looks like spider mites.
i agree. check under the leaves with trich scope and look for the mites.
http://www.drugs-forum.com/growfaq//1234.htm
i had them once and i used doktor doom fogger a couple times and it held them in check until harvest. but there are many threads here on mites.
in addition to dr doom get some azamax...feed it to plants will stop mites from maturing.
your also going to need to reapply in about 7 - 10 days to kill the hatchlings from the eggs you didnt kill on this nuke attack.
remember to clean your grow room up in good shape and spray it after this grow is done. you dont want any carry over to the next grow.