. Agriculture for southern schools. leaves. It has never been found possible to in-troduce any poison into the sap of a plant so as to destroythe sucking insects upon it. The principal thing thatcan be done to destroy such insects is to apply somethingwhich will not injure the plant, but which, coming intocontact with the insects body, will cause its death. Thereare two kinds of such treatment that can be used. Thefirst kind includes many substances which cause death bycovering with soap or with oil the openings through which !56 AGRICULTURE the insect breathes, which keep out the air and real
. Agriculture for southern schools. leaves. It has never been found possible to in-troduce any poison into the sap of a plant so as to destroythe sucking insects upon it. The principal thing thatcan be done to destroy such insects is to apply somethingwhich will not injure the plant, but which, coming intocontact with the insects body, will cause its death. Thereare two kinds of such treatment that can be used. Thefirst kind includes many substances which cause death bycovering with soap or with oil the openings through which !56 AGRICULTURE the insect breathes, which keep out the air and reallysuffocate it. These are called contact insecticides. Kero-sene emulsion is one of the most common of such sub-stances. (See ) The second class includesthose gases that are either poisonous in themselves orcause the death of the insect by replacing the air andthus causing suffocation. Exercise. — Watch caterpillars feedins; on foliage, and mosquitoesand flies sucking blood or sweets, and describe what you see them Fig. 167, — Leaf-eating Caterpillars at work SECTION XLVII. INSECT ENEMIES OF THEFARMER While the injurious kinds number but a very smallfraction of the great group of insects, they are exceed-ingly important both to our wealth and health. Perhapsmore than a tenth of all the crops raised each year in ourentire country is eaten or destroyed ,by insects. Thisdamage amounts to a direct cost of about ten dollars forevery man, woman, and child in the United States. The Hessian fly. — Perhaps the most injurious speciesof all is the Hessian fly, a minute insect which lives onthe stems of wheat and other grains. In some placeswheat cannot be grown because of the presence of thisinsect and the injury it causes. The only remedy con-sists in burning over the stubble after the crop has beenharvested and in delaying the planting of the fall wheatuntil after frosts have occurred. The chinch-bug. — This is another very important insectthat attacks graing. It
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