Extensive investigations have demonstrated that the cotton boll-weevil can be controlled only by cultural methods. Profitable crops of cotton can be grown in spite of the weevil by planting early maturing varieties farther apart and earlier, by thorough cultivation, by plowing up and destroying all the old stalks in early autumn, and by a more liberal use of fertilizers—all these are "farm prac tices." By burning fruit-tree prunings before spring, the hibernating stage of several fruit pests, as plant-lice eggs and bud-moth larv, may be de stroyed. The application of a little quick-acting commercial fertilizer will sometimes stimulate a plant to overcome or outgrow the onslaught of its insect enemies ; but when used in practicable or fertilizing quantities, these fertilizers will not kill the insects.
It is an alluring thought that we may be able to develop insect-resisting varieties of many kinds of agricultural plants. The resistance of certain American native grape roots to the phylloxera plant-louse is proving to be the salvation of the grape industry in Europe. Promising efforts are now being made to develop a boll-weevil-resisting variety of cotton. Sometimes certain varieties of wheat seem to be resistant to the hessian fly.
Much can be done around farmhouses to reduce the numbers of house-flies and mosquitos. Put the hors:e manure in tight sheds so that flies can not breed, or spread it on the fields every two or three days in summer. Drain off or fill in low places where water stands continually or after showers, as such places breed "wigglers" or mosquito larvm.
If the rain-barrel is also screened with wire netting, it will not become the breeding-place of thousands of mosquitos. House-flies may bring to human food the germs of typhoid fever on their feet or mouth parts, and the only way one can get malaria is through the agency of certain kinds of mosquitos (Anopheles) that may have sucked the diseased blood from some malarial patient, which they then inject into the body of another when they "bite." (Vol. I, p. 297.) Spraying and ether insecticidal methods.
For a half century before 1875, the materials used by American farmers to kill insects consisted largely of whale-oil soap, hellebore, lime, tobacco, sulfur and salt. These materials were dusted or sprinkled or syringed on the plants. With the ap pearance and rapid march of the Colorado potato beetle across the country from 1860 to 1870, there came into use Paris green poison, which was des tined to revolutionize insecticidal methods. In
1S72, it was suggested that a Paris green spray be applied on cotton plants for the cotton worm and on apple trees to kill canker-worms. Six years later it was found that the poison spray effectively checked the codling-moth, and this gave a new impetus to the warfare against insects, which has finally resulted in the modern formidable array of insecticide materials and elaborate machinery for their application.
The materials used as insecticides may be divided into three groups, based largely on the two differ ent ways in which insects eat. Some insects, as caterpillars, potato-beetles, and many others, have their mouth-parts provided with strong jaws which enable them to bite off and swallow solid particles of their food-plants. (Figs. 53, 65.) Many other insects, of which the plant-lice, stink-bugs, scale insects and mosquitos are familiar examples, have their mouth-parts drawn out into fine threads which are forced into the plant-tissues along a stiff, sup porting beak ; such sucking insects are unable to eat solid particles and hence cannot be fed a poison sprayed on the surface, for they can suck only liquid food from the inner tissues of the plant-host. (Figs. 54, 66.) To kill biting or chewing insects, it is necessary only to apply a poison on the surface of the plant where they are going to feed. But each individual sucking insect and not a certain part of the plant must be hit with some material that will soak into its body and kill, or that may smother by covering the breathing holes along the sides of the body. The third method is fumigation.
Biting insects.—The insecticides used for killing biting insects consist mostly of poisons which have for their basis white arsenic. This substance can not be used alone, as it dissolves slowly, and this canoes it to burn foliage severely. But it can be combined with salsoda and lime to form a cheap and effective poison spray. Boil 1 pound of arsenic and 2 pounds of salsoda in 4 quarts of water until dissolved ; then slake 2 pounds of stone lime with this solution, and add 2 gallons of water. Use about quarts of this stock mixture in 40 gallons of water or Bordeaux mixture, for general orchard spraying ; for potato-beetles, double the dose of poison.