Pests That Attack Man

by E. F. KNIPLING

EXPERIENCE during the war demonstrated that we must concern ourselves with insect problems throughout the world, especially the control of insects that affect man.

Hundreds of species of insects, ticks, and mites annoy man. But of far greater importance is that they transmit dangerous diseases. During the war, when our men and women Were stationed all over the world, we had to fight lice, which carry louse-borne typhus and relapsing fever; mosquitoes, vectors of malaria, filariasis, dengue fever, yellow fever, and encephalitis; flies, transmitters of typhoid, dysentery, and cholera; fleas, which spread plague and murine typhus; mites, carriers of a disease known as tsutsugamushi or scrub typhus; and ticks, which carry tularemia and spotted and relapsing fevers. There are other important disease carriers among the insects and arachnids (ticks and mites), but the ones mentioned are all of vital importance throughout the world, particularly in this age of fast and complicated transportation. All of them occur in this country. Fortunately, most of the diseases I listed are not present here, although malaria, encephalitis, relapsing fever, spotted fever, tularemia and murine typhus, and certain fly-borne diseases are a problem in the United States.

As a result of the research by entomologists, chemists, and engineers, new insecticides and more effective control methods were developed for every important pest that affects man's health. The Army, Navy, United States Public Health Service, Tennessee Valley Authority, Rockefeller Foundation, National Defense Research Committee, and various other agencies and industrial concerns have made valuable contributions. The advances that have resulted will make this country and other parts of the world healthier and more pleasant places in which to live. They will also be a tremendous factor in the economic development of many areas that are now handicapped because of insects and insect-borne diseases.

Among the thousands of chemicals tested by the entomologists, one material, DDT, proved outstanding. Most of my discussion is devoted to DDT, but other materials have been found useful for certain purposes, and studies on a number of promising insecticides are under way in efforts to determine their full potentialities.

The body louse (Pediculus humanus corporis Deg.) carries the dreaded typhus disease; often it is found in the clothing of destitute peoples, hidden in the seams and folds. Although a common and important parasite of man through the ages, control methods until recently were generally ineffective or unsatisfactory.

A colony of body lice was maintained under the direction of G. H. Culpepper in the Bureau of Entomology and Plant Quarantine Laboratory at Orlando, Fla. Human subjects, serving as guinea pigs, fed the lice and allowed themselves to be infested in order to test new insecticides. R. C. Bushland, Gaines W. Eddy, and associates in the same laboratory found that DDT was many times more effective than the best of the other materials known at that time. One ounce of a powder containing 10 percent of DDT, dusted on the inside of the underwear