SQL Insert Performance
Some quick remarks:


Naming is important. I don't consider dt, conString, con, etc. proper names since they don't tell me enough. There's no need to abbreviate. selectcmd doesn't follow Microsoft's rules about compound words, and dttemp even contains the dreaded "temp".
You haven't encapsulated the MySqlCommand which creates the table in a using.
MySqlCommandBuilder inherits from a class that implements IDisposable, so perhaps that too should be used in a using statement?