Hi Chris,
It is caused by Varroa. You may find this article by the Maine State bee
inspector Tony Jadczak interesting.
http://mainebeekeepers.org/information-for-beekeepers/when-disaster-strikes/

I have had people bringing me bees from dead hives wanting me to check them
for Nosema, it seems they always blame that first, over half the bees in
their sample have had stunted abdomens so I know it was Varroa collapse. I
ask them the product they used and the dates they used it. In many cases
the product is used incorrectly, not a fault of the miticide and/or they
treated to late in the season.

Karen T-K
Maine

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