Migratory butterflies

Butterfly in the rockmelon patch

It was difficult to catch a photograph of this butterfly, though at this time of year I feel I am being bombarded with them.  They are a non-native species, apparently introduced, I think from the US, in the late 19th century.  They are 'Wanderer' or 'Monarch' butterflies.  Big and spectacular, I feel a little bit uncertain about them,
a) because they are toxic, adult and caterpillar, and make animals who try to eat them sick, and
b) because the plant they breed on is a noxious weed and should be pulled out.
 But they are beautiful, and even the weed the caterpillars rely on is an interesting plant.  So, depending how vicious I am feeling, sometimes I leave the weed, and sometimes I pull it out.