Here's a USDA summary on neem oil soil drenches for control of root weevil:
http://www.nps.ars.usda.gov/publicat..._NO_115=124481
It's rather technical, but it boils down to fairly high saturations
seriously retarding the growth rate of root weevil larvae. Neem in feed
baits, however, worked better without having to change the character of
the soil.

Most claims that neem as a soil drench functions as a systemic pesticide
come from industry propoganda & gardeners who've believed mere labels. A
few good studies exist which suggest that when soil drenches when used for
specific purposes they may have some positive benefit. A general soaking
for no reason than there MIGHT be insects to get rid of is not something
that should ever be done to a healthy garden, though packagers &
manufacturers of neem products would sure like you to do it anyway.

Because neem is relatively recent in American gardening, it may seem odd
to realize that in Asia Minor neem tree growers & extract manufacturers
are the equivalent of the old Rubber Barons, politically very powerful,
regarded as Brahmanic religious leaders even when they are primarily
businessmen seeking riches, & funding even major universities in India for
the sole purpose to find out nice things, & cover up bad things, about the
Neem Barons' source of billions. Last year when Australia decided to put
it on the list as a poisons as a Schedule 5 toxin (equivalent of RoundUp)
to be used with caution (and as Schedule 7 dangerous poison in high
concentrations), the Rubber Barons got their lobbying "experts" &
religious wackos out by the hundreds to pose as organic farmers &
ecologists whose only desire to was to keep access to this toxin
unrestricted. Toxic information can be downloaded from he
http://www.health.gov.au/tga/ndpsc/neem.htm

Use of neem as for any other chemical pesticide or fungicide should be as
needed or for specific purposes, for example: Spring surface-coating of
neem oil has also been used under roses to reduce the amount of blackspot
that reaches the rose leaves, but seems to be a matter of gluing the
spoors to the soil rather than outright fungicide.

I wouldn't use it as a systemic insecticide as soil drench or for any
other purpose, as it would harm as many benificial as harmful insects, &
cast an imbalance over the garden that would in the long run cause
population explosions of harmful insects. That's certainly what happens in
gardens that make themselves reliant on chemical pesticides, & no reason
to suppose neem by right of being a "natural" chemical would be any
different. For example, a North Carolina State University study found that
flea beetles which attack some root-vegetables such as radishes can be
controlled with neem soil drench, though what would work better is
parasitic nematodes, which neem will also kill -- better to encourage a
healthful nematode population than to kill the beneficial with the
harmful. If one starts using neem or any other insecticidal product
willynilly even when there is no evidence of harmful insects, that leaves
mainly what is beneficial to kill.

Organic gardening should not just mean finding "natural" toxic chemicals
with which to continue the same unhealthful practices encouraged by the
pesticides & chemical industry generally.
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A RELIGION CALLED NEEM WORSHIP

Throughout India today it is believed that small pox has vanished because
of the use of Neem, due to the protection Neem imparts from Sitala the Pox
Mother. Anyone thinking straight would say that small pox vaccinations had
something to do with it too, but that's not the common belief. Malaria,
measels, & most other diseases are treated with neem. It is the
first-choice of cure for measils & chicken pox all sorts of illnesses &
skin conditions, treated by means of bathing in neem, & pretty much
everyone believes in it though it has no ability to treat these
illnesses.

If a sick person recovers (when the otherwise untreated disease runs its
course), than Mahadevi did it, because She is Great! If the sick person
does not get well, must've ****ed off Mahadevi somehow, she can be one
mean mother after all with human blood all over her fangs. Nearly
everything out of India about Neem is inspired by this faith, & even
scientists must recite the litany "It has been used for thousands of years
without ill effect." Yes, & neem garlands have been woven for the necks of
shrine divinities for thousands of years, & that's why India still exists
but Rome has fallen.

The chemical toxin in neem is called azadarachin (ADA) & is by no means
harmless to people or animals. If you read the small print on the neem
labeling, those are not put there because the manufacturer was feeling
cuddly & wanted to warn against extremely unlikely possibilities; they are
forced by law to include these warnings because the product is toxic.
There are many other probable problems with Neem that have not been
sufficiently studied outside of religiously-motivated neem-baron-financed
studies in India, so the list of things to worry about may get longer over
time. Here are some of the harmful effects the manufacturers have already
copped to:

1) Neem easiily damages & occasionally kills plants, & is especially
harmful to impatiens, hibiscus, some roses, dianthus species, & olive
trees.

2) Neem is not recommended for use on edible plants or in areas where food
is prepared, because poisonous when injested. (In India they eat it as a
religioius sacrament to the Goddesses Sitala & Durga & Marriamme, use it
in all sorts of herbal medicines, but these are not safe uses.)

3) Harmful when absorbed through skin (old recommendations to rub it on
your arms & legs as a mosquito repellant are no longer on the labels; it
is still nevertheless to be found in skin creams & shampoos imported from
India). Much contradictory information can be found on the web about the
safety of skin contact -- promotors want to say its safe for ANYthing --
but on the label itself, the only place where the information counts
legally, it states categorically that it is harmful when absorbed through
skin.

4) Harmful to bees & many other pollinators.

5) Contaminates water, banned for deliver use in irrigation systems.

