With most of lawn care, timing is important. July and early August is the ideal time to prevent grubs. Applying imidacloprid (Merit for example) each year will prevent grubs from occurring. Otherwise, grubs may cause considerable damage to your lawn.
An understanding of the life cycle is important. In June & July the adult stage is the beetle (Japanese, Oriental, Chafer). They will lay their eggs in July and August.
Imidacloprid will prevent the eggs from hatching. Without this control, eggs will hatch and develop into grubs. Grubs will feed, and cause damage, thru the Fall and then move deeper into the soil for protection against the winter freeze. In the Springtime as the soil thaws, grubs will come back up to start feeding on the roots again until late Spring when they become the adult beetle... and the cycle is complete.
Grubs will feed on the roots of your lawn and death will occur. Grubs will also feed on other garden plants root systems. Often you will find a lawn in early Spring or Fall that looks like it has been rototilled. Well, it has been tilled by animals (skunks for example) searching and digging for food (grubs).
Imidacloprid is the most widely used insecticide worldwide. It is toxic to insects, and less so to mammals. In fact, a Veterinarian may prescribe a flea and tick treatment for a dog that will contain imidacloprid.
Most importantly, before using any lawn care product, read and understand the product label. Proper use and calibration is very important. Or, alternatively, hire a licensed lawn care professional.