The ants are probably after the honeydew, which is excreted by the scale insects. Scale insects, when mature, are motionless and enclosed in a waxy cover, or scale. They suck the sap from the plant, causing the plant tissues to lose color and wither. Use a soft cotton cloth dipped in mild detergent solution (1/2 teaspoon per quart of lukewarm water) to wipe small numbers of scales off the plant. You can also use a cotton ball or swab soaked in rubbing alcohol to kill pests on contact, or you can remove them. For more information on pest control read: Houseplant Insect Control.

Have you fertilized the plant? The light green leaves may be an indication of iron chlorosis. Citrus plants perform best when fertilized with a fertilizer for acid-loving plants. You can use a liquid soluble fertilizer from spring until early fall. Check the product label before using and follow the label instructions.