First you need to identify the pest. Grub worms would be eating the roots so you would not see leaf damage. There are many more common reasons for plants to die in an area instead of underground larvae. The drainage and quality of the soil is the most common reason for failure. Either heavy clay so the roots rot or too much sand so the roots dry out. To check for grubs cut up a potato into sections and pierce each section with a stick. Now bury the potato sections at different levels around the area leaving the stick protruding from the ground. After a few weeks you can pull up the potatoes by grabbing the sticks and check to see if there are grubs burrowing into the potato chunks. If you find larvae send an image and I can better help to identify what is causing the problem. You might find that the problem is cultural (wrong plants for the area, poor drainage or too dry) rather than an underground pest. Keep growing, Marianne Binetti