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Plant Pests

( Originally Published 1920 )


Red Spider. Tetranchyus bimaculatus Harw.

RED spiders or spinning mites (fig. 76, b) are very troublesome to greenhouse crops; cucumbers are especially attacked by them. Eggplants and tomatoes are next in preference. Of the flowering plants, roses, violets, sweet peas, carnations, and chrysanthemums are also favorite hosts. The plants in the vicinity of greenhouses which are subject to the at-tacks of red spiders are beans, eggplants, celery, tomatoes, strawberries, clover, grasses, and weeds.

Nature of Injury. The red spiders (fig. 76, a) feed on the under side of the leaves by puncturing and extracting the chlorophyll and plant juices in the cells of the punctured area. This soon results in small dead areas which become apparent as small whitish specks on the upper part of the leaf. In advanced stages, affected foliage become pale, whitish, transparent, and covered with minute dead pitted specks usually arranged in clusters.

Control. Red spiders are at their best in hot, dry houses. They may be readily controlled by syringing with water. A strong but fine water spray destroys their web and drives them off since red spiders cannot thrive under moist conditions. Plants with hardy foliage may successfully be rid of the spider by spraying with a solution composed of one-half pint of "Nicofume" liquid, and two quarts of concentrated lime sulphur in 25 gallons of water. One-half ounce of salt dissolved in one gallon of water seems to control red spider on carnations. This, however, may burn the foliage of most other plants.

Investigations by Vinal seem to show that no fumigant was efficient in killing red spiders without severely hurting cucumber plants. Sulphur burned to form sulphur dioxide proved very effective in killing all stages of mites. However, since this gas is very deadly to plant life, it can only be used as a fumigant to free empty houses from spider infestation. For the control of all stages above the egg stage, spraying with lemon oil or linseed oil emulsion proved very effective. Lemon oil may be used at the rate of 1 part in 20 parts of water, and applied thoroughly. For directions to prepare linseed oil emulsion, see p. 399.

MITES (Tarsonemus pallidus Banks)

Mites (fig. 78, a and b) are really closely related to red spiders. The species Tarsonemus pallidus is of particular interest to greenhouse growers because it attacks cyclamens, snapdragons, geraniums, and chrysanthemums.

Nature of Injury. On the cyclamen (fig. 77, b), the work of the mites produces a gall. Usually both leaf and flower buds are badly affected. Infested plants are stunted, the foliage distorted, and the blossoms discolored. Instead of the normal soft pink or red, the petals become blotched and streaked; ultimately the flower wilts and dies pre-maturely. On the geranium (fig. 77, a, and fig. 79, b) the attack by the mite causes the foliage to curl and to drop prematurely. Often, too, the injury becomes *noticeable as scorched spots on the leaves. Injury is most severe when the plants are crowded, the leaves touching each other, and the humidity high. In this respect, therefore, the mite differs from the red spider, in that the latter only thrives under droughty conditions. On the snapdragons (fig. 78, a and b, fig. 79, a) and chrysanthemums the attacked foliage becomes curled and distorted, and the flower buds, too, swell somewhat, and become distorted and useless. The same pest also attacks the blackberry out of doors (fig. 79, c.).

Control. At first, mites seem to attack the cyclamen during dry weather. Later, however, poor cultivation, poor ventilation, and excessive moisture in the house seem to encourage the work of the pest.

With geraniums some varieties seem to be more resistant than others. Garman states that the varieties Le Pilote, Jean Vroud, S. A. Nutt, Alphonse Ricard, Madame Kowalevski, Baron Grubissich, Maryland, Beaute Poitevine, Mme. Laudry are all susceptible to the attacks of the mites. On the other hand, the variety Le Favorite seems to be immune. It is not known whether or not there are resistant varieties of cyclamen,, chrysanthemum, or snapdragon. Because of the extremely primitive respiratory system of mites,, it is difficult to keep them in check by fumigation with various gases. It is safer therefore to resort to spraying. However, when a plant becomes badly infested, no attempt should be made to save it, It will be cheaper to destroy it by fire. Dusting with tobacco or sulphur will do little good. Moznette obtained good results by dipping the plants in an oil emulsion called Yelros mixed in the ratio of one part of Yel-ros to forty parts of water. Yelros contains a good deal of Xylol which is penetrating. This treatment, however, is recommended for older plants. Young plants may be greatly injured by burning. "Black leaf 40" and water in the ratio of 1 to 1,000, or of one teaspoonful to a gallon of water will effectively control the mite. By adding three to four pounds of ordinary soap to each 100 gallons of the above solution, it will be made to adhere better to the treated plants. Spraying should begin before the mites attack the plants and should be continued every ten days until the flower buds are ready to open. At this stage the spraying should cease as otherwise the petals may be discolored.

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