Good question Andy. Traditionally grub treatments are most successful when applied in fall when grubs are present and actively feeding on grass roots. In addition, there are some grub preventative products available that are best applied in late May and June. These preventatives establish their presence in the soil and kill the grubs right as the eggs hatch and the grubs begin feeding. Here is a link to a great grub control posting from Michigan State University Extension that gives you the best information on year-round grub control: https://www.canr.msu.edu/news/how_to_choose_and_when_to_apply_grub_control_products_for_your_lawn.
  Good luck with your lawn and I hope this gives you all the information you need.