I think the beetles love living under cover, in the tunnels where I use newspaper covering with mowings as mulch, if you lift that there are loads of black beetles of all descriptions under there. The paths are kind of composted as they were just grass that I mowed short and then covered with the fabric and I guess the bed material moves around. I garden on a steep slope so there is always movement of organic material . I also have quite a lot of NZ flatworms and I have found that where there are flatworm eggs there are beetles which I think are waiting for the moment they hatch out and they can get a lovely succulent meal of baby flatworms.
I don’t grow celeriac, in fact I hate anything that tastes of celery,