The affix āru comes, in the sense of 'agent having such a habit etc.' after the verbs śṝ 'to to injure' and vand 'to praise, to salute'.,
As शरारुः 'noxious, a mischievous animal'; वन्दारुः 'praising, a bard'.,
शॄ-वन्द्योर् आरुः
Adhyāya 3 · Pāda 2 · Rule 173