This section is from the book "The Florists' Manual", by William Scott. Also available from Amazon: The Florist's Manual.
The nicotine extracts are now largely used and save the florist the trouble of making his own solution out of the stems, and it is doubtless better. They can be used greatly diluted and will rid plants of aphis, red spider and thrips. Rose growers who heat with hot water and cannot vaporize as described above can use this extract to great advantage. In a 20 or 22-foot house, at every thirty feet, you can place a pie dish, say 12x6 inches and six inches deep. Dilute some extract in ten parts of water and pour into the dishes to the depth of one inch. Have some old pieces of iron, 8x4 inches and two or three inches thick. We use pieces of old railroad iron of the old style, cut into 8-inch or 9-inch lengths. These are made red hot in the fire and carried with the help of a coal scuttle and tongs to the dishes on the walk, and when one is dropped into the tobacco extract there is directly a cloud of vapor which is very effectual in killing the aphis, and of no possible harm to the roses unless it be the softening of the growth.
The vaporizing of the extract by diluting it in ten parts of water and placing in small tin troughs that are made to lie on the upper pipe of a steam coil is a method used by many growers and answers the purpose well. The tins are replenished frequently and while you have steam in the pipes evaporation is continuous. It is too slight to any more than just notice, but so continuous that the aphis gets no chance to thrive.
We have seen excellent results from using the nicotine extract and the Eclipse compressed air pump. A 2-inch pot full of extract and water enough to fill the pump two-thirds full was the right proportion in this case. The compressed air pump sends out a fine, colorless spray that will keep chrysanthemums, carnations or roses clean of everything in the insect line.
The several extracts on the market vary in strength, so that the dilution must be different for each, but each manufacturer gives definite and generally reliable directions for applying in the several ways described.