Question

Grub  

Bark/Tree Damage   I have a mature walnut tree in my yard.  It has always had a large round area on the bark and I never paid much attention to it. Today I noticed it had grown to an area about twenty-four inches in diameter and upon further examination found that it was populated with white grubs and elder bugs.  The grubs were about an inch in length, a quarter inch in diameter and are mining in a vertical, concentric pattern on one side of the tree.  The tree does not produce.  Can the grub be identified from the photos and is there a method of treatment that will preserve the tree?

Answer
Looks like one of the borer type insects. It is difficult to ID the exact species from the larvae but from what you describe it is a borer. I would spray the bark area with an insecticide called Merit or Onyx. This will kill the insects under the bark. Borers usually attack trees under stress so I would fertilize the tree with 10-10-10 fertilizer at the rate of 1 lb per inch of trunk diameter scattered around the tree and watered in good. Apply just before a rain storm and you will not need to water. Apply now and again in the spring. This will increase the overall health of the tree. Check with your local garden type store for these insecticides.