There have been a few reports of spider mites and a little bit of spraying in cotton.  This does not come as a big surprise as we have been hot, dry and spraying aggressively for thrips.  The spider mite treatment threshold in cotton is when 30-50% of plants are affected and spider mites are present.  Honestly, it takes a little professional judgement on whether a miticide application is needed based on the severity of the symptoms and how much stress the plants are under.  Cotton that is not under drought stress can tolerate more spider mites.  There are a number of good miticides listed below.  Many people will choose one of the abamectin products such as Agri-Mek, Abba or Zoro because they are economical (and they work).  Dicofol 4F at 32 oz/acre also works well, especially early in the season, and is relatively inexpensive.  Bifenthrin products such as Brigade will usually not provide adequate control in early season cotton.

Note:  The herbicide Liberty (formerly Ignite) has pretty good activity on twospotted spider mites.  You should expect some control of spider mites when it is used for control of weeds in Liberty tolerant cotton.  Liberty is not labeled or recommended for this purpose!!!  However, you may be able to skip or delay treating for spider mites if you are making an application of Liberty. 

Recommended Miticides for Cotton (click to enlarge)