Every lawn has grubs in the soil.  However, in order to cause damage to the lawn there needs to be ten or more grubs per square foot.  For most of the year, grubs remain deep in the soil and in sunny areas.  Once they become active, they feed on the root system of the grass and cause major damage to your lawn, sometimes irreversible.  In order to prevent grubs from destroying your lawn, a granular product is applied in mid summer.