I had mites only once with my Green Trees when I first received them.
I used 1ml per litre of water. I removed the water bowl from their cage and sprayed the cage as well as the snakes.
The vet recommended doing this for 3 weeks at 3 day intervals. Making sure you remove water bowl every time. You can put the bowl back once snake and cage are dry. I also cleaned and disinfected there enclosure each time as well.
This is to eradicate any newly hatched mites etc.
The injections work from what I have heard, but what do you do about the ones you may have in the enclosure, also it does not have an effect on unhatched eggs, which are generally deposited somewhere dark and nice and warm in the enclosure.
I sprayed my snakes and enclosure 3 times over 9 days and then stopped. I have never encountered a mite since then. That was a year and half ago.
Also my vet said that the diluted ivermectin mixture must be remixed each time. After 24 hours the invermectin will begin to degrade within the water and therefore reduce the effectiveness.
I am not an expert and only had to go through this once ( knock on wood) but from my experience it worked very well and I have had no problems since.
The vet I went to has a practice that does see alot of reptile cases and she is very knowledgable.
I hope this helps you out,