Pests in the Greenhouse
Many of the outdoor pests mentioned in this section, such as slugs, woodlice, millipedes, earwigs, thrips, capsid bugs, leaf-hoppers, aphids, caterpillars and leaf-miners, are also found under glass. Different species of these pests may be found but the control measures are the same, with the added advantage that in some cases insecticides can be applied as a smoke, thus saving much time and labour. This section will, therefore, be confined to those pests which are generally found only under glass.
Potato Root Eelworms attack tomatoes grown under glass. Infested plants make poor growth because of the damaged roots, and the foliage droops and is purple on the underside. Such symptoms can, however, be the result of bad root action due to other causes, and expert advice should be sought if eelworm is suspected.
Root Knot Eelworms produce similar symptoms on tomatoes, but they are comparatively easy to identify by the hard, elongated swellings which appear on the roots. Many other greenhouse plants may be attacked by this eelworm, especially cucumber,, coleus and carnation.
General symptoms are the retarding of growth and pale yellow foliage which tends to wilt in warm weather. The shape and consistency of the root swellings vary on different plants.
If the infestation is found after planting, feed heavily and mulch with a thick layer of moist peat to encourage the growth of new roots. Under glass it is possible, though expensive, to sterilizeand with metham-sodium (‘Vapam’ or ‘Sistan’) before planting, but this may have to be done yearly to keep the eelworm in check. Alternatively, discard infested soil and replace it after staging, pots, etc., have been thoroughly washed with cresylic acid. Follow the manufacturer’s instructions for rates of application.
GLASSHOUSE RED SPIDER MITE
The Glasshouse Red Spider Mite, one of the most serious greenhouse pests, is extremely small, and a strong magnifying glass is required to see it clearly, but it breeds quickly and can cause a lot of damage in a short time.
It attacks most plants, feeding on the underside of the leaves where it lays its round, reddish eggs, and the leaves become finely speckled with yellow marks which spread until the whole leaf turns yellow and withers. When plants are heavily infested, the mites spin fine webbing over the leaves. These pests will also infest outdoor plants, particularly those growing in sheltered places or against walls.
Spray with malathion or derris two or three times at seven- to ten-day intervals, or fumigate with azobenzene smoke, repeating the treatment seven days later. The mites dislike humid conditions and frequent sprays of water will help to prevent serious infestations.
Although some species of whitefly are found out-of-doors, on brassicas andprincipally, the most common and troublesome is the Glasshouse Whitefly. The adults, which resemble miniature moths, are pure white and about one-twentieth in. long, and can be found clustering on the underside of leaves of a wide variety of plants. They flutter about when disturbed. The young, which are flat, scale-like creatures, difficult to see because of their small size and transparent greenish colour, are also found on the underside of leaves. The pests suck the sap, causing lack of vigour, wilting and, on some plants, yellow mottling of the leaves. The foliage also becomes sticky with honeydew excreted by the insects and sooty moulds may then grow on this material.
Spray or fumigate with D.D.T., lindane or malathion. Only the adults are killed, so that two to three applications are necessary at ten- to fourteen-day intervals before good control is achieved.
Mealy bugs are pale pink or yellow insects covered with a white mealy substance, which congregate in sheltered corners of the plants, such as leaf-axils, and suck the sap. Like many sucking insects they excrete large quantities of honeydew, and the leaves may become covered by sooty moulds. They are normally found only under glass where they can infest a wide variety of plants.
On deciduous glasshouse plants, such as vines and peaches, a winter wash of tar oil will give good control. On other plants spray with malathion or nicotine-white-oil emulsion (l teaspoon 95 to 98 per cent nicotine, and 2 tablespoons summer white oil in l gal. water). Repeat two to three times at 14-day intervals.
Scale insects resemble small, discoloured blisters on the stem or leaf and are often not recognized as insects. Their bodies are protected by a tough scale which varies in shape and colour according to the species, and they remain motionless for most of their lives sucking the sap through elongated mouth-parts thrust into the plant tissues. Infested plants lose their vigour due to loss of sap and, as honeydew is excreted, the plant is liable to be disfigured by sooty moulds. In severe cases, leaves may turn yellow and drop off, and the plant may die.
Some important outdoor species, already discussed, are also common under glass where conditions are ideal for this type of insect.
The Cushion Scale, about one-sixth in. long, is oval and easily recognized by the white woolly mass which is pushed out behind it. This scale attacks orchid, camellia and magnolia but it is less common than the following species.
The Oleander Scale is a round, flat, whitish scale about one-twelfth in. in diameter, which attacks the foliage of cyclamen, acacia, oleander and.
The Soft Scale, the most frequently found greenhouse species, is very flat, oval, about one-sixth in. long and yellowish-brown in colour. It attacks citrus, fig, vine, ivy and many other tender plants, and is found on the underside of the leaves, usually near the mid-rib.
On deciduous glasshouse plants, such as vines and peaches, a winter wash of tar oil will give good control. On other plants wipe or brush off as much of the scale as possible with soapy water, then spray with malathion or nicotine-white-oil emulsion (1 teaspoon 95 to 98 per cent nicotine, and 2 tablespoons summer white oil in 1 gal. water). Repeat two to three times at H-day intervals.
The Vine Weevil, also found on outdoor plants, has a curved, white grub, which is an important pest of pot plants. It attacks many plants and tunnels into the corms and tubers of cyclamen and begonia, causing the death of the plants.
Mix aldrin dust with the potting soil. Some control can be obtained on established plants by drenching with liquid aldrin, but this is only a temporary measure.