First, I applaud all local efforts to re-establish or maintain healthy native ecosystems. I hope these efforts increase widely and succeed greatly. This does not mean that I applaud any and every profit motivated introduction of a biological control agent that may spread across the entire continent. I do not greatly condemn this particular introduction. It is understandable, yet regrettable for it's effect on me and at least a few others, perhaps more than a few.
The site in question is a collection of strips and patches beside and between a major divided highway, entrance roads, a parking lot, an abandoned road, and a few stone and concrete "observation points". The plants are mostly alien. In sunny areas, close to 100 % alien. Fortunately, along one side of this collection there is a long strip of forest, mostly native but not completely. I suppose this juxtaposition is what makes this location the all around best site I know in the NYC area for easy observation of a variety of interesting insects, especially wasps. In late summer the clumps of Knapweed have been by far the most generally productive plant for bug-watching. The nearby Porcelain Berry, (another frequently reviled alien) is its rival for wasps. I will miss the Knotweed. It is also one of my favorite cut flowers, beautiful and long lasting. I imagine that there are many places such as this one, all to be a little worse if the Knapweed is eradicated. Is this a tragedy? Of course not. Is it regrettable? Certainly, to some.
Now let me asked a question. Do you think there's a chance in hell of the Knapweed in this location being replaced by a native plant? If it is, I will apologize for my regrets.
Finally, attempts at humor often fail on the internet. My wanting ID confirmation so I could bill the western farmers is an example. I must also note that although I received two replies from much respected experts, this amateur did not in fact get ID confirmation. If anyone thinks this is a serious complaint, I must learn to use smiley faces.
BTW, I only cut"waste place weed " flowers to take home.
Thanks for your reply. Charles

of biological controls. They are often released before all the science is in. Someone just mentioned the other day here that something released as a Canada Thistle control now seems to have moved over to feed on the lovely native Cirsium muticum!
We do hope that some native wildflower group does "adopt" your site and help the native species gain a foothold. We just visited a site within the city limits of the city of Chicago today and the place is filled with Rattlesnake master which was teeming with all the bees and wasps a person could want to study. We would hope some Eryngium or maybe some Pycnanthemum could be introduced at your place by a native plant society to help make up for the loss of Knapweed.
Finally, we must confess to being far from experts. We're just amateurs here learning our way through the bugs, and we simply do not know the fly that you posted, so we can't confirm your ID. But one of the Dipterists here will recognize it and you should get your ID (or we'll bug them to get it for you!)
Thanks for the thoughtful discussion

Here, here, John and Jane. Hey, I appreciate that I can see lots of great bugs on the blooms of Japanese knotweed, but if I have to look a little harder, on native plants, for wasps and bees, so what?