This is a hard one to answer because it depends on your local environment and where the scale came from in the first place. The oil usually just suffocates any insects it lands on, assuming there was nothing else added to the oil that was more deadly. Since some scale have a hard shell under which the female produces young and protects them until they decide to crawl to another location. It is during this crawler stage that the oil is most effective. Once they settle in a new location and produce a shell of their own, the oil is less effective.
The systemic injections passes through the circulation of the tree and any insects which draw from the tree are then exposed and hopefully die from the pesticide.
You need to get further information before you decide on paying for another treatment next year. Do a search on line for magnolia scale and read what some of the extension sites say about the life cycle of the insect. Pay attention to dates so you can judge how relevant the information is to present day. Talk with your county extension people to see if they have more local information. Environment is important when you are dealing with pests. You will get a better understanding of what treatments are available and have a more educated discussion with the person applying the control measures. You may even decide to contact a different applicator if one exists in your area.
Think about and try to answer the following questions. You may think of other questions as you educate yourself on this pest.
here is one link to info from Cornell which actually comes from Virginia. Do not rely on just one site. Our climate is changing and it takes time for sites to incorporate the effects on the information they provide when it comes to pest species and the plants they attack.
Questions you can consider before asking the Applicator to return
1. Are there other sources of the scale from which they can again move to your tree? Are there any secondary host? How do they over winter? Is there an environment in your area which will let them over winter?
2. What is the effectiveness of the Merit with respect to how long it remains effective in the tree at killing any scale that feed?
3. Based on the life cycle of the scale, will this be long enough to kill two or more crawler generations before becoming ineffective?
4. Can the scale adapt to the Merit and no longer be killed by the material at the concentrations applied?
5. How damaging is the scale to your tree? Does it transmit know diseases to this species of tree and how resistant is the tree to the infections?
6. If the tree dies, will that be a significant loss? Could you or would you be able to replace it with the same species or a different tree? What would that replacement cost?
7. Is this a consistent problem each year? Sometimes weather plays a significant roll in the life cycle of many insect and will reduce or enhance the level of infestation from year to year.
Some trees have a great emotional significance to a family or individuals which can influence your willingness to pay the price for treatment or take risks. You could tell the applicator that you would like to first see if the tree is cured. If not, you can call them back and pay for another treatment. Two years of treatments could be better than one but if you have other sources from which the pest can infest your tree again, you may need to repeat this treatment periodically for the life of the tree. It partly depends on some of your answers to the questions I presented to you.