Deni, 

Thank you for your question and picture. 

Can you please submit a picture of the entire tree. Also, a  picture of the dropping branches. 

Please answer these additional questions;

Are all of the leaves affected? What area of El Paso County do you line in? 

Kindly, 

Kerry P

El Paso County Colorado Master Gardener

Hello Deni,

We have been  reviewing your question from 8/5  regarding the spots on your aspen leaves. I believe you were talking with Keri P on this. Another master gardener, Trudi, found the link below which we believe is the issue. It is damage from the Aspen Leaf Blotch Miner Moth. Although the damage is visible, it isn't much threat to the life of the tree. The article below will tell you more about it.

Thank you for contacting the Master Gardener help desk and please contact us again if you need anything else.

Happy Gardening!

Jasper B apprentice and Adrian H Master Gardener

https://somethingscrawlinginmyhair.com/2011/01/08/aspen-leaf-blotch-miner-moth/

Hi Deni, 

This is Kerry, again. Great question. 

Below is what [W.S. Cranshaw, Colorado State University Extension entomologist and professor, bioagricultural sciences and pest management; D.A. Leatherman, entomologist, Colorado State Forest Service; and J.R. Feucht, former Extension landscape plants specialist and professor, horticulture. 3/96. Revised 7/14.] Colorado State University suggests. 

Management of Leafminers of Trees and Shrubs

Few, if any, leafminers pose any significant threat to the health of trees or shrubs. Injuries are cosmetic and treatment decisions are based on plant appearance.

Furthermore most leafminers have numerous natural enemies that normally well regulate their populations. Outbreaks, if they occur, are usually very short-lived. However, exceptions to this can occur among the non-native species that are not as well regulated by natural enemies in areas such as Colorado where they have been recently introduced. Examples of non-native leafminers are those found on elm (elm leafminer, European elm flea weevil), birch (birch leafminer), and the tentiform leafminers of apple.

If controls are attempted with insecticides there are two approaches to consider. The first involves sprays applied to leaves that are timed to coincide with periods when egg laying occurs – usually shortly after leaves have expanded in spring. Persisting contact insecticides can be used for this purpose, such as the various pyrethroids that currently predominate for control of insects that chew on leaves (permethrin, bifenthrin, lambda-cyhalothrin, cyfluthrin, deltamethrin), None of these will move systemically in plants so active larvae within mines will not be affected. Sprays of systemic neonicotinoid insecticides (imidacloprid, dinotefuran) can provide some control of these stages among leafminers that are beetles (Coleoptera) or sawflies (Hymenoptera). Neonicotinoids tend to be poor in control of caterpillars, the larval stage of moths (Lepidoptera).

Alternately, neonicotinoid insecticides can be applied to the soil for root uptake. Imidacloprid (Merit, various generics) is widely available for this application both through most nursery outlets and by commercial applicators. Dinotefuran (Safari) is presently only available through commercial applicators.

Hope this helps. Let us know if you have any additional questions. 

Kindly, 

Kerry P. 

Colorado Master Gardener, El Paso County