Many of the household fly sprays now on the market have from 1/2 to 3 percent of DDT added to the toxic ingredients already present in the spray. Such sprays will give a quick kill of crawling insects that can be hit directly with the spray. Their best use is as a space spray, where the air is filled with a fine floating mist of spray, to obtain quick, temporary relief by killing the flying insects in your home. You should not use preparations containing more than 3 percent of DDT as space sprays. Although there may be some slight residual effect from these weak DDT solutions, you should not depend on them for applying an effective DDT residual deposit.

The gas-propelled aerosols now available commercially are a war development and are another form of space spray. Most of them contain DDT and pyrethrins and they are excellent for obtaining a quick kill of flying insects in a closed room. When used at many times the normal rate of application they may be helpful in the control of crawling insects. At these high concentrations they may be irritating, and they have no lasting effect. For permanent results repeated applications are required.

DDT can be ground and diluted with inert carriers such as talc or clay, and in this form it is a good insecticidal preparation for applying a residual deposit, especially against some of the crawling insects. Some DDT dusts for agricultural purposes contain only 1 to 3 percent of DDT, but for use as a household insecticide a dust should contain 10 Percent of DDT.

In order to obtain the greatest benefit from the use of DDT, you should take advantage of its long-lasting effect and apply it to surfaces as a residual treatment. The important thing to remember is that you should treat surfaces where the insects to be controlled will come in contact with the DDT deposit. It is evident that the treatment will have to be applied in different places for the control of different household pests, because of their varying habits. A rather complete coverage of all surfaces can be made for general insect control. It is not often necessary to make such a general application to control any one insect problem. The usual procedure will be to treat only selected areas to control certain insects.

A 5-percent solution of DDT in refined kerosene is most suitable for the control of flying insects such as flies, mosquitoes, and wasps. It is important to treat screens on windows and doors. In localities where insects are not abundant this may be sufficient. Even where insects are abundant it is not necessary to spray the entire interior. For the control of flies, spray such favored resting places as hanging light fixtures and drop cords; projections or uneven places on walls and ceiling; and edges of beams, arches, and door and window frames. Pay particular attention to treating the kitchen and dining room, where flies may be attracted by food odors. It will be helpful to spray outside the house on the porch, around doors, and around garbage cans. If this is done, many flies will be killed before they get a chance to enter the house. For controlling mosquitoes spray in dark corners or secluded spots, and under beds or other furniture where they may hide during the day.

A paint brush may be used for applying the DDT solution to screens. Any good sprayer that produces a moderately coarse spray can be used to apply the solution in other places. Most ordinary household sprayers will do. Put on just enough spray to moisten surfaces thoroughly. One quart of solution will cover 250 to 500 square feet, depending upon how absorbent the surface is.

For the control of crawling insects such as roaches, silverfish, clothes moths, carpet beetles, ants, bedbugs, fleas, and brown dog ticks, the solution may have to be sprayed on floors, lower parts of walls, around baseboards or moldings, around door and window frames, in corners, in cracks or crevices, on beds, behind or underneath objects, or within cabinets, cupboards, drawer spaces, closets, or storage spaces. Remember the habits of the insect to be controlled so a DDT residue will be left in places where they develop, feed, seek shelter, or will crawl over the deposit in the course of their normal activities.

The 10-percent DDT dust is useful against many crawling insects. It should be applied with a small hand duster of the bellows, bulb, or plunger type, using the nozzle to blow the powder into cracks or crevices, and behind or underneath objects where it would be difficult to reach with a spray. The spray should be applied in exposed places where the appearance of the white powder would be objectionable, and to vertical or underneath surfaces where a powder would not adhere. Although most infestations can be controlled with either the liquid or powder, the combined use of the two, each in the places to which it is most suited, will often give more rapid or more satisfactory control.

Precautions

Although DDT is a poison, it is not so dangerous as sodium fluoride or arsenicals, which are commonly used. It is perfectly safe to use if a few simple precautions are observed.

Do not contaminate food or utensils when using DDT. Do not store DDT containers where they might be mistaken for food packages, or where children can reach them. Food in packages can be stored safely in sprayed places after the spray has dried.

Do not spray an oil solution near open fires. Avoid excessive or prolonged contact of the skin with an oil solution of DDT. Wash with soap and water when you are through spraying. Do not breathe large amounts of spray mist.

Caged birds and goldfish should be removed from rooms being treated with DDT insecticides. Any oil spray is likely to injure house plants.

THEAUTHOR

L. S. Henderson is an assistant chief of the Division of Insects Affecting Man and Animals in the Bureau of Entomology and Plant Quarantine. A graduate of the University of Kansas, Dr. Henderson joined the Department in 1938. After 4 years in Alabama, where he tested insecticides against the white-fringed beetle, he transferred to the Agricultural Research Center at Beltsville. He did the preliminary work on a method of testing and evaluating liquid roach insecticides and was in charge of the household insect project before he was brought into the divisional office in Washington in his present capacity.