Q From Kari Frisch: I was wondering if you knew the origin of the idiom open a can of worms. I’ve heard that it might be associated with Pandora’s box.
A To open a metaphorical can of worms is to examine some complicated state of affairs, the investigation of which is likely to cause trouble or scandal and which you would much prefer was left alone. A recent example appeared in the Washington Times: “Uncovering past ownership, however, can open its own legal can of worms in exposing art theft.”
It has long since become an overused and overrated journalistic cliché, which rather lends itself to mixed metaphors: “Africa is a huge can of worms and we can no longer stick our heads in the sand” (from the Daily Record of Glasgow of 17 March 2006). The earliest example I’ve found is from a syndicated article in the Ironwood Daily Globe of Michigan in 1951: “The question of command for Middle East defense against Soviet aggression is still regarded as ‘a can of worms’ at General Eisenhower’s SHAPE headquarters here.”
The evidence suggests that the original cans of worms were real cans with actual worms in them, collected as bait for fishing. (The can in this case being a smallish metal container with a handle and a lid.) Here’s one in a largely forgotten work of 1914, Diane of the Green Van, by Leona Dalrymple: “There are times, alas, when even fish are perverse! Thoroughly out of patience, Diane presently unjointed her rod, emptied the can of worms upon the bank, and returned to camp.” Fishermen have told me that the most annoying aspect of opening a can of worms is that, being live bait, they crawl out and are difficult to put back. So there may well be an association with the idea of Pandora's Box, as you suspect.
It’s easy to see how an angler — more probably a non-fishing friend or relative of an angler — who opened a can containing a wriggling mass of worms would see it as something that was best left closed and unexamined.