My dad grew his teeth in the dirt, but he was self educated and knew about genetics.
He worried about the danger of genetic replication, especially in hybrid corn strains.
Bringing in a non specific genetic strain is always kinda dangerous. It's a matter of genetic diversity. If one breed of bees has a problem, another might not. I seriously doubt it's the genetically altered crops. It's possibble, but unlikely.
Pesticides and herbicides are not as widely used today. Bo Weevils in cotton and Aphids still are pests tho. They need to find the bees to see if they are diseased, or poisioned. Sounds like they vanish like some UFO abduction or something. I'm sure they check the field and aroud the hive. Damned if I know. I am thinking disease by genetic sigularity.