Spraying AphidsSpraying to kill plant lice with a caustic, tobacco, pyrethrum or an oil emulsion (as directed by the manufacturer) early in the day will give best results.
The reason is that the solution will naturally become stronger with the evaporation of its water; whereas the same solution applied in the evening will tend to become weaker because of its dilution with dew. For the same reason the morning application dries up and sticks to the leaves and at night the dew liquefies it again, but the evening application not only becomes more dilute but it may drop off and be lost.
Applications of contact sprays should also be made early in the season to control aphids before the leaves begin to curl in order to kill or knock off every plant louse. Tobacco in all commercial insecticidal forms seems to be an excellent repellent as long as its fumes may be detected by plant lice. Hence the advisability of making applications as early in the season as the first louse can be discovered; also of thoroughly covering the under side of every leaf with the solution. In that location it will stick longer than on the upper side as it is there more protected from washing by rain and dew.
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See Also: Garden Pests