蟊 refers to a type of insect pest that eats the roots of rice seedlings, but in modern Chinese it's almost never used alone — it appears mostly as a bound morpheme (b.f.) within the compound 蟊贼 (máo zéi, 'a pest; a harmful person'). You won't encounter it as a standalone noun in everyday speech or writing. Learning it helps with reading classical or formal texts where 蟊贼 is used metaphorically for corrupt officials or troublemakers.