Normally we only worry about spider mites in soybean, but when conditions are just wrong they can impact corn as well. Corn that has been treated with some fungicides and insecticides (particularly a broad-spectrum pyrethroid) are more likely to experience spider mite outbreaks under hot, dry conditions. A general guide for treatment of twospotted spider mites in corn is 15-20% of leaf area covered in mite colonies, with continued dry weather expected. Control is most likely to be economical from pre-tassel through soft dough, and unlikely to provide a return from dent onward.
Most-excellent field crop entomologist Chris DiFonzo at Michigan State has put together a nice table listing spider mite products for corn, soybean, dry bean, and sugarbeet. Always check your labels!