I've had this Serissa for about a year and a half. It's been struggling due to environmental conditions but I have it in a good spot now with it's own light (8hrs; humidity ~80; temp ~20 celcius). Recently I noticed foliage turning black and dying. Upon closer inspection, I saw these bumps on the branches which almost look like insects. I picked a few off and there is a sap bubble underneath, and it appears to be a shell. The leaves are sticky and around the base of the tree is all sticky, so I thought maybe it was aphids, but I don't see any live bugs anywhere. I'm hoping someone can identify what's happening so I can save the tree! It's lost most of its foliage. There is new growth coming in, but I'm concerned it will turn black and die back like the rest has. Thank you in advance.

Scale insects a big infestation. It has all but killed the plant, you need to removed the scale with your finger and thumb and hope it survives, a systemic insecticide may do it but it may damage the tree so I would just rub them off

Indeed scale insect. But I am not sure they are responsible for the weak tree or just one of the symptoms.

I have never tried this species myself since I have never seen any long time healthy one. In bonsai we need long term health for development. Those experienced people I know have tried all states that it is a very finicky species.

But given the distribution range for this species, all year humid subtropical climate, I would asume the most common problem is high temperature and low humidity in winter. Especially in the early winter, that first cold spell when our central heating kicks in and dry up the air indoors.

Week trees gets bugs, that is natural. Fighting the bugs may not help the tree if the reason for the week tree and the bugs is solved.

I think caring for it as citrus trees is the key. Outdoors in summer and protected from frost in winter. 5c is proboably better than 20c in winter.

According to my observation an infestation of scale insects at some plants can be caused by too much watering for a longer time in an organic soil (standard soil).
The black leaves could also be a sign of too much water !?
I think the scale insects will not kill your plant, you can pick it by hand or you remove it with a small paint brush with firm bristles and warm soapwater.
If the bristles are not firm enough, cut off 20-40% from the tip ,.. cover the soil with 3-4 layers newspaper or foil ,... use a coffeecup fill it with warm water and a little dishwashing soap ,.. now you push the scale insects with the brush from the side a couple times, until they fall off ,.. the sticky parts is their excretions you ca removed that also with a paint brush and soapwater (a softer paintbrush do a better job for that step). Make sure that no soapwater comes into the soil, you covered the soil and you can also tilt the tree to the side.