mealy bug vs mealybug

mealy bug and mealybug both are nouns.

Word	Noun	Adjective	Verb	Adverb
mealy bug	Yes	No	No	No
mealybug	Yes	No	No	No

As nouns, mealy bug and mealybug are synonyms defined as:

mealy bug and mealybug: scalelike plant-eating insect coated with a powdery waxy secretion; destructive especially of fruit trees

mealy bug (noun)	mealybug (noun)
scalelike plant-eating insect coated with a powdery waxy secretion; destructive especially of fruit trees	scalelike plant-eating insect coated with a powdery waxy secretion; destructive especially of fruit trees