Aspen is a beautiful tree, but must be kept healthy
It is hard to find the perfect tree, they all have positive and negative attributes. As an Extension agent Winger says problems with aspen are high on the question list "what's the matter with my tree?Most aspen trees have damage caused by borers that lead into other problems. There are many different insects that bore into aspens. The most common Saperda calcarata is, an insect in the long-horned or round-headed beetle group. This insect problem is very common. The injury is caused by the larvae feeding on the inner wood of the trunk. Their tunneling weakens the wood and allows the invasion of canker and decay fungi. Borer wounds typically "sap" or "bleed."
Aspen is an attractive host for the insect, especially when the aspen tree is already under some kind of stress or decline. Common stresses for aspen trees in our area are iron deficiency (yellow leaves), poor watering regime (too much or too little), aspen leaf spot (fungus causing leaf spot - common following wet springs or when sprinklers water the leaves of trees), oystershell scale (insect that feeds on limbs and trunk of tree, sucking out sap), and aspen borer (beetle larvae that bore holes into trunks of trees). Aspen is not well adapted to living in the valley areas. They do much better in the mountains.
Suggestions to reduce aspen borer problems:
Don't plant aspen, select another tree species that is well suited to our valley growing areas and not as susceptible to insect injury.
Grow aspen in a grove with continual renewal of trunks rather than as single specimen trees in your landscape. Aspens have a strong ability to sucker and send up new shoots. Remove aspen trunks when signs of borer injury are observed (sap running down trunk, accumulation of sawdust at base of hole or tree) and allow new trunks to fill in the open spaces - aspen are quick growers.
Keep aspen trees as healthy as humanly possible - treat with a chelated iron product (Sprint 138) in the spring to reduce iron chlorosis, prevent sprinkler water from getting onto leaves (promotes aspen leaf spot disease), prune out dead and dying limbs, water properly (water as a tree, not your lawn - this means infrequent, deep waterings), remove trunks infected with borers as soon as they are observed. A healthy, vigorous tree will resist borer attack.
You cannot remove the damage that the borers have already done. You can help (sometimes) reduce further damage to the injured trunk and others in the area by well-timed insecticide treatments to the trunks. This is costly, time consuming and involves some risks (pesticides should always be used properly - follow the label directions and wear appropriate protective clothing).
You must treat before the adult beetles lay their eggs in the late spring through summer. Once eggs hatch and larvae bore into the trunk, the damage is done. Sevin and Thiodan insecticide products labeled for shade trees are the recommended treatment. Apply a spray that drenches the main trunk of the trees every 3-4 weeks from May-July.