Man's physical universe

xanabras

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.

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