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Greenhouse Thrips

( Originally Published 1920 )

GREENHOUSE thrips often cause considerable damage to ornamentals, while their presence may not always be detected. The damage caused by thrips is confined to the foliage. The adult and the larvae of the thrips feed by puncturing and lacerating the epidermis and by sucking the plant juices. The injury at first becomes apparent on the older leaves, and later spreads to the younger ones. The trouble first appears on the under side of the leaves in the form of minute white epidermal spots. Later, as the spots become more numerous, they unite, forming large blotches. At this stage, the injury becomes apparent on the upper part of the leaves as a distortion between the lateral veins, and by wilting and dying of the edges. Both sides of the infested foliage soon become covered with minute drops of a reddish fluid which finally changes to black. These drops are voided by the thrips. Affected foliage be-comes white and drops off prematurely. In the greenhouse, thrips may attack azalea, aspidium, Croton, dahlia, phlox, verbena, pink, ferns, palms, ficus, and fuchias.

Control. The easiest way to keep this pest in check is in burning nicotine papers at the rate of 2 sheets to every 1,000 cubic feet of greenhouse space. Fumigation is best performed at night in a moist atmosphere. Early in the morning the treated house is opened and thoroughly aired. Fumigation with nicotine liquid extracts is also effective in controlling thrips. Russell * recommends the use of one ounce of liquid nicotine (containing 40 per cent. nicotine) to every 1,000 cubic feet of greenhouse space, or from one and a half to two ounces of the weaker strengths. The liquid is evaporated over small lamps or stoves, and to prevent scorching is diluted with water. Greenhouse thrips may also be kept in check by fumigation with hydrocyanic-acid gas. For directions, see p. 385.

MEALY BUGS (Pseudococcus sp.)

Mealy bugs are really scales without armor. They are sprinkled over with a white mealy wax or powder which gives them the name. In the greenhouse there seems to be but two species of importance, namely, Pseudococcus citri and P. longispinus.

Control. The easiest way to keep this pest in check is to liberally syringe the plants with water at a fair pressure. This will wash the insects off the plants and permanently dislodge them. Spraying with a mixture of one pact nicotine sulphate to 750 parts of water will also keep them in check. Fumigation with five ounces of cyanide to each I,000 cubic feet of space for one hour will also be effective.


There are various scale insects which attack green-house plants. The black scale, Lecanium oleae, attacks the geranium and roses. Lecanium tessellatum attacks palms. Aspidiatus nerii attacks a large and varied number of house plants.

Control. The safest method of treatment is fumigation with hydrocyanic-acid gas. For directions, see p. 385.

WHITE FLY (Alleyrodes vaporaiorum West)

The white fly is considered one of the worst pests of some greenhouse crops. The plants most affected are cucumber, lettuce, tomatoes, eggplants, chrysanthemums, coleus, geraniums, and roses. The pest injures the plants by attacking the underside of the leaves and by sucking the tissue juices. The insect deposits a sweetish, sticky substance which tends to clog the stomata of the leaves, and frequently serves as the host for a sooty mold.

Control. White flies may be controlled by fumigating with hydrocyanic-acid gas. For directions, see p. 385.


Plant lice (fig. 80, a) derive their food by sucking the plant juice. In the greenhouse there seem to be four important aphids which attack plants., The melon aphis (Aphis gossypii) is a general feeder and attacks a large number of plants. The green aphis (Nectorophora rosa) seems to attack the rose only. The brown aphis (Rhopalosiphum viola) attacks the violets. The black aphis (Nectorophora chrysanthemicoleus) attacks the chrysanthemum. The following table by Davis is a list of plant lice which are known to attack ornamentals in the greenhouse.

Control. Aphids may be controlled in the same way as thrips (see p. 369). They are also kept in check in nature by lady beetles (fig. 8o, c)' which feed on them, and by a fly, Aphidius testaceipes (fig. 8o, b and d). The adult female of this fly lays its eggs in the body of the aphis. The eggs upon hatching give birth to a small legless larva which feeds upon the interior vital parts of the plant louse. When the larva is fully developed it pupates and cuts a circular hole on the top of the body, emerging as a winged insect ready to attack other aphids.


Soils infested with insect pests are as sick as when infested with eelworm or parasitic fungi. The green-house man in sowing his seed has often great difficulty in obtaining a good and even stand. Frequently the stand is reduced by fifty per cent. in spite of the many resowings. The cause of this may be traced to the presence in the soil of certain insect pests. Among those dreaded most are: Cut-worms (Agrotis sp.), (Lycophotia sp.), (Peridroma sp.) ; wireworms (Melanotus sp.), and white grubs (Phyllophaga sp.).

Control. Spraying the soil will be of little value in the control of underground insect pests. Fortunately, however, we have more effective means for dealing with them. To destroy wireworms, sow corn which has been soaked for ten days in water containing arsenic or strychnine sulphate before planting the regular crop. The larvŠ will feed on the poisonous corn kernels and die. Another way is to treat the seed with gas (coal) tar.

White grubs may be controlled by the use of bisulphide of carbon.

Cutworms may be controlled by the use of a poisoned bran made as follows: to three ounces of molasses add one gallon of water and sufficient bran to make a fairly stiffened mixture. To this add Paris green or arsenic and stir well into a paste. A heaping teaspoonful of the mixture is scattered here and there over the infested beds.


Ants are often troublesome in the greenhouse. These may feed on germinating seed or on growing tips of tender plants. A more pernicious habit is the care and protection which they afford to plant lice and mealy bugs.

Control. Ants may be controlled by being fed with a poisoned bait prepared as follows : To 7 1/2 pints of water add one-fourth of an ounce of tartaric acid (crystals), 15 pounds of granulated sugar. Boil this mixture slowly for 30 minutes and allow it to cool. Then slowly dissolve three-fourths of an ounce of sodium arsenate in one-half pint of hot water and allow it to cool. After this is well stirred into the sirup above mentioned, add one and a half pounds of pure honey. This mixture may now be sprayed on paper or boards on the beds. The ants being fond of sweet foods will be attracted, and upon feeding on it will be poisoned.

HOT HOUSE MILLIPED (Oxidus gracilis)

These "thousand legged" little creatures are very common in beds in which have been mixed large quantities of rotted manure or leaf mold. They often attack sprouting seeds, either devouring or seriously injuring them. These millipeds may occasion considerable damage in cucumber beds. The common species is Oxidus gracilis. However, an-other injurious form often met with is Julia virgatus.

Control. Gossard of the Ohio Experiment Station recommends the application of tobacco dust mulch to the infested bed. This is spread out evenly and worked in one-half to one inch deep. A mixture of nicotine sulphate and water in the proportion of I to 700 when applied to the infested soil will also help to destroy the millipeds in the beds. They may also be poisoned by placing at various intervals on the beds slices of pumpkin previously dipped with phosphorus rat paste.


Sow bugs are abundant under potted plants or in damp secluded places. Besides feeding on dead animal and vegetable matter, they also feed on living plants. They prefer orchids and similar plants because they can hide in the moss and feed on the fibrous roots. Carnations also are often attacked by the same pest.

Control. Tobacco dust and nicotine solutions are ineffective in controlling sow bugs. Sliced pumpkin treated with phosphorus rat paste is an effective remedy.


Slugs or snails often cause considerable damage to greenhouse plants. These pests feed at night and usually hide during the day. They secrete a slimy mucus as they travel. By dusting the plants with ashes, lime, or any caustic dust, the latter will ad-here to the slime of the body, which upon drying soon exhausts the creatures in their struggle to be freed.

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