Question
We have several Aspen trees in our neighborhood that have leaves dripping with what seems to be sap. The Aspens have never done this before and it seems to be a new occurance after talking to the neighbors. The elevation is 7000' here and the Aspen trees have always done well. No signs of diease and the trees all look healthy.

Answer
Sounds like an insect called aphids. These soft bodied insects suck the plant juices from the plant and secrete a substance called honeydew. When they do this sap drips like a fine mist. Honeydew is high in sugar content and is sticky. To stop the sap you will need to treat the aphids. But this late in the year it would do little good. The leaves will be turning soon and falling off. I would wait until next year and see if the aphid population returns and is so then treat the tree with a soil applied insecticide called Bayer Advanced Tree and Shrub insect control. This is applied to the soil and the tree roots will carry it to the foliage and when the aphid sucks the juices the insect will be killed. If the aphids return next year use this treatment.