The idiom “worm in the apple” means something that was thought to be good turns out to be bad, something that looks good on the outside but is rotten on the inside. The phrase originates from the literal presence of worms, specifically the larvae of the codling moth, within apples. They burrow inside the fruit, making them undesirable and inedible. The earliest use of the term is found in an 1850 issue of the newspaper New England Farmer.
