If you are going to do plant health care, you need to know the common insects and diseases of your area. There are probably only about 20-30 that are common enough to have treatment protocols developed and registered for use.
Generally, around here, there is only one insect and one disease for each tree species that we can treat. This doesn't mean there aren't more, but only a few that we have products that are registered for use.
So, once you know what the probable pests and treatments are, you can determine:
- how prevalent
- demand for the service
- how much to charge for the service
- what the success rate is
Generally for diseases, we only claim to be able to control the disease, not cure it.
Insects will depend on whether we can use a contact insecticide only or a systemic. Generally we don't claim a 100% control rate. Don't set yourself up for trouble with exaggerated success claims.
When you are selling PHC, it's more effective to say, this is the disease diagnosis, this is the treatment protocol, this is the expected outcome/success and this is the price. I have found that some people will pay on the hope that something will work to save their tree.
To answer your question, slightly paraphrased, does PHC (disease and insect control - injections and sprays, fertilization, vertical mulching and radial trenching) make money. Uncategorically, yes. But you have to invest some time and effort into planning. It's not something that you can go and pick up as a fill in job.