Milky spore is very effective for controlling the beetles.
You will not see full control until the second year as the spore needs to complete the lifecycle and discharge the future spores into the ground, after killing the beetle larvae from the beetles you see the first year.
Then, you may not need to treat for another 20 years or so. The spores are regenerated in the soil by other grubs completing the spore cycle. But at a drastically reduced number of beetles. You will always have some fly in from outside the treated areas, but the numbers will be reduced from what you are seeing now.