• Mealybugs may be a problem indoors
• Mites may be a problem indoors
• Aphids may be a minor problem outdoors
• Older leaves turning yellow is first a sign of water stress and second light stress
• Light stress plants will have a very straight appearance as they try reaching for a light source
• Disease should not be an issue indoors

     Mealybugs can be removed with a systemic insecticide drench and hand cleaning.  If pesticides are not an option, try horticultural oil directly sprayed onto the insects.  A second and third treatment, if using oils,  will be necessary.  Space out the sprays at 3 week intervals to break up the mealybug life cycle.  Live mealybugs are most likely buried in newly emerging foliage especially in new leaves that are just starting to uncurl. 

     Mites like the Sterling much more than they like the Dieffenbachia 'Camouflage'.  In fact, all insects seem to attack the Sterling more than the Camouflage, at lease in the nursery setting.  Mites should be primarily controlled with predator mites.  Neoseiulus californicus,(Spical, Spical Plus, Californiline) and Phytoseiulus persimilis,(Spidex, Bio Persimilis, Phytoline) are two excellent choices for spider mite control.  If a mite infestation is exceptionally bad, prior to the release of predator mites, spray the plant with water or horticultural oil.  Both will provide some knockdown control of spider mites and will not leave a toxic residue that may effect predator mites.  Red colored spider mites, most likely the southern red mite Oligonychus ilicis or a red phase of the two spotted spider mite, are more difficult to control with predator mites and this plant does attract them.  Neoseiulus californicus will have limited impact on the red color spider mite, and Phytoseiulus persimilis will provide no control on these pests.  At least this has been our experience in the nursery.  The red mite is more active in the cooler months of the year, mainly October through March, in South Florida.  Miticides registered for interior use and compatible with predator mites include Sultan, Shuttle O and Floramite SC.

    Aphids can be controlled with the same treatment as mealybugs.

Other Links:

Progression of Low Light Stress on Dieffenbachia 'Sterling'