Got a call from my brother yesterday saying he needed DH to come look at his hive as he was sure the bees were all dead and he wanted to know what happened to them. Here's what he found.

Don't wax moths usually move in after the bees have left or are very weak?

That's my understanding Tabby. Something did kill the bees though. DH said there were thousands of bees dead in the bottom. The hive was doing fine a week ago so not sure what actually happened to them.
My brother is going to try again next spring.

Thousands of dead bees sounds like they died too fast to do house cleaning.
Pesticide?

Not sure there are some farms in the area that may have sprayed pesticide but my brother isn't aware of anyone doing so.
My research says wax moths do not kill honey bees directly but each of those cocoons has a honey bee in it along with a worm that fits the description of a wax moth worm.