Fortunately xylophene is still for sale in France, it is banned pretty much everywhere else. It is the most efficient insecticide ever.
Do wear protective clothing and do not go in the room for at least a week.
You do not need to inject it inside the beam. Give a good vacuum to your wood then a thorough painting or spraying of xylophene once every 4 years is enough unless you see new frass (saw dust like bug excrements) .
Do not listen to insecticide companies as far as I can see they are all gangsters. I have studied insects pests and how to get rid of them quite thoroughly as a preventive museum conservator.
Good luck