DDT in the Home

by L. S. HENDERSON

EVER SINCE man began to live under a shelter, insects have followed him into that shelter. Whether he has lived in a cave, a grass hut, a tent, a log cabin, a frame house, or a palatial mansion, insect pests have been present. They may be only annoying, or they may be injurious. Insects in man's home may bite him, spread disease, contaminate or destroy his food, damage his clothing, ruin his household furnishings and possessions, or even attack the structure of the dwelling.

From the time man was first troubled by these pests in his home, he has spent a great deal of time and effort in attempting to develop new and better ways of getting rid of them. Early methods were often crude and ineffective. Modern advances in biology have revealed information that helps us to know when and where we can best apply control measures. Physics and engineering developments have helped us devise new equipment and new ways of applying insecticides. The chemists have found or made up new compounds that are more effective for controlling insects.

One such compound is DDT. Its discovery has opened the way to new and better methods for controlling insects in the household. Chemists are developing other similar or closely related compounds, and already some have been found that may prove even more useful than DDT.

The older sprays available for the control of household insects had to be used so the insects were hit directly with the spray. Such sprays were useful in obtaining an immediate reduction in the number of insects present, but the effect of the spray lasted for a few minutes and then more insects would begin to come in again. Often it was impossible to hit all the insects and many of them escaped or were unharmed in protected places. Repeated applications of spray had to be made in an attempt to control such pests as roaches, bedbugs, carpet beetles, silverfish, fleas, brown dog ticks, flies, and mosquitoes.

Investigators found that DDT sprays had an unusual property not possessed by the common household sprays. They found that when DDT insecticides were sprayed on surfaces, insects crawling over the remaining residual deposit of DDT were killed. This residue continued to be effective for several months, and sometimes for a year or more. In practical tests in homes where repeated applications of an ordinary household spray would have been required to eliminate insect infestations, a single DDT residual treatment did the job. Insects in protected places not hit by the spray have to crawl over a DDT deposit when they come out into a treated room in search of food and water, and are killed. Not only is the existing infestation eliminated but for some time after that any insects that happen to find their way into the treated area are killed before they can establish a new infestation.

Insects are not killed immediately upon contact with a DDT residue. As they crawl over it, however, the DDT is absorbed into the body through the feet and body wall. Exceedingly small amounts in the insect body begin to affect the nervous system. The first symptom is a restlessness or excitement which may cause the insect to crawl or fly away. Later the insect is seized with tremors and convulsions, loses its ability to make coordinated movements of flight or crawling, and eventually rolls over on its back. Kicking and twitching of the legs continues for some time, but gradually the insect grows weaker, and death follows.

Death may occur a few minutes or several hours after contact with DDT, depending upon the resistance of the insect and the condition of the deposit. Death may not occur for several days in the case of some of the more resistant insects that touch only small amounts of residue.

DDT has no fumigating effect and insects will not be killed simply by being in the same room where DDT has been used. They do not have to eat the material to be killed, but they must actually come in contact with it, although continuous contact is not necessary. During the first brief contact with a DDT residue a fatal amount of DDT is absorbed by the insect, and it will die, even if it moves off to an untreated area. During the period of excitement some insects may fly or crawl for a considerable distance before they become inactive. In some cases very few dead insects may be found immediately in a treated area. This does not mean that the DDT is not effective. You should look for its results in the absence of live insects, not in the presence of dead ones.

Although the DDT residue is slower in killing insects than are some other insecticides and although it is not effective against all kinds of insects, it does have advantages that other insecticides do not have. One is its long-lasting effect. Another is that DDT is effective against more different kinds of insects than most other insecticides, and its use provides the most effective and simplest method of control yet discovered for some household insects. As is true of the best of insecticides, however, DDT must be used properly in order to give satisfactory results.

Another advantage is its adaptability to use under different conditions and by different methods. It can be formulated into a number of different kinds of sprays and dusts, although it is not a good insecticide in its original form. Preparation of most DDT formulations requires special equipment or technical knowledge and skill and cannot be done satisfactorily in the home.

There are several DDT sprays that may be used in the home for the application of a DDT residual deposit. The most suitable one is a 5-percent solution of DDT in deodorized kerosene. An emulsion containing 5 percent of DDT can be prepared from commercially available concentrates. If the concentrate contains 25 percent of DDT, mix one part of concentrate with four parts of water to form a 5-percent spray. Wettable powders are also available for the preparation of DDT suspensions. These powders have been specially prepared so they can be mixed with water, and usually contain 50 percent of DDT. Follow the directions on the container label for preparing the spray mixture.