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.
The insect pests that invade greenhouses are perhaps as difficult to eradicate as any. There are so many chinks and crevices in walls and floors for them to breed in, and they are so difficult to reach that it is not to be wondered at that they escape the effects of washes, vaporizers, and fumigators. While it is probably true that thousands of insects in an active state must succumb to the fumes and washes, on the other hand there must be thousands at the same time in a dormant stage that are not affected in the slightest degree, being protected by a covering that seems to be impervious to everything except fire. In due course such pests come forth, after the danger is past, and play havoc with the various crops, much to the surprise of the gardener, who thought he had disposed of them.
The practical question is: How best are these enemies to be destroyed? Certainly more drastic measures must be employed than those at present in force. If the pests nest in the soil of a greenhouse, the gardener cannot expect any assistance from birds of the air to lessen their numbers, as a bird in a plant house is literally a vara avis, and is too frightened and flustered to search for the grubs or eggs of obnoxious insects. The grower of crops under glass must therefore rely upon other remedies. Besides using solutions made from nicotine, quassia chips, soft soap, arsenic, etc, on the plants themselves as preventives, the grower would be wise to cleanse his houses thoroughly after they have been cleared of the crops. The walls should be covered with hot limewash, and the woodwork should be painted at least once a year, but more frequently if possible; and if some paraffin and cement be churned up in the limewash, a thin covering will be applied to the walls that will seal up effectually the eggs of any pests that may be hidden in the crevices. In addition to this, sulphur or brimstone should always be burnt in an empty house before a fresh crop of plants is brought in. A strong sulphur vapour is not only fatal to insect pests but also to fungoid diseases. By this means such stove and greenhouse pests as scale, mealy-bug, red spider, thrips, slugs, snails, wood-lice, ants, etc, may be reduced almost to vanishing point. The keynote to immunity from pests in the greenhouse is cleanliness, not only of the structures themselves, but also in the methods of cultivation. A certain expense will be incurred, but it is better to spend it in this way than in trying to secure freedom from attack by artificial means.