psyllid vs plant louse

psyllid and plant louse both are nouns.

Word	Noun	Adjective	Verb	Adverb
psyllid	Yes	No	No	No
plant louse	Yes	No	No	No

As nouns, plant louse is a hypernym of psyllid; that is, plant louse is a word with a broader meaning than psyllid:

psyllid: small active cicada-like insect with hind legs adapted for leaping; feeds on plant juices
plant louse: any of several small insects especially aphids that feed by sucking the juices from plants

Other hypernyms of psyllid include louse.

psyllid (noun)	plant louse (noun)
small active cicada-like insect with hind legs adapted for leaping; feeds on plant juices	any of several small insects especially aphids that feed by sucking the juices from plants