There is a free PDF of a new Washington state study on varroa thresholds:
Optimum Timing of Miticide Applications for Control of Varroa destructor (Acari: Varroidae) in Apis mellifera (Hymenoptera: Apidae) in Washington State, USA. James P. Strange and Walter S. Sheppard
http://esa.edoc.com/economic/v94n6/v94n6p1324.pdf