psylla vs plant louse

psylla and plant louse both are nouns.

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

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

psylla: 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 psylla include louse.

psylla (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