CHAPTER XXIV.
GREENHOUSE VERMIN.
The insects which infest the plants usually grown in the greenhouse are few in number, but capable of doing an immense amount of mischief if not kept under with a strong hand.
Those which occasion the most trouble are green-fly, black fly, thrips, red-spider, and scale. They are all more or less destructive, and when once they obtain a firm footing in the house are very difficult to eradicate. The first three are, perhaps, the most easily disposed of, as they can be effectually destroyed with tobacco-smoke, or by immersing the plants in tobacco-water. The other two are the most persistent, and a considerable amount of labour and perseverance will be found necessary, as they cannot be dispersed with tobacco-smoke. First of all, it is necessary to consider what the most favourable conditions are for the development of these pests, and these may be said to consist in a dry and confined atmosphere, and an unhealthy state of the plants, especially when brought about by neglect in watering. In fact, it is the easiest matter possible to produce a swarm of green-fly, or to have the foliage of the plants smothered with red-spider, for it is simply necessary to neglect the air-giving and watering for a few days to ensure either of these results. Allow the ventilators to remain closed for two or three hours every day after they should be opened, and insufficiently supply the plants with water, and the cultivator will soon be supplied with sufficient work to employ his head and his hands, and, moreover, tax his patience, for, probably, the remaining part of the season. The opposite of these conditions will, it may be readily imagined, promote a vigorous growth, and check the development of these pests. With a careful system of watering and proper attention to air-givmg, there will not be much trouble with either green-fly or red spider. The keeping
