I have two questions about fumigation:
First; is it right to make it in a preemptive way?

No. Spray pesticides on EXISTING insects. It does no good
(except to the pocketbook of the pesticide company). to spray a
bug that isn't there. Even systemics should not be use unless
one of the target insects is present. Today's pesticides do not
persist (thankfully!) in the soil and on the leaves and in the
tree's system.

If so, could anybody
explain me when to make it and agains what kind of
insects/fungi/spiders/etc
and with what agents?

Read
The
Label

We are not licensed pesticide applicators (most of us -- Nina is
an exception). The label for each poison you bu will specify
where it can and can NOT be used. It is against the law (in most
countries, at least) to spray in any any different manner than
the label instructions.

And another question about the use of oil based pesticides
against spiders and aphids: Is it safe for our bonsai to use
them? Does
it affect in any way our soil if it becomes wet with that kind of
solution
(maybe the water retention?)?

If you use THAT much oil spray you will poison your plant.
Read the Label.
Spray ONTO the bug, and don't overdo.

Jim Lewis - - Tallahassee, FL - Bonsaiests
are like genealogists: We know our roots!

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