is not possible
to calculate the size of the varroa population
parasitising a colony simply by examining
the infestation rate of drone brood. This
is probably influenced in part by the cycles
of drone brood production in each colony
and in part by the cyclical nature of the
infestation of cells by varroa. The parasite
load of drone cells was seen to vary from
one- to six- times in the space of a week,
without any relation to the actual varroa
population. This confirms the observations
of Ritter & Ruttner who also observed the
weakness of the infestation of drone brood
as a measure of colony infestation.

From: "The removal of capped drone brood: an effective means of reducing the infestation of varroa in honey bee colonies"
JEAN-DANIEL CHARRIÈRE, et al. Bee World 84(3): 117–124 (2003) © IBRA

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