The best time to apply a grub control solution to a lawn is generally in July. This is the point in the grub worm's life cycle when it is small and feeding close to the surface of the lawn, making it the most vulnerable to chemical and biological insecticides.
The Michigan State University Extension suggests using a grub control product that includes either chlorantraniliprole or imidacloprid, both in granular form. These solutions should be followed immediately by watering, either via irrigation system or a lawn sprinkler. This allows the solution to soak into the lawn's root system where the grubs feed.