Spraying Aphids

Spraying 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