My first thought was spider mites--or mites of some kind.
They are pretty invisible...
Using a Systemic on the soil as well as on the plant may help.
"Bayer has a good spray called "Insect, Mite and disease control" Not 100%
sure of the name. Home Depot carries it.
http://www.homedepot.com/p/Bay...
Hope it helps.... Gita