In message , Chris Hogg
writes
Nicotine is a powerful alkaloid insecticide and was available to
amateur gardeners fifty or so years ago. However it is also very
poisonous to humans as well as being addictive, and is now only
available to commercial growers (AIUI). Even they have to adopt
stringent safety precautions when using it.

Tobacco leaves typically contain about 5% nicotine. Someone on a
recent thread (I think it was one on red spider mite) mentioned almost
jokingly that they'd heard you could make an insecticide by soaking
old cigarette butts in water. Is there an old-established recipe for
doing this (so many ounces of tobacco per gallon, soaked overnight, or
whatever) that gives a nicotine solution of sufficient concentration
to be insecticidal but harmless to us mortals?

Homeopathic levels ;-)

Basically you do not mess with the stuff. A half decent chemist can
extract enough nicotine from a couple of cigarettes for human LD50.
There are recipes out on the web that will get you into serious trouble
unless you are used to handling dangerous chemicals. Natural toxins are
not "safe". And the kitchen sink chemistry recipes to extract them are
positively dangerous (besides being unlicensed use).

Is there a harmless
concentration? After all, smokers happily puff their way through
umpteen cigarettes a day, and whatever long-term illness they die
from, acute nicotine poisoning isn't it.

Only because burning the stuff is an incredibly inefficient process.

The other problem with nicotine as an insecticide is that it generally
comes laced with tobacco mosaic virus which despite its name will also
infect lots of other valuable plants. TMV can even be viable in the
smoke from tobacco.

Regards,
--
Martin Brown