This section is from the book "Commercial Gardening Vol1", by John Weathers (the Editor). Also available from Amazon: Commercial Gardening, A Practical & Scientific Treatise For Market Gardeners.
These are far more numerous than those afflicting plants under glass. There is scarcely a fruit or vegetable, flower, tree, or shrub that is not subject to attack from one or more pests. Unfortunately, few growers realize the mischief the various insects can do until some crop is almost destroyed by an epidemic. When the danger is discovered, then washes of all sorts and descriptions are tried, but they are often too late in their application to be of any value.
If one must use washes and sprays it is wiser to use them as preventives rather than as cures, and before there is any sign of the crop being attacked.
It is now well known that many leaf-eating and leaf-mining insects can be foiled by the early application of some good insecticide. Thus, aphides of all sorts, leaf-miners, caterpillars, and most soft-bodied pests are prevented from doing mischief if the plants are syringed or sprayed some time in advance of the usual period of attack. The various washes and insecticides are mentioned in connection with the crops they attack. As there is a right time and a wrong time for doing everything, the intelligent grower will naturally make himself acquainted with the period when certain insects are likely to commence their depredations, and spray in advance. It would evidently be useless to spray after the insects have eaten their fill and disappeared; applying insecticides under such conditions would be equivalent to locking the stable door after the horse had been stolen.