[Igelau]

You need better reasons than that. I doubt this is the case with monarch butterflies, but there are certainly lots of plant/animal species that are aesthetically pleasing but become invasive and wholly detrimental.

If I'm not mistaken, monarchs are poisonous due to their diet, so there's some potential for negative impact if they overrun a niche that some other animal was planning to eat.

[redis_mlc]

Monarchs have migrated for thousands of years between remote locations, long before humans had a large impact on the environment.