Preventative grub control is best done on small larvae after eggs have hatched in July. Curative treatments can be done later if infestations are found. The following are some grub treatmetn decision considerations from Rutgers University.
That time of the year is fast approaching. The beetles will be flying and checking out your turf for nice egg-laying sites. Oriental beetles will come first followed by the more in-your-face Japanese beetles within the next three weeks. This is the time to think about white grub management. The question is whether to apply a preventive application that is more expensive and often not necessary, or to take your chances and apply a curative application later if monitoring suggests damaging grub densities.
July would be the best time to bring out any preventive application. However, such preventive applications should be restricted to areas with very low damage threshold and tolerance, areas with a history of white grub infestations, and areas with high beetle activity (egg-laying) in June-July. Remember that the dominant white grub species may vary in Delaware and may include the oriental beetle, Japanese beetle and Asiatic garden beetle. Other species like northern masked chafer and green June beetle can also cause damage in some locations. Keep in mind that species attacking turf can vary to some extent from year to year and can vary dramatically from site to site.
Information adapted from "To Treat or not to Treat: Update on Preventive White Grub Treatments" by Albrecht M. Koppenhöfer, Ph.D., Specialist in Turfgrass Entomology, Rutgers University in the June 26, 2008 edition of the Plant and Pest Advisory, Landscape, Nursery & Turf Edition, Rutgers Cooperative Extension.