Most likely leatherjacket infestation, hopefully not chafer grubs. You can buy a nematode solution to water onto the grass to kill them off, but it can only be used when the temperature is above about 60 deg. F. Nemesys make it I think, only available on the internet. Otherwise, try laying a sheet of thick plastic on the grass in the evening and then lift it up in the morning - you should see the leatherjackets on top of the grass, when you can pick them off, although if you've had no rain recently, this might not be successful.