Science: Careful with DDT

DDT, the “wonder insecticide” that had millions of dollars worth of free advertising, is a very good insecticide. But it will not do everything. In too small quantities, it will not do anything. To eager manufacturers of bug sprays with “flagrant” claims, the Department of Agriculture has said sternly: label your products properly and give accurate directions. To the bug-plagued public, the department has offered some advice:

¶ DDT is best as a “residual insecticide.” It stays on a treated surface for several weeks or even months, killing most insects which come in contact with it. But a good strong treatment is necessary. Oil solutions or water emulsions for residual spraying should contain at least 5% DDT. Weaker solutions do not leave enough of the stuff behind.

¶ The famous “bug bombs” (aerosol sprays) are excellent for a limited purpose. They should contain both DDT and pyrethrum (or some similar toxicant). The pyrethrum knocks flies and mosquitoes down quickly. The slower DDT keeps them from rising again. But bug bombs leave no appreciable residue. Bugs that arrive in the room after the air has cleared remain in good health.

¶ DDT is definitely—though not violently—poisonous. It should not be breathed or kept near the skin. After working with it, hands should be washed with soap. Only special dust mixtures should be used on pets or human beings.

¶ Not all insects are easily killed by DDT. Roaches are the outstanding resisters. The best technique on roaches: attack often and leave a heavy residue on the places where they hang out.

¶ Not much is known as yet about the full effect of DDT on large areas. Thus far, wholesale sprayings have had some surprising—and bad—effects on birds, fish and desirable insects. People who plan to treat whole islands or townships had better wait until the scientists experiment some more.