"Charles T. Smith" wrote:

Cement? I was thinking along the lines of tar. I have an unused bag of
cement, and I've seen trees with cement plugs, now that you mention it...
But it seems extreme to me ... for example, cement gets warm when it
hardens. That won't be a problem? Would cement be less of a chemical
toxen to the tree than tar, for example?

Were it an old hollow tree or relatively large cavity then cement will
cause problems. But yours is a very young tree, which in a few years
will grow enough that it reduces that borer hole to mere
insignificance. As to heat, trees easily tolerate far higher
temperatures over much longer periods simply by growing in the sun.
And that is why filling a void with tar is not a good idea, it's fine
to "cauterize" a small surface wound in order to prevent entrance by
insects and water. Whenever the sun strikes that tree the tar will
reliquify to some degree and leach throughout the tree even into the
living portions especially downwards (gravity) to the root area and
that may interfere with sap flow... in a small tree that can have the
same effect as a blood clot in your leg, death. Patching cement
doesn't generate much heat, you can run a small test batch and check
the temperature with an insta-read thermometer... I seriously doubt it
will reach more than 90F during the curing, and only for a very short
duration, not nearly as hot as the surface of a tree gets from the
sun, and thats all day every every sunny summer day.

And if you're still tentative go to a plant nursery and ask if they
can recommend an arborist's cement, calking compound or some such.
Plumbers putty would work too, or spackle patching compound (probably
easier to work with than cement, a small $2 can from The Depot will
do, all you want to do is fill the void until the tree heals itself.
Let us know if you get that borer out.