Insecticides for Cotton

by R. W. HARNED

COTTON, an important crop since before the dawn of history and one that has been grown in North America since the earliest colonial days, has always had serious insect enemies. Against them, most of the materials known to control insects attacking plants in the United States have been tried. The boll weevil, bollworm, cotton leafworm, cotton aphid, cotton flea hopper, and other pests continue, however, to reduce yields and destroy cotton crops.

The first potent weapon was paris green, the use of which was recommended in 1872 by Prof. C. V. Riley, then the State Entomologist of Missouri, to control the cotton leafworm. Its use spread rapidly, especially in the Gulf Coast States, and during the next 40 years thousands of tons were used against the cotton leafworm and the bollworm.

Next came the discovery in 1916 by workers in the Department of Agriculture that calcium arsenate dust of certain specifications was effective and practical for the control of the boll weevil. Later investigations showed it to be useful against the bollworm, and in mixtures with sulfur to be of great practical value in the control of the cotton flea hopper, tarnished plant bug, and other plant bugs and stinkbugs that attack cotton. Because of its wide use for boll weevil control and its lower cost, it largely replaced paris green for the control of the cotton leafworm and bollworm.

For the control of cotton insects about 50 million pounds of this arsenical insecticide are now used each year. The need for applying calcium arsenate to large acreages of cotton stimulated improvement of dusting Machines, but no ground machines could be satisfactory for use when the Soils were saturated with water. In 1922 airplanes were first used for the application of calcium arsenate to cotton. This method was so satisfactory that within a few years there were several dozen companies operating hundreds of airplanes used largely for dusting cotton.

During the decade 1920-30 two other materials, sulfur dust and nicotine, were first used extensively for the control of cotton insects. Sulfur had long been known as an insecticide, but was not used on cotton until it was found to be helpful in reducing losses caused by the red spider and the cotton flea hopper. Sulfur later was found to be useful in reducing infestations of the tarnished plant bug and rapid plant bug and its use helped to check the reduction in quality of cotton in fields where injurious stinkbugs were abundant. Mixtures of sulfur and arsenicals, such as calcium arsenate and paris green, were found to be more effective against the cotton flea hopper and the other plant bugs and stinkbugs that attack cotton, and in recent years millions of pounds of these mixtures have been dusted on cotton in Texas, Arizona, New Mexico, and California. The cotton aphid had always been a minor pest of cotton until the extensive use of calcium arsenate for boll weevil control, about 1920, caused it to become a major problem. The heavy aphid infestations develop because calcium arsenate decreases the acidity of the plant juices and there is some evidence that this favors the rapid development and reproduction of the aphids. The arsenical dust also kills the beneficial parasitic and predaceous insects that normally destroy the aphids and prevent serious infestations. Nicotine was found to be fairly satisfactory for the control of the cotton aphid and its use for this purpose has steadily increased during the past 25 years. The method of application generally used is to mix nicotine sulfate with calcium arsenate or with lime for dusting cotton. During recent years the supply of nicotine available for use on cotton has not been sufficient to meet the demand.

During the 1930's the insecticides recommended and widely used for the control of cotton insects were calcium arsenate, sulfur, lead arsenate, paris green, nicotine sulfate, and mixtures of them.

Recent research has developed several materials that may be as effective as the insecticides developed in the years since paris green was first used in the cotton fields, or even more effective. Experiments have shown that mixtures of basic copper arsenate and sulfur have good dusting qualities and when applied at the rate of 12 to 15 pounds an acre are more effective than calcium arsenate for the control of the bollworm. They are more effective than sulfur or mixtures of sulfur and calcium arsenate for the control of the cotton flea hopper, nearly equal to calcium arsenate against the boll weevil and less likely to cause injurious aphid infestations, and better than calcium arsenate and equal to lead arsenate against the cotton leafworm because they adhere to the foliage and are effective for weeks after application. Their chief disadvantage is that they cost more than calcium arsenate or mixtures of calcium arsenate and sulfur.

Cryolite applied at the rate of 8 to 10 pounds an acre is more effective against the bollworm than calcium arsenate, but is much less effective against the boll weevil and cotton leafworm. Research had shown that rotenone has possibilities against the cotton aphid, but the war interfered with investigations with this material and it was not available for use on cotton. Additional research may disclose that it may be useful against aphids and possibly other insects on cotton.