752 MAN'S PHYSICAL WELFARE Insects Are Combated According to Their Methods of Attack. Certain materials will destroy some insects and be of no avail against others. It has been found that insects may be divided into two classes based on their feeding habits. Those of one class, called chewing insects, eat the foliage, stems, and roots, while those of the other, called suckiyig insects, puncture the fruit, leaf, twigs, stems, or roots to suck the juice. Insecticides are thus classified according to the type of insect to be attacked, as stomach poisons and contact poisons. Chemicals are also used as fumigants, baits, and attractants in the warfare on insects. flG. olo. Airplane OnMuit; Lui Luii lui umi \% cc\ 11 v_uii Li ui. ^^i^^Jui ic^\ v)i LilC U.O. Department of .Agriculture, Bureau of Entomology and Plant Quarantine.) Various Types of Stomach Poisons Are Used to Kill Insects. is Arsenical poisons are the most important stomach poisons. Arsenic used in such forms as calcium arsenate, lead arsenate, basic copper arsenate, and Paris green. These products are either sprayed or dusted on the plants. Unfortunately, they leave residues on food products which have a cumulatively harmful effect on the human body, so that the chemist is now called in to determine the amounts to be used on various fruits and vegetables and to work out methods of removing the poisons before the products are marketed.^ The use of lead arsenate was first introduced in 1894 by F. C. Moulton, chemist for the Massachusetts Gypsy Moth Commission. Since that time its use has increased to millions of pounds a year. • Consumers should find ways to exert pressure to compel adequate protection from poisons.
CHEMICAL WARFARE USED TO COMBAT INSECTS 753 Calcium arsenate, a cheaper compound than lead arsenate, is used in huge quantities in the cotton belt, where it is dusted on by airplane to control the boll weevil. Thrips on lemon trees are successfully combated with a spray of tartar emetic. Recently rotenone has been found to be toxic to many insects and yet not poisonous to the higher animals. In 1940, 3,220,972 pounds of Derris were imported from the East Indies, while 3,345,843 pounds of Lonchocarpus were imported from Brazil and Peru for extraction of rotenone. Fig. 319. Difference in yield between unpoisoned and poisoned plots. Poisoned on right. Results of boll weevil control. (Courtesy of the U. S. Department of Agriculture, Bureau of Entomology and Plant Quarantine.) Contact Insecticides Are Familiar Today. Common pyrethrum plants yield two chemicals that are deadly to insects but only slightly toxic, if at all, to the higher animals. Pyrethrum is one of the first substances to be successfully used as a contact insecticide. Pyrethrum powder has been known in Europe for more than a century as Dalmatian or Persian insect powder. Early in the nineteenth century, the jealously guarded secret nature of this powder became known, when an Armenian merchant observed that it was prepared from the powdered flower heads of certain plants of the genus chrysanthemum.