Indoor orange trees in the mid winter months often suffer from red spider mite. Its a very small spider very hard to see that attacks the softer areas of the foliage i.e the newest growth. You can treat it with a organic soft soap spray or you can you an insecticide such as Provado which is systemic and is more effective. Also Red spider can also be effectively controlled with biological control using the predatory mite Phytoseiulus persimilis.
Its also a good practice in the summer place the plants out side to a harden off the growth and the rain will help to remove the red spider mite as well as the dust on the leaves