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If your speaking of the sill plate on top of the foundation, you need to support the joists if its the one that they rest on, cut it out and hammer in new one. In some cases when we do these
In any case this appears to be a support beam. You need to construct a support wall on each side of this beam and if possible slightly raise the flooring above, or attic floor about a 1/16 of an inch to take the pressure off, then remove this damaged joist up to either a column that may not be seen in the wall or up to another structural wall. What ever is closest.
I would suggest because this is a structural member, or at least appears to be, that you get a carpenter that does this type of work. Not a handy man type. You can get a good reference from any local realtor in your area, as they deal with insect damage wood all the time and have a list of qualified contractors who just do this type of work.
It may look expensive to do, but experienced contractors who do nothing else but insect repair can have this project done in a few hours. While if you were to do it yourself, you may end up spending the entire weekend with this project and still many not have it done properly. Most likely your only looking at around $250 for the whole project.