Ed Rosenthal say there are negative ROOT MITES, though!
http://www.cannabisculture.com/v2/articles/4664.html
From what you tell us, this seems most likely the case! This may necessitate a completely different regime, as mites are not susceptible to imidacloprid. Spinosad in high strength is minority effective I think - probably also Azatrol/Azamax and other mite treatments. I would also try Spectracide Triazicide, as that is supposed to have effect on mites as well.
http://www.cannabisculture.com/v2/articles/4664.html
From what you tell us, this seems most likely the case! This may necessitate a completely different regime, as mites are not susceptible to imidacloprid. Spinosad in high strength is minority effective I think - probably also Azatrol/Azamax and other mite treatments. I would also try Spectracide Triazicide, as that is supposed to have effect on mites as well.