Borers in wisteria

What is best to use to kill borers in a mature wisteria. It was infested with curl grubs to start which have been dealt with, and now has borers, it is about 30 years old, dug from a large garden about 3 years ago, grown on for 2 then potted a year ago

Inject or spray insecticide into the holes then plug the holes with cut paste or clay.

what is the best insecticide to use, so far I have only tried pyrethrum

Based upon some web searches it appears that borers in wisteria are hard to eliminate. The best recommendation is to keep the wisteria healthy so that the borers don't attack it. After that there seem to be two approaches.
1. Coat and recoat the entrance holes with a long acting insecticide so that when the beetles come out they are poisoned and can't mate.
2. Use a long acting systemic that gets into the wisteria and kills the borers when they eat the xylem/phloem. The problem with this is that it will kill beneficial insects such as bees. If you chose this route, I would not allow the wisteria to flower for a couple of years after the last application of the systemic so that it can be flushed out of the tree. This is probably doable on a bonsai, but not very easy on a wisteria in the landscape which is why it is not recommended in most of the sources I found.

thank you it is dormant now and I have trimmed to a compact size so there will be no flowers, I will work on the borers coming into spring

I am not sure, but I am thinking that the borers may be active even when the tree is dormant, particularly during the early fall. They would be feeding so they can then molt into beetles that emerge and mate in the spring to lay more eggs and create more borers. I would probably inject the borer holes with an appropriate insecticide and then cover and be prepared to dose with a systemic as soon as the buds start to swell in the spring.

I had the same problem and after all the gentle treatments eg Pyrethrum I took a hard line and used Confidor (active ingredient Imadacloprid) tablets inserted in the soil, now have new growth and a very eaten old looking trunk. Should have done this in the first place!