Dawn, You may want to contact the Extension office in Clackamas County to follow up on your situation. Ask for Glenn Ahrens, who is the Forestry Extension Agent. Trees die for many reasons, but most insects do not kill trees. Many only go after trees that are already under stress (like from compacted soils or overcrowding). Thinning sounds like a good idea, but will take a while for trees to respond. Keeping your horses away from the trees is another good step.