Just about all plumeria plants will drop all their leaves by this time of year unless you are growing them under artificial lighting. The winter is the plumeria plants dormancy period. Leafs will start to grow back in spring. Remember to reduce watering of the plant and protect it from cold weather.
During the dormancy period, we water our plumerias about once every week (our winters are very mild). In other parts of the USA we recommend watering them about once every other week. Check for stem softness and if it too soft give a bit more water. Use time released fertilizer during the winter and then a bloom at first signs of leaf growing for a month (late winter/early spring).
If you are growing your plumeria plants in containers hold off transplanting them until next spring after you have a few leaves to prevent too much plant stress. Keep the plants out of weather under 55 degrees and always make sure the soil drains well when watered.
Plumeria plant pests in our area occur less during the winter time than during the summer. Currently we are using a soap based insecticide once a month as a preventative measure during the growing period and once twice (every other month) over the winter period. The most common problem are spider mites. If you get spiders mites on your plumerias during growing season – remove as many leaves as possible and then spray the plant once every 4 days for three weeks. Remember to spray the branches and under the leaves too not just the top.