Grass grub adults emerge in October and are active until mid-December. The cooler the temperature, the later they emerge. The adults will start to emerge once the soil temperature reaches over 10 degrees.
The grubs that you see in the lawn are the larvae of beetles. These grubs are C shaped, off white in colour with a dark head. They eat the roots of the grass, causing irregularly shaped patches of wilted, dead or dying grass in April and May and again in August to mid-October. Lawns that are heavily damaged by grubs will have a yellowish tinge and will feel spongy when walked on.