Spider Mites are a challange for any gardener. They are a special challange for a butterfly
gardener. Chemicals which kill spider mites will also kill caterpillars.
There IS one thing which spider mites hate with a passion - water.
The use of this two-step 'treatment' is almost always a
totally effective means of eliminating them. First, cut the infected
plant and dispose of the leaves and cuttings in a sealed container. Second, place a water sprinkler on the plant
for two - three days. Your problems should be over!
Be sure to dispose of all cut plant material properly if it has disease or pests. Place the cuttings, empty pots, and leaves
in a black plastic garbage bag, seal it totally
(you can spray throughly inside the bag with insect spray if you wish), and place in the garbage.    
Do not compost this material!
Although we use Malathion for milkweed aphids, it does not seem to affect spider mites.
Insecticidal soap doesn't make a noticable difference at our farm.
We encourage use of soap or insecticidal soap for your first attempt to rid plants of any pests before using poisionous chemicals.