What is clearly the case is that the standard industry claim that "Neem's
natural properties pose no danger of toxic reactions" is a falsehood. At
best it is a matter of degree of toxicity. Many other problems may turn up
in the near future as testing continues in America and Australia withut
the Indian Neem Barons' political mightiness securing exclusively positive
findings.

Some industry claims are already known to be disengenuous. It is widely
asserted that birds that eat insects killed by neem are not harmed. What
they don't mention is that birds that eat seeds & berries that have been
sprayed with neem ARE harmed.

An Indian study indicated neem might benifit the livers of rats that had
had their livers experimentally damaged -- a study conducted by a man who
believes that he can go to a special heaven called Suryalok & live there
for three times longer than the age of the earth, if only he plants a neem
tree -- & the result of this study is that neem is now marketed world-wide
as a cure for every conceivable liver ailment.

EPA is behind the curve on this one, having relied on too many India
"studies" unquestioned even when they literally begin with with prayers to
the Goddess of the Neem Tree, at health & horticultural institutes whose
"scientists" & "doctors" all have theology degrees exclusively. I kid you
not. EPA granted tolerance exemptions for clarified hydrophobic extracts
from neem oil on food & produce (Reg. No. 11688-8), on the basis of
studies conducted & selectively submitted by neem manufacturers. This was
in 1995 & the EPA has still not sought out independent evidence of safety,
as the Australian government HAS done & found the safety claims more than
a little suspicious.

EPA was suspicious enough to leave neem off the list of mosquito
repellants that can be sprayed on the skin. This restriction against use
annoys the neem promoters a great deal, who have produced literature
proving citronella is a deadly toxin but neem is safe. Reality is that
extracts of pine, citronella, or neem all have the possibility of grim
effects especially if used outside clearly defined dilustions & approved
purposes. Neem manufacturers want you to believe all the warnings against
trival products are extremely true, but none of the warnings against their
own product are true. They'd like to be able to tell you to eat it, bathe
in it, & incredibly even stuff it up your ass -- which they do tell people
to use it for in India & Africa, but cannot legally tell Aussies & Yanks
to do the same. In America, the EPA won't allow it, even though, as the
neem promoters keep squeeling, it's been eaten, bathed in, & stuffed up
the asses of people in India for thousands of years, & that litany is all
they think anyone should know.

The Neem Tree as earthly embodiment of the adeviam, or Mahadevi (Great
Goddess), is inherently expected (in India) to also be dangerous, as
Mahadevi is the Great Slayer as well as the Great Healer. In India it is
no big deal that by using neem for fifty different things every day of
your life might cause injury, as one risks the wrath of ones Adeviam in
order to earn Her Blessing. Indian research on the Neem is rational in the
same way that "scientific" archeology done on the actual Noah's Ark is
oh-so-credible credible because it was undertaken by archeologists from
Brigham Young University. The Brigham Young mythifiers were motivated by
the Book of Mormon; the hindu mythifiers are motivated by the Vedas. The
first purpose is religious; the second goal is to find out something nice,
that can be pounded into a scientific context, that conforms wholy to the
religion.

The primary Great Mother of Neem is named Sitala, though Durga also
sometimes bares a Neem leaf in one of her many hands, & just about any god
or goddess (but especially those associated with death & illness) can be
honored by being garlanded with neem leaves & berries & flowers. Sitala
has fangs & a ferocious face & she causes pox diseases, drought, & general
harship, but she also has the power to avert pox diseases, drought, &
hardship. After funerals, for strictly superstitious reasons, a neem leaf
is chewed to ward off deathly spirits, & neem is used for all kinds of
spiritual cleansing & as proof against demons, & burned so that the smoke
will scare away demonic microbes. "Scientists" end up recommending the
scattering of neem leaves at the doorways of houses after a funeral or
family illness or as a healthful fumigant to kill germs.

Neem propogandists such as at www.neemfoundation.org (which funds neem
research IF it conforms closely to the Neem Foundations glowy religious
believer-attitude) & scores of others have flooded the world wide web with
intentionally misleading information (it was a conscious decision by by
the Neem Barons to do this, emulating the long-term Monsanto program to
flood the net with misleading, paid-for, false, & selective data). Never
trust any information that is not backed with a specific study from
independent university health sciences departments or horticultural
extensions or toxicologists.

In the end it may hold up as way safer than similar chemicals if one must
resort to chemicals. If it were a choice between pine sol or neem, I'm
willing to believe neem would be the better choice -- but the evidence is
not that promoted by companies & organizations with financial & religious
motivations. More non-religious studies are going to be required before we
know for sure. The horticultural studies I've read on neem have found it
useful in some few specific cases, most of which find it less useful than
other alternatives however (it can treat powdery mildew for instance, but
not nearly so well as can a spray of dilute milk). In India the REAL value
of neem is in scattering demons. All other uses in cosmetics, shampoos,
herbal remedies, baths, & agriculture are informed by this belief that it
gets rid of demons & death-spirits. In the west, we are likely to find its
uses somewhat fewer.

-paghat the ratgirl

--
"Of what are you afraid, my child?" inquired the kindly teacher.
"Oh, sir! The flowers, they are wild," replied the timid creature.
-from Peter Newell's "Wild Flowers"
See the Garden of Paghat the Ratgirl: http://www.paghat.com/