A sharp, unwelcome rebuke. To have a flea literally inserted in your ear would be an unwanted nuisance, just as being scolded, even if deserved, would be. The British use the phrase to mean “put a bug in the ear”: to plant a suspicion. The French “put a flea in the ear” to arouse amatory feelings, hardly an aphrodisiacal image (any more than a Spanish fly would be).