leaf bug vs plant bug

leaf bug and plant bug both are nouns.

Word	Noun	Adjective	Verb	Adverb
leaf bug	Yes	No	No	No
plant bug	Yes	No	No	No

As nouns, leaf bug and plant bug are synonyms defined as:

leaf bug and plant bug: small bright-colored insect that feeds on plant juices

leaf bug (noun)	plant bug (noun)
small bright-colored insect that feeds on plant juices	small bright-colored insect that feeds on plant juices