I've always had a stoner theory that bees visit cannabis plants much like how sharks visit cleaner fish stations. Sharks have learned that there are small fish that will eat the harmful things on their body, so they don't try to eat them.

Well cannabis is said to have some mild antibiotic and antiviral properties.. mild enough to not really be considered one of its benefits for humans. I think it provided a benefit to insects, especially bees. Those mild antibiotic properties wouldn't be mild to bees.. and the time period when cannabis was outlawed was close to when the sharp increase in colony collapse happened. Even though cannabis evolved to be wind pollinated, I think it would provide a great service to insect populations by providing them a cleaning station to rid themselves of harmful bacteria.

Granted correlation does not equal causation and there are other likely factors as well, I feel pretty sure that when we took cannabis out of the wild, we basically gut punched the insect world. With the increase in harsher environments where harmful bacteria and viruses thrive, we took away something that helped them fight them.