Contact your local Extension office and ask about its plant diagnostic services. It is best to know what the problem is before attempting controls.

Because Hoya is a tropical evergreen, mostly climbing shrub belonging to the Milkweed family, it is possible that some insect larvae (worm) or beetle has been eating the leaves. For larvae control, you can use the organic insecticide that contains a Baccillus named Bt sold as Thuricide, Dipel, or Biological Worm Control. This product will not work for beetles, so if you use it and the damage continues you will have to get an insecticide labeled for beetle control.