The petroleum is the best disinfectant. It can safely be applied to any
cutting or stem, as long as it is not planted, but is one of the most
invidious substances when applied to vegetation in the garden, or
fields. If effectively applied, it can not be prevented from running
down the bark of the tree and entering the ground, where every drop
binds a certain amount of earth to an insoluble substance, in which
state it remains for ever. With every application the quantity of these
insoluble compounds is augmented and sterility added.
If I am not mistaken, it was near Antwerp--at least I am certain it was
in Belgium--where the first experience of this kind is recorded.
In France, preparations of coal tar have been recommended and have
been lately used in the form of a paint. May be that in this form the
substance is not so apt to enter into combinations with the soil. At any
rate, the method is of too recent a date to permit any conclusions about
the final result of these applications, as the invidious nature of the
substance produces, by gradual accumulation, its effects, which are not
perceived until they are irreparable.
2. Lye or soap. The application of these insecticides requires more
care, and is therefore more troublesome. But instead of attracting
fertility from the soil, they add to it. In Southern Europe soap
and water has been for many years the remedy against the Lecanium
Hesperidum. The method applied by the farmers in Portugal, as described
to me by Dr. Bleasdale, is perhaps the most perfect one. The Portuguese
have very well observed that the colonization of scalebugs always begins
at the lowest end of the trunk and pretend, therefore, that the scalebug
comes out of the ground. This, of course, is not the case, but may their
interpretation be an error, they have been practical enough in utilizing
their observation about the invasion beginning near the roots. They
knead a ring of clay round the tree, in which ring the soap water runs
when they wash the tree, and besides, they fill frequently the little
ditch formed by this ring.
This arrangement of course is only possible in climates of a rainy
summer.
As it is our object to make our knowledge as available as possible for
practical purposes, I repeat for the benefit of cultivators the advice,
without repeating the reasoning: