how to get rid of knotweed

Don't know about a beetle, but if you google Japanese Knotweed, one of the options it presents you with is the Environment Agency's site. This weed has to be disposed of in a particular manner, and usually a specialist company is needed to get it out and get rid of it. Google it and see.

Government agencys are carrying out tests with some beetle that is partial to Japanese Knotweed. It is not generally available.

If you go to a blog by Bulbaholic you will find out a bit more of what you are dealing with. Click on Bulbaholic and then on his blogs section.