It is very likely the adult form (beetle) of the grass grub. A spray of natural pyrethrum (available from organic suppliers) applied in the evening (when the bees have stopped foraging and the grass grub beetles come out of hiding to feed), best on a warm night and repeated once or twice after three days, will reduce the damage. To affect the grub stage of this insect, biological control can be utilised in applying a product called Invade (which contains a naturally occurring disease for the grubs). Avoid rye-grass and clover mixes. Lucerne and some other grasses are less attractive and palatable to the grass grub beetle.