I'm pleased you're keeping your tree with character and wildlife value. Don't fill or block the hole with anything impermeable. That will lead to rot in the tree. You could try cutting some fine metal mesh to size and putting it over the entrance.

You could buy some black plastic window screen and wrap the trunk at that location. Tie it with plastic twine or the like.

anything you add.. like foam.. cement.. etc ... will actually trap moisture inside... and speed the decomposition process .. that it probably usually dries out is actually a positive ...

if you have the space ... i would plant another apple tree .. to invest in your future...

as long as the tree wont fall on anything important ... then there is no reason to remove it ... not unlike randomly appearing mushrooms ... this is a free show from ma nature ... as she reclaims it ...

a pic would be interesting.. if you can add one or two ...

i wouldnt worry about the wasps or whatever.. they usually move on year after year .;.. but a healthy respect for them.. never hurts ...

ken

Thanks everyone. never thought about the window screening, ken, thanks for the info on the moisture - was worried about that.

Window screen sounds just right. I didn't suggest it because we don't use it here. I googled it and it looks like the kind of mesh I was thinking of.