This is a toughy. I always get swarms of them every year, and though I know they lead to a deline of some plants, I don't do anything about it. However, I would try some insecticidal soap for starters. Spray on a cloudy day or early in the evening so that the product will stay wet longer. I would suspect that neem oil would be of benefit in this case, too.

Hey Rhizo,
Thanks for responding. I was a little paranoid due to I planted some china asters this year without knowing that they catch things easy such as aster yellows.
But the story is , I received some aster plugs last year the perennial type but no one could give me the name nor could they tell me what color the flower was . :( ( I aquired many perennials this way and it drove me crazy to have to wait till this season to see what I actually had.!) Life is fun !
This year it grew into a healthy looking bush 3 feet wide and 3 feet tall . Actually there are 2 of them make up the width.
Anyway I was waiting paitiently to discover what type I had and was relying on the bloom for clarification.
Some buds opened up and what I was seeing was very scanty petal that looked more like sepals (see photo link below)
It is a very unasuming flower. Being that I am brandy new to perennials I was thinking it may have aster yellows.
But the plant had no other characteristics of this virus.
Also I was thinking who would grow an aster such as this?)
Not knowing nor seeing what aster yellows does or looks like I was really nervous.
Like I said I have grasshoppers , katydids , crickets and many leafhoppers. Despite this kingdom of munchers my plants are not suffering and I figure it is good food for the birds. But I feared Aster Yellow.
Today I found out that the aster I have is probably a native and grown for ecological reasons. I was able to speak with someone over a PENN State who viewed the plant and she said the foliage looks wonderful and she feels that it is most likely a native aster grown for the eco sys.
Put my mind at ease and I sprayed neem only on the 3 clumps of China Aster that I have.
Today I was rewarded , when looking out my kitchen window at the garden I observed a Scarlet Tanger rumaging in my stand of Becky daisy feasting on the assorted population of the forementioned kingdom!!! I was mesmerised for I have never seen one of these birds ever and I think they are stunning as well as entertaining to watch.
The day previous to this my daughter was able to observe a ruby throat humming bird at the lobelia Cardinalis. I missed this one :( but the fact that the visit happened means I am somewhere along it's route and likely to see him/her once again.
I called a friend who knows about birds and their names to help me Identify this red beauty that visited the Becky today and she said that was a great bird to have visit and it is most likely he/she will return. I hope to see this bird again and would be extremely happy to get a picture.
Anyway sorry for the long winded post but I wanted to share the benefits of Chem free gardening. Sometimes it can get scary but nature seems to balance and this is nice to actually experience firsthand.
Rhiz let me know if you know the name of this aster . Also what do you think of this variety ? Would you plant it ?
I am going to keep it but it will be moved to another area in fall.

In this first photo the two buds in the lower left as well as the upper right corner are what the flower appears like in full bloom! The detail and showiness of this flower escapes language . LOL

{{gwi:1003860}}

The second it a picture of the leaves.
{{gwi:1003861}}
Thanks Rhiz.
Laura