Larva in my greenhouse??

I was helping my mom dig in our greenhouse in Netherlands and we dug up alot of our plants roots looked rotten and eaten, we kept digging up these ugly larva and after bit research i came to find out they are a big pest and the larva of a May bug or something like that called "Engerling". How do i get rid of them, in only a 100x10cm area we dug up 9 of these bastards. Pls help