Grubs of cockchafers can kill plants by feeding on their roots and destroying them. Early symptoms include yellowing canopies and wilting leaves.  The adults, on the other hand, eat the flowers and leaves. Too many of these beetles can quickly cause significant damage to your garden plants. In fact, these pests were so feared that in the 1300s, they were once tried in the French court and ordered to stop destroying crops! Of course, these bugs didn't give two hoots to the human courts.