lady beetle vs ladybug

lady beetle and ladybug both are nouns.

Word	Noun	Adjective	Verb	Adverb
lady beetle	Yes	No	No	No
ladybug	Yes	No	No	No

As nouns, lady beetle and ladybug are synonyms defined as:

lady beetle and ladybug: small round bright-colored and spotted beetle that usually feeds on aphids and other insect pests

Other synonyms of lady beetle include ladybeetle, ladybird, ladybird beetle.

lady beetle (noun)	ladybug (noun)
small round bright-colored and spotted beetle that usually feeds on aphids and other insect pests	small round bright-colored and spotted beetle that usually feeds on aphids and other insect pests