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.
Farmers, gardeners, and fruit-growers are indebted to the late Miss Ormerod and to the late John Curtis, and more recently to Professor F. V. Theobald, of Wye College, and Professor Walter Collinge, for the valuable information they have placed on record with regard to the habits of the various insect pests. Generally speaking, most of these have four different stages of existence:
1. The egg - a dormant stage.
2. The maggot, larva, grub or caterpillar - usually the most destructive stage.
3. The chrysalis or pupa - the dormant and non-destructive stage.
4. The perfect insect, which in many cases may possibly help to fertilize certain flowers at times.
The female insect is naturally more to be feared than the male, because in many species she is capable of depositing numerous eggs, from which in due course arise a devastating horde of hungry larvae. There are thus two dormant stages in the life-history of an insect, namely, the egg stage and the chrysalis stage, and two active stages, viz. the larva and perfect insect. Some pests, however, notably the green fly or aphis, are not only egg-bearing but also viviparous, i.e. at certain seasons they bring forth young the females amongst which soon mature and bring forth families with amazing rapidity.
In the active stage it is sometimes difficult to catch and even to see some of the pests, as they assume many forms closely resembling in appearance and colour the leaves and shoots upon which they are feeding. The cultivator, however, with a keen eye will often detect the presence of insect pests when others may be oblivious to their presence. While washes and sprays applied at this stage will no doubt disable a large number of pests, many must escape destruction, being thus saved to do further mischief at some future time.
In the dormant stages of egg and chrysalis, however, the grower has the pests at his mercy, and then is the time to make war upon them. By destroying the eggs, future generations of caterpillars, etc, are suppressed, and by destroying the chrysalides the future perfect insects are prevented from giving rise to new families.