Question
I have a 60+ year old Crepe Myrtle in my yard that has some sluglike "creatures"  eating away in an existing knothole.  I have tried various treatments, snail/slug  killer, a pesticide, but I cannot kill them all.  I suspect they may be hatching further down in the rotting wood in the knothole and I cannot get to them.  Help!  This tree is one reason I bought this house and I don't want to lose it.

Answer
It may not be a slug (snail) but an insect larvae. I would spray the area with an insecticide--Merit or Onyx are good ones. Or you could use Orthene. These are systemic and will penetrate the bark and wood killing any insect there. I would also fertilize the tree with 10-10-10 fertilizer at the rate of 1 lb per inch of trunk diameter. Scatter it around the tree and water it in--apply just before a rain storm and you will not need to water. This will increase the health of the tree and ward off insects. Apply fertilizer now and again in the spring.