Here are a few non-toxic suggestions. In order of nuisance required
to accomplish the goal.
1- This is the first and best thing to do first of all, especially as
you are using green, fresh stumps. Remove any bark. Most of the bugs
live in or just under the bark. If that is a daunting task to you,
ask for help from a woodworker. It isn't a hard job if the stump is
green. It's a little harder if it has seasoned and dried - but still
do-able with simple tools; A side-ax, a drawknife, a barking spud, or
even a fairly sharp square-ended garden spade can do a lot of the
job, depending on what kind of tree. A mallet and a big chisel can do
it too - a little slower. Or if it is a thin- barked tree, you might
even ask someone with an angle grinder and some really coarse grit
sanding discs ( 60 grit or even 40 or 36 grit) to grind off the bark.
Do it outside - a messy job.
2 - Wrap the stump in a garbage bag and put it into a deep freezer
and freeze it good and cold - below zero fahrenheit - for a long
time - a week - to kill any eggs, larvae, beetles and other critters
that are inside waiting to hatch. If it's too big for your freezer
ask at your friendly butcher or supermarket for a little corner of
their walk-in freezer.
There are some bugs that'll sleep in a piece of dead wood for years
and years, hot or cold - like buprestid beetles - but they don't
come in great numbers so don't worry about them.
3 - After de-barking and freezing you could get a big pot or tub and
boil the stump for a while to cook out some of the sap. That tasty,
sugary sap is what them little buggies love to eat. So new ones will
come to search it out if they smell that yummy sap in your studio.
Remove the temptation. Might as well add some ordinary salt and borax
to the boil water.
4 - When the stump has had a chance to dry - Powder post beetles can
be killed by mixing up a good strong saturated solution of common
household borax - cheap at the supermarket - 20 Mule Team - yeee-
haw! - add a little bit of dish detergent as a wetting agent and
paint on a few applications and let it soak in and dry. Paint it on
while the solution is good and hot. Especially put a lot on the ends
of the stump. That'll kill the beetles when they try to eat their way
out to go forth and multiply.
Don't use ANY of the wood preservatives which you can buy at the
hardware store - whether green, brown, or clear in colour. They all
have horrible dangerous chemicals which will ruin your life and kill
your baby - and besides that they are evil-smelling forever.
5 - My son-in-law at the Boston Museum of Fine Arts has invented a
method which, for your purposes is probably overkill (pun intended)
whereby he puts wooden objects inside a sealed enclosure ( a garbage
bag would do) and replaces all the air inside with nitrogen. He
leaves things in that atmosphere for a considerable time, which
suffocates critters inside the wood eventually - probably too much
trouble - renting a nitrogen tank etc.
I'm sure you'll get some other ideas. But I know the ones above do
work for their intended purposes.
Marty Hykin in Victoria BC where the bugs all hate my very name but
most other folks like me OK.