As explained in the Jan 2018 Bee Culture magazine.
Samuel Ramsey, Ph.D student from Univ. of Maryland has turned up some rather astonishing research on Varroa: in a nutshell, Varroa do not feed on Honey Bee blood as most including myself believed. Varroa feed on the bees fatbody or liver as commonly called in mammals. Insects do not have a liver, I make the analogy to keep things simple.
This explains how Varroa can meet the energy requirements to lay an egg 1/3 the size of the mite. Scientists have been scrambling to figure how Varroa could acquire so much energy from blood which is mostly water. Now we know, the fat bodies are high in energy so this makes total sense to me.
Blessings