In the case of the hop the most effective remedy is a solution of
quassia and soft soap. The caustic potash in the soap neutralizes the
oily integument of the lice and dries them up, but the quassia
supplies a bitter principle not unlike that of the hop, though without
its grateful aroma, which acts as a protection in the absence of the
bitter of the hop itself. So closely does the hop bitter resemble that
of quassia, that in seasons of hop failure it is said to be employed
as a substitute in brewing, and at one time its use for that purpose
was prohibited by law.
As a further proof that the bitter principle of the hop is distasteful
to the aphis, it is noticeable that when the fly first arrives it
always attacks the topmost shoots of the bine where the leaves have
not developed, and where the active principle is likely to be weakest.
The same position is selected by the aphis of the rose, the bean, and
every plant or tree subject to aphis attack--it is the undeveloped and
therefore unprotected part which is chosen.
It is remarkable that when a destructive blight is
proceeding--generally in a wet and cold time--and a sudden change
occurs to really hot dry weather, the hop plant often recovers its
tone automatically, shakes off the disease, and the blight dies away,
a fact which strengthens the assumption that in normal weather the
plant can protect itself.
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