It's time to treat for grubs

Beneath your feet, working diligently against your perennials and ornamental shrubs... are the grubs.
Getting rid of this pest involves more than just attacking the grub larvae. As with many insects, there are several stages in the grub life cycle.
"White grubs are the larvae of various scarab (or chafer) beetles, including Japanese beetles, June, and May beetles," according to www.MichiganGardener.com
The larvae overwinter in the soil. Then in late June and early July, the adult beetles emerge. 
Beetles can be treated with spray insecticide, but it needs to be applied often because the beetles keep coming. If there's not too many of them, they can be hand-picked and dropped in a container of soapy water. 
If not killed, the beetles will continue to lay eggs in the soil.
"The eggs are invulnerable. The grub larvae are susceptible to several kinds of granular insecticide sold specifically for lawn treatments for grub control. You can work this granular formulation into your flower beds as well," http://www.michigangardener.com/grubs-in-lawn-beds/ 
In addition to granular insecticides such as Dylox 6.2G, there are organic alternatives including parasitic nematodes or milky spore. Both are living bacteria that kill grubs.

Parasitic nematodes also kill cutworms, borers (including squash vine, peach tree and iris borers), corn earworms, cabbage root maggots, weevils (including strawberry, carrot and black vine weevils), wireworms, armyworms and even flea larvae.