Note:  most Plumeria seem resistant to scale insects, 
but the best treatment is high pressure, water removal.

1.  Keep the growing area clean and free of fallen leaves.
(Keep the Greenhouse area as clean as possible)

2.  Carefully remove and place badly infected leaves into trash bags.

3.  Blast all existing leaves clean with a water hose.

4.  Move plants into a more open and sunny area.

5.  Improve the air circulation in greenhouse area, and as weather permits remove plants, and blast them clean and free of all scale insects and mites.

6.  Insecticides : Bug-B-Gon Multi-Purpose Garden Dust 
or comparable insect dusting product.