My recently bought rose plant is dying. It was looking week from the time I bought (it was the last plant in the shelf, and I did not want to miss that one, a rare variety). I dug it up today to find that the root was surrounded with many small insects (see image below) and some worms. I currently have the plant in a container of water.

I would appreciate your advice on two things:
1. Whats the best way to kill those insects?
2. Should i plant the rose plant back in a pot or let it be in the water container for a while? Someone on a blog suggested keeping plant whose roots were attacked by ants to be kept in water container for a while, but I'm not sure if that will be a good idea for my young rose plant.

Thanks in advance!