It has two giant wisteria vines, over 150 years old, ringing the balustrades of two staircases that descend 6 meters down into the main garden. I've known the vines now for five years, and they have largely been the picture of health.

One has been somewhat ailing since last year, when we suffered a drought.
It seemed to improve vastly this last spring, into July, putting up new shoots from its base, and showing lots of new growth.

Since two weeks, they have BOTH taken suddenly very ill. The weak one has dropped ALL its leaves, the stronger one has kept its lower half, but totally lost its top third. Completely out of character, in my experience of them.

I initially suspected root damage had occurred by accident backj in July, far across the yard, when an unrelated stump was deeply dug out and removed. But could 2 vines share the same root system? Could wisteria that old rely on roots 45 feet northwards, and four feet down?

The wisteria typically re-flower this time of year, and are fully green until November 1. The weather has been a bit wetter than average.

A woodpile in the yard was also moved in July... which I gave no thought to.

But, I have just now found bore-holes, perhaps from wood-boring beetles? in the body of both wisteria trunks, big enough to put my pinkie into. On the stronger one, the holes correspond to the height where leaves are now being lost. On the weak one, there is no apparent connection... it's totally lifeless, with just a few hanging seedpods. The new growth from the base has dropped dead.

I would think beetles eat only rotten wood, not live wood.

I honestly don't know what to do, or what to suspect, or what else to say.
Can anyone help me make my next move, to save my wisteria?
Thanks

If you have taken any stems from the Wisteria - was there any discoloration internally that might lead you to think in terms of a fungal disease which could have come in during the drought? If it is a fungal attack then it may well spread plant to plant underground. Is anything else in your garden or local area similarly affected?

Boring beetles have no qualms at all about attacking either live or dead wood - depending on their preferences.

Other possibles include a change in the water levels or a seepage of some kind; an accidental poisoning from some source including paving, roadworks (although Wisteria are usually indifferent to surface spraying of glyphosate in my experience.)

I'm not sure about roots several feet down but I've met them many yards away from the main plant, sneaking off to scramble victorious through the neighbour's cherished hedge. For such large and ancient plants to travel the distance you indicate seems possible - but not to be so greatly impacted by an excavation.

Having seen some seriously decrepit Wisteria, full of dead wood, and borer holes, with a few sprouts still clinging to life, I think I'd be patient for a while longer - but I'd probably protect the base of each plant if you have severe winters.