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.
In most cases these are to be found at rest in the soil. The chrysalis, or pupa as it is also called, is the stage of development following that of the larva, maggot, or caterpillar, and preceding that of the perfect insect. When the larva has eaten and destroyed a certain amount of plant tissue, and has attained its full size, it then prepares to take a rest for a certain period. It exudes a secretion out of which a leathery protecting coat is formed, and it proceeds by a series of jerks to pull this coat over its body from the bottom upwards, much in the same way as if a man tried to pull a tight-fitting sack over himself from the feet upwards, until he could tie it over his head. While this process is going on, many larvae hang by a silken cord from the bough of a tree, or shrub, or leaf, afterwards dropping down to the ground and burying themselves in the soil at certain depths. Other larvee, however, spin cocoons in which they pupate and go to rest in the soil, in crevices of walls, etc.
The periods at which various insects go to rest in the soil vary according to their nature and habits, some being dormant either in spring, summer, autumn, or winter, while others are active and destructive. It is in this period of inactivity that the cultivator has the key to destroying the pests. There they are resting quietly in the soil, and so long as they are undisturbed there is every chance that they will come forth in the perfect insect stage to carry on mischief. Not only are the pests free from severe frosty weather by being buried in the soil, but what is of more importance is that they are also out of the reach and out of sight of the birds, whose beaks in most cases are either too short or too tender to pierce the soil covering the pupae.
It is therefore to the treatment of the soil that the cultivator must pay more attention if he wishes to stop the mischief of these pests at the fountain head. So long as the soil is left uncultivated, so long are the pests quite safe from frost or birds. As soon, however, as the spade, fork, plough, or hoe is used to turn up the ground, then and not till then will the grower receive the assistance of nature's pest destroyers, the birds - thrushes, blackbirds, starlings, rooks, robins, sparrows, magpies, finches, owls, swallows, poultry, etc, that are ever on the watch to pick up any choice morsels of diet in the way of chrysalides or grubs that are brought within their reach. Birds are of the utmost assistance to the gardener; they render him valuable services free of charge, and are only too glad of having a free feed placed at their disposal. A thrush or blackbird will probably account for hundreds of grubs of various insect pests in the course of a day, if the ground has been turned up so that they are readily detected. Even the cost of digging the soil should not be debited to the birds, but to the cultivator himself, as it is he who obtains the additional advantage of having a larger supply of nitrates, phosphates, potash, fresh air, and other essential plant foods placed at the disposal of his crops. As some pests are dormant in the soil at every season of the year the wisest plan therefore to secure their eradication is to keep the upper layer stirred with the fork, spade, hoe, or scarified as often as the growing crops will permit. The hoe is probably the most convenient for keeping the surface of the soil in a loose and friable condition after digging or trenching. Its constant use will prevent any pests from going to sleep too long, as they will be brought to the surface and exposed to the keen sight of the birds. The hoe, therefore, may be regarded not only as better than the hose pipe or water pot for keeping moisture in the soil, but it must also be considered as a far superior and more effectual destroyer of ground pests than most of the insecticides or earth powders recommended for this purpose. The work of hoeing between the crops will of course entail expense, but the money spent in this way will be found to yield more satisfactory results than twice the amount spent in misapplying insecticides of doubtful efficacy. The cultivator has to consider whether it will be better for him to stir the soil frequently, so as to expose the various grubs to his friends the birds (at the same time liberating food, keeping down weeds, and conserving moisture), or whether he will allow the soil to remain untilled and infested with pests that will in due course compel him to spend a good deal of money in washes and sprays, or lose his crops altogether. After all, the whole question is a matter of pounds, shillings, and pence, and the cultivator will find it more advantageous in every way to spend his money in frequent digging and hoeing operations, if he wishes to secure clean, healthy crops that are free from attacks of noxious insects. The advantages of cultural operations have been already discussed at pp. 101 to 107.