Pest information courtesy of Robert Cox - Colorado State University Cooperative extension:
"Aspen trees are affected by numerous insects, diseases and cultural problems. While there are plenty of good-looking aspen around the region, it also is the most common problem tree discussed in calls or samples brought to Colorado State University Cooperative Extension's Plant Diagnostic Clinic...
Aspen trees are short-lived trees, as expected from their role in forest ecology. In the urban landscape, even properly cared-for aspen may not reach 20 years. Life spans can be shortened further by one or more of several insects or diseases that attack aspen. Fungal diseases, such as Cytospora or other cankers which attack the trunk, are common, as are diseases of the foliage such as rusts, or leaf spots. Of the many insects that attack urban plantings of aspen, oystershell scale, aphids and aspen twiggall fly are most prevalent."
Remember that aspens are very sensitive to many environmental problems and are host to more than five hundred species of parasites, berbivores, diseases, and other harmfull agents. Aspen has been a disapointment to many when planted in the landscape.
More on Tree Insects and Disease