There are two reasons to need a grub control fertilizer applied to your lawn. The first is protection if you have had a problems with grubs in the past. The second is if there was an active population of grubs in your yard within the previous year.
Much of the fertilizer portion of a grub control fertilizer is the same as a regular summer fertilizer application. If you would like to read more on the fertilizer portion, the previous blog post covers that topic. The difference between regular summer fertilizer and a grub control fertilizer is the grub control insecticide that is on the fertilizer.
This grub control application is just a preventative application. During the mid-summer, beetles are mating and getting ready to lay their eggs for the next generation. Once you have applied and watered in the grub control application, a grub from a recently hatched egg will not be to survive in your lawn. This is how your lawn is protected with this application.
You are ultimately stopping the grubs before they are big enough to cause any damage. Since grubs only lay eggs once per year, once a timely application is completed, you will have all of the protection your lawn needs from grubs until next summer.
A grub control fertilizer should be used if there is a long history of grub problems or if there has been a problems with grubs within the past year. It is best to try to minimize the use of chemicals. Just remember, any time that you do you a pesticide, including a grub control product, allows read and follow the instructions on the label.