A few days ago I noticed divots of grass and soil turned over on the lawn that fronts to the greenhouses at the Botanic Gardens. I asked and the gardener said it was the work of rooks and crows digging for chafer grubs. I didn’t get an answer yet to my full question, which was that the damage seemed to follow the location of the fairy circles that I saw last year (and the year before that).
Chafer grubs are about 2cm long and are the larvae of the chafer beetle. The grubs feed on the roots of grass plants and then in Spring they surface and emerge as beetles. They swarm and fly into the trees to mate, and then the females come down to lay eggs in the soil. The eggs hatch into grubs and begin feeding. And that completes the cycle.
Gardeners control chafer grubs on lawns by soaking the ground with nematodes. They are microscopic creatures that act as parasites on other insects by releasing bacteria into the host’s body to kill them. The nematode then eats the host.