Don't know for sure about root grubs, but I've gotten to a point where I will not plant beets without planting marigolds with them. Everything eats beets! And they eat beets before anything else in the garden! Leaf borers are the worst on my beets, usually. But when I have marigolds right in there with em, my beets generally do very well and are bug-free. I scatter marigold seed wantonly around my gardens every year and encourage them to go bonkers.