Gently lift up the sod. If it easily pulls away from the ground the root system may have been eaten by grubs. Cut into the soil and look for their presence. More than 10 per square foot of lawn could indicate a problem.
The presence of beetles may be an indication that they are laying eggs in your lawn. Tan colored chafer beetles are active just after sundown and Japanese beetles can be seen flying during the day feeding on ornamentals.