Heavily damaged turf will feel spongy underfoot. It will peel back from the soil like a carpet because the root system has been chewed away from the leaf. Above ground, there will be thin, stressed turf. This stress decreases drought tolerance and, ultimately, increases the chance of weed invasion.
Even when grub populations do not cause visible damage, their predators might. The grubbing activities of vertebrates like raccoons, skunks and crows can be highly problematic. It is common for this feeding damage to be more troublesome than direct grub damage.
Whether the damage is directly caused by the grubs or predators, on sports field it results in an increased risk of sports injuries.