Its a shame. The left over patch is now in full bloom, and is just
crawling with bees. All sorts of bees and flying insects; some that
I've never seen elsewhere. Still, its going to have to go. I can see
for myself that it can't be controlled.

Swyck

Spray your herbicides late in the evening when bee foraging is at a
minimum that way you kill far fewer bees as they mostly have to be
directly hit with the herbicide and it's surfactants for it to kill
them.
This late in the season there is no time for a hive to recover from
poisoning.
With Japanese Knotweed expect to have to reapply often.
If you spray actively foraging bees and kill them you are misapplying.