1042 DISEASES AND INSECTS 



DISEASES AND INSECTS 



root-knot and is more prevalent in light soils. It is 

 especially troublesome in greenhouses. The adult 

 female worm is flask-shaped, .5 to 1 mm. in length, 

 pearly white in color, and is found within the knots on 

 the roots. Each female lays several hundred eggs. 

 The young worms may continue within the same 

 root or migrate through the soil to others. Nematode 

 root-galls have been found on nearly 500 different 

 species of plants. It is especially destructive to okra, 

 hollyhock, Amarantus tricolor, peach, snapdragon, 

 celery, heart-leaved basil, wax gourd, beet, rape, red 

 pepper, balloon vine, melon papaw, catalpa, endive, 

 watermelon, coffee, muskmelon, cucumber, squash, 

 pumpkin, carrot, deutzia, California poppy, fig, soy- 

 bean, pecan, morning-glory, lettuce, gourd, sweet pea, 

 flax, tomato, tobacco, peony,, ginseng, passiflora, petu- 

 nia, tuberose, cherry, pomegranate, eggplant, potato, 

 salsify, clovers, violet, Old World grape. See page 1023. 

 This pest may be controlled in greenhouses by the 

 use of live steam to sterilize the soil or by a weak solu- 

 tion of formaldehyde, one part, 36 to 40 per cent 

 formaldehyde , to one hundred parts water, applied at 

 the rate of one to one and one-half gallons to every 

 square yard of soil surface of shallow beds. After the 

 application, the soil should be thoroughly stirred and 

 planting should not be done till at least ten days later. 

 Under field conditions, the problem is more difficult. 

 The most feasible method is a system of crop-rotation 

 in which an immune crop is grown for at least two 

 years between susceptible crops. One of the most 

 resistant crops is the Iron variety of cowpea. Clean 

 cultivation should be practised so as to destroy all 

 susceptible plants. 



Insecticides. 



Insecticides are substances used to kill insects, as 

 poisons, washes and gases. Insects are subject to many 

 natural checks, such as wind, rains, sudden changes of 

 temperature, the attacks of parasites and predaceous 

 enemies, and are often destroyed in great numbers by 

 bacterial and fungous diseases. In spite of these 

 natural checks it is, however, usually necessary to 

 resort to a spray or some other artificial insecticide for 

 the protection of our crops. 



The essential requirements for a satisfactory insecti- 

 cide are: efficient killing power, safety to the foliage, 

 cheapness and ease of application. The choice of an 

 insecticide for any particular case will depend upon a 

 number of factors: upon the structure, habits, and 

 life-history of the insect to be killed; and upon the 

 susceptibility of the host plant to injury, its mode of 

 growth and the conditions under which it is cultivated. 

 Some insects, as the plant-lice, are soft-bodied and pro- 

 vided with a thin and delicate integument; others, 

 like the beetles and wireworms, have hard, horny shells 

 impervious to ordinary spray liquids; some insects bite 

 off and swallow portions of the plant, while others 

 merely suck out the sap by means of a slender tube; 

 some are injurious in the larval stage, others as adults; 

 some attack the roots, some the foliage and fruit, while 

 others burrow in the trunk and branches. Plants 

 vary greatly in their susceptibility to injury from the 

 use of insecticides; the peach and Japan plum have 

 especially tender foliage, while the apple is not so easily 

 injured. All these points and many more must be con- 

 sidered in selecting an insecticide which will be adapted 

 to the control of any injurious insect. Our methods of 

 fighting insects are constantly changing as new facts 

 are discovered, new methods devised and new insecti- 

 cides invented. Our present methods are the results 

 of a more or less unconscious cooperation extending 

 over many years between the practical grower, the 

 student of insect life and the progressive manufacturers 

 of spraying materials and spray machinery. 



Insecticides may be classed into those which are 

 eaten with the food and kill by poisoning; those that 



kill by contact with the insect's body; and fumes of 

 gases used for fumigation. The poisons are effective 

 against the biting or chewing and lapping (fruit flies) 

 insects; the contact insecticides are used as a rule 

 against sucking insects; and fumes and gases are 

 employed principally in greenhouses and for the fumi- 

 gation of nursery stock, stored seeds, and citrus trees. 



Poisoning insecticides. 



The most widely used substance for the poisoning of insects is 

 arsenic in its various compounds. For this purpose only compounds 

 insoluble in water can be used, as soluble arsenic is very injurious 

 to foliage. 



White arsenic. This is the cheapest form in which arsenic can 

 be obtained. It is a white powder, soluble in water and very inju- 

 rious to foliage. A cheap and efficient insecticide may be prepared 

 from it as follows: 



For use with bordeaux mixture only. Sal-soda, two pounds; 

 water, one gallon; arsenic, one pound. Mix the white arsenic into 

 a paste and then add the sal-soda and water, and boil until dis- 

 solved. Add water to replace any that has boiled away, so that 

 one gallon of stock solution is the result. Use one quart of this 

 stock solution to fifty gallons of bordeaux mixture for fruit trees. 

 Make sure that there is enough lime in the mixture to prevent the 

 caustic action of the arsenic. 



For use without bordeaux mixture. Sal-soda, one pound; water, 

 one gallon; white arsenic, one pound; quicklime, two pounds. 

 Dissolve the white arsenic with the water and sal-soda as above, 

 and use this solution while hot to slake the two pounds of lime. 

 Add enough water to make two gallons. Use two quarts of this 

 stock solution in fifty gallons of water. 



As there is always some danger of foliage injury from the use 

 of these home-made arsenic compounds, and as they cannot be 

 safely combined with the dilute lime-sulfur when used as a summer 

 spray, they are now rarely employed in commercial orchard 

 spraying. 



Paris green. Paris green is composed of copper oxid, acetic 

 acid and arsenious oxid chemically combined as copper-aceto- 

 arsenite. By the National Insecticide Law of 1910, paris green 

 must contain at least 50 per cent arsenious oxid and must not 

 contain arsenic in water-soluble form equivalent to more than 3 Yi 

 per cent arsenious oxid. For many years paris green has been the 

 standard insecticide for orchard use, but is now largely replaced 

 by the safer and more adhesive arsenate of lead. In spraying 

 apples, paris green is used at the rate of one-half pound to one 

 hundred gallons of water or bordeaux mixture. When used with 

 water, lime twice the bulk of the paris green should be added to 

 lessen the danger of foliage injury. Paris green cannot safely be 

 used with either the dilute lime-sulfur as used for summer spray- 

 ing or with the self-boiled lime-sulfur. 



London purple. London purple is an arsenite of lime and is a 

 by-product in the manufacture of aniline dyes. Its composition 

 is variable, the arsenic content varying from 30 to 50 per cent. 

 Owing to the presence of much soluble arsenic it is likely to cause 

 foliage injury, and it is now little used in commercial spraying. 



Arsenate of lead. Arsenate of lead was first used as an insecti- 

 cide in 1893, in Massachusetts. It has now almost entirely re- 

 placed paris green for orchard work throughout the country. It 

 adheres better to the leaves, may be used at considerably greater 

 strength without injuring the foliage and may be combined with 

 a dilute lime-sulfur solution or with the self-boiled lime-sulfur. 

 Chemicallj , arsenate of lead may be either triplumbic arsenate 

 or plumbic-hydrogen arssnate. The commercial product usually 

 consists of a mixture of these two forms, the proportion depending 

 on the method of manufacture employed. It is usually sold in the 

 form of a thick paste, but for some purposes the powdered form is 

 preferred. Under the National Insecticide Law of 1910, arsenate 

 of lead paste must not contain more than 50 per cent water and 

 must contain the arsenic equivalent of at least 12% per cent 

 arsenious oxid. The water-soluble arsenic must not exceed an 

 equivalent of three-fourths of 1 per cent of arsenic oxid. In the 

 best grades of arsenate of lead paste the chemical is in a finely 

 divided condition, and thus when diluted for use remains in sus- 

 pension for a considerable time. Arsenate of lead is used at various 

 strengths, depending upon the insect to be killed and on the sus- 

 ceptibility of the foliage to injury. Four pounds in one hundred 

 gallons can be used on the peach if combined with the self-boiled 

 lime-sulfur; on apple, four or five pounds in one hundred gallons 

 is usually sufficient; on grapes for killing the grape root-worm 

 beetles and the rose-chafer, eight to ten pounds in one hundred 

 gallons have been found necessary. The poison is more readily 

 eaten by these beetles if sweetened by two gallons of molasses in 

 one hundred gallons, but, unfortunately, the addition of molasses 

 greatly decreases the adhesiveness of the poison. Some species of 

 fruit flies may be controlled by the use of sweetened arsenate of 

 lead sprayed on the foliage of the plants at the first appearance of 

 the flies. They lap up the poison with their fleshy tongue-like 

 mouth-parts and succumb before ovipositing. 



Arsenite of zinc. Arsenite of zinc is a light fluffy powder and 

 contains the equivalent of about 40 per cent arsenious oxid. It 

 has been used extensively on the Pacific slope as a substitute for 

 arsenate of lead. It kills somewhat more quickly and is fairly 

 safe on apple foliage when used with bordeaux mixture or with 

 lime. When sweetened with molasses, it is injurious to foliage. 

 One pound of zinc arsenate is equivalent to about three pounds 

 of arsenate of lead. In orchard experiments, as a rule, it has not 

 shown that it is superior to the latter. 



