• Spider mites may be a problem indoors 
• Mealybugs may be a minor problem indoors
• Scale may be a minor problem indoors
• Aphids may be a minor problem outdoors or on a patio
• Drooping older leaves and a dull appearance may indicate water stress

Mite damage on the Congo has a distinctive look that makes it easy to diagnose this problem.  Small round spots on the leaves usually indicate mites are present.  See the link below for a picture.  In the nursery we have never seen a large population of mites on a Congo leaf, but we have seen a large number of spots caused by only a few mites.   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.  Miticides registered for interior use and compatible with predator mites include Sultan, Shuttle O and Floramite SC.    

Mealybugs and scale 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 disrupt the insect's  life cycle.  

On green Congo's, scale is a bigger problem than mites, at least in the nursery.

The green Congo will grow larger than the Rojo Congo.  This philodendron remains upright as it grows larger producing a very nice looking large plant.  The Rojo Congo tends to lean and sprawl as it grows larger so that it never quite reaches the same dimensions of a green Congo plant.  We have grown the  green Congo up to 55 inches tall in a 7 gallon pot in the nursery.  

Aphids may be found on the newest growth even before the new leaf uncurls.  These insects can be controlled with the same treatment as mealybugs and scale.

Other Links:

Mite Damage on Philodendron 'Rojo Congo'

Horticultural Oils 

Safari Pesticide Label