754 MAN'S PHYSICAL WELFARE Over three million pounds of insect powder are used in the United States each year. In 1940 we imported 12,000,000 pounds of pyrethrum flowers. Many fly sprays consist of a solution of pyrethrins, the active ingredients of pyrethrum in a petroleum fraction similar to kerosene. Every year about a million pounds of nicotine are extracted from waste materials of the tobacco industry for use as an insecticide. It is especially valuable as a contact insecticide in killing aphids. Lime-sulfur spray used in the control of scale insects is prepared by boiling together a mixture of sulfur, lime, and water. Recently developed contact poisons are 4,6-dinitro-ortho-cresol and 2-cyclohexyl- 4,6-dinitrophenol. Fumigants Are Also Used to Kill Insects. Hydrogen cyanide gas, HCN, is used to fumigate warehouses and sacked grain, although carbon disulfide is also effective for dry grain and seeds. Carbon disulfide presents a serious fire hazard because of its ready inflammability. A mixture of ethylene dichloride and carbon tetrachloride, although it is a little more expensive for this purpose, is not inflammable. Paradichlorobenzene, so widely used as a deodorant, is also used to kill moths in clothes, as well as borers in fruit trees. Methyl bromide has been found to be an effective fumigant for the potato tuber moth, the codling moth, and the Japanese beetle, while dichlorethyl ether, ethylene dichloride, hexachloroethane, ethylene oxide, and methyl formate are used for other insects. New invasions of insects are reported frequently, and new methods of attack are being developed. One of the most destructive insect pests is the termite, which gets into wood and eats the heart out of it before it is detected. Insect Repellents Are Greatly Needed. In many communities where the expense of mosquito control is too great, the home owners who have spent a great deal of money in developing and caring for their lawns and gardens would welcome an efficient repellent for mosquitoes that would permit them to enjoy warm evenings in their yards. A few repellents have been discovered. For example, butyl oxide oxalate is a fair repellent, while tetramethyethylthiuran disulfide is an excellent repellent for the Japanese beetle. Adult houseflies are repelled by a mixture of diethylene glycol monobutyl ether acetate and triethanolamine. A still better repellent
CHEMICAL WARFARE USED TO COMBAT INSECTS 755 is a mixture of diethylene glycol monobutyl ether acetate, diethylene glycol monoethyl ether, alcohol, and corn oil. This mixture is also very repellent against mosquitoes. Mosquitoes are also repelled by a mixture of thyme oil and pyrethrum extract in castor oil. In the treatment of insect pests, it would often be desirable to attract insects. When insects such as the Japanese beetle or grasshoppers are to be killed in quantities of hundreds of tons, it would help a great deal if substances could be used to attract the insects into containers where they could be killed on a large scale. It has been found that male June beetles are attracted by isoamylamine and that Japanese beetles are attracted by geraniol or a mixture of geraniol and eugenol. STUDY QUESTIONS 1. Discuss the methods by which man combats his insect enemies. 2. What are man's chief competitors for food? 3. What is the objection to the use of arsenic and lead poisons for spraying plants? 4. What is the active ingredient in many fly sprays? 5. List two methods other than the use of poisons for killing insects. 6. In what respects are insects superior to man? 7. What would be the advantages of substances which would attract or repel insects? 8. Why is rotenone such a valuable insecticide? Where is it obtained? 9. Prepare a list of diseases and their insect carriers. 10. Why is the promiscuous destruction of insects undesirable? IL Would you say that insects are winning in their warfare against man? 12. Compare international wars with man's war against insect pests.