To trouble (someone, or a group of people) through repeated military-style attacks.
To persistently bother (someone, or a group of people) physically or psychologically when such behaviour is illegal and/or unwanted, especially over an extended period.
To annoy (someone) frequently or systematically; to pester.
To put excessive burdens upon (someone); to subject (someone) to anxieties.
To inhabit a place in unpleasantly large numbers; to plague, harass.
To invade a host plant or animal.