Butterfly Armegeddon

So one morning last week I noticed thousands (literally!) of yellow butterflies in a field one the way home from my kid's school.

I dreaded going to and from school because of the amount of butterflies I hit each way.  However, as I was getting this post ready, I read that they can actually be a nuisance pest on alfalfa fields - which is exactly where I see them.  I still don't like to hit them, but now I don't feel quite as bad.
They are called Colias, Clouded Yellows, or Sulfurs.  You can read more here.