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.
But how are these eggs and chrysalides to be destroyed? Entomologists tell us that the eggs of many insect pests are protected by a covering impervious to most, if not all, of the insecticides on the market. If that is so it would be waste of time and money to apply these washes. In a cold state possibly many washes may be harmless to the eggs of insect pests, but if applied hot or warm, in the form of fine spray, the liquid would probably soften the coat of the eggs and render them pervious to the destructive properties of the insecticide. The embryo larva would thus be destroyed. It may be stated that there is absolutely no danger in applying boiling-hot solutions to plants in the open air, provided they are applied in the form of a fine misty spray, and with as much force as possible. Even tender leaves of plants under glass will not be injured by hot washes applied in this way, because the minute globules of liquid are considerably reduced in temperature almost immediately they reach the surface of the plant. For outdoor work the only difficulty would be to maintain a large supply of liquid at a sufficiently high temperature to render it effective when applied to the eggs of insect pests.