Has it been too wet or too dry?

Leaf drop can be normal due to lower light and dry air indoors. Mandevilla can also fall victim to insects such as whitefly, scale, mealybugs and red spider mites. From your description, I can’t determine which one. My guess is that it could be spider mites-which form webbing in the leaf axils. They are very tiny and difficult to see. If so, try an insecticidal soap spray as that will be the safest to use indoors. If not give me a description of the insect so that we can determine what course of action to take.

Jen
Thanks for responding to my e-mail dated 11-25. You are correct about the insect. Can spider mites kill or harm my plant? How often/long should I treat the plant for? Since its indoor how often should I water? You have been very helpful. Hope you and your family have a safe and enjoyable holidays.

Some of the symptoms of spider mite infestation are tiny dots or stippling that causes a bronze, grayish mottled look. Eventually the leaves will start to turn yellow/brown along the veins and drop. In severe infections you will also see webbing on the plant. To be sure you have spider mites, take a white sheet of paper, place it on the underside side of the leaves and tap on the leaf. If you see tiny pindots moving on the paper, then you have spider mites. Isolate your plant from any others because they spread easily. To control the infestation, spray the plant with insecticidal soap every 3-5 days for two weeks. Be sure throughly spray the plant getting the underside of the leaves. If insecticidal soap does not do the trick try Neem oil. Spider mites love the hot, dry environment that is indoors during the winter. Water your mandevilla just enough to keep it from wilting-keeping it on the dry side and mist it to increase the humidity. If the infestation gets out of control, cut back the plant, place it in a cool(garage) place and let it go dormant for the winter. More on insecticidal soap