Well, look at it this way.
If you try to fight them, and beat them, then they'll just come back stronger than ever.
Which is what bothers me.
Slow adaptation to new environments is fine, draining energy to feed is fine, living for a long time due to changing forms is fine especially for a mythical creature...
“She'd become a governess. It was one of the few jobs a known lady could do. And she'd taken to it well. She'd sworn that if she did indeed ever find herself dancing on rooftops with chimney sweeps she'd beat herself to death with her own umbrella.”
― Hogfather