Thanks for your question.

I believe that the asters are infected by spider mites. Spider mites are tiny arachnids that can cause damage to plants by sucking out their sap. Generally plants recover from such an infestation provided it is neither severe nor prolonged. There are two distinguishing features of such an infection: tiny holes in the leaves and black droppings of their excrement deposited on leaves. Both are present in your pictures. These black droppings could be mistaken for eggs, but they are not.

Other indications of a spider mite infestation on asters include:

1). Look for fine webs on the undersides of leaves and between stems. These may not be visible if the infection is recent or minor.

2). Leaves may appear speckled, stippled, or bronzed due to the loss of chlorophyll. Some leaf yellowing may also be present. I think that I see some indication of these symptoms in your pictures.

Remedial procedures include:

1). Use a strong jet of water from a hose to dislodge mites from the plant. This can reduce their numbers significantly. Do two to three times a day for a week or so.

2). Apply an insecticidal soap to the affected plants, making sure to cover the undersides of the leaves. A soap like this should be available at most gardening centers.

3). If the occurrence of leaves having tiny holes continues to increase, consider using a miticide. Again, this would be available at gardening centers. Be sure the label indicates it is effective against spider mites.

Here is some additional information on these and related points:

https://extension.umn.edu/yard-and-garden-insects/spider-mites

https://opticfoliar.ca/how-to-get-rid-of-spider-mites/ - :~:text=Apply neem oil liberally with,signs of infestation have disappeared.

https://www.gardeningknowhow.com/ornamental/flowers/asters/managing-problems-with-asters.htm

https://ipm.ucanr.edu/PMG/PESTNOTES/pn7405.html

Good luck. Please get back if you have any further, related questions. Thanks for using our forum.