Question
we found a white ribbed worm in the base of a tree in our yard. It was about 5 inches long maybe as big around as a Sharpie marker. There were 10 or 20 of these worms in the tree (which we had to take down because they had killed it) can you tell me what this is and how we can stop it from causing futher damage to our trees?

Answer
It was hard from your discription of the "worm" to ID it. I am going to guess it is the larvae form of a large beetle such as a Rhino or Hercules  beetle type. Here is a web site that discribes with picture this type insect: http://www.uky.edu/Agriculture/CritterFiles/casefile/insects/beetles/hercules/he...  
IF it is this is not what killed your tree--these type beetle larvae feed on fungi in the deadwood adn do not cause mortatlity but feed on dying trees. I would suspect that something else killed the tree and these were feeding in the inter wood area of the stump or lower part of the trunk. This is the best I can do without more information. In any case they will not harm your other trees.