liquid derris 5 will get rid of your spider mites better than miracle gro bug spray ever will no bother.. as derris 5 is made to get rid of red spider mite as well as other things...plus you make your own solution of it per litre of water so you can have it that bit stronger if you wish..miracle grow will just be your standard bug spray already watered down....and it shouldnt damage the plant either..
http://images.google.co.uk/imgres?im...%3Den%26sa%3DG
you can get liquid derris 5 at any decent garden centre - even the big sheds..focus, b&q etc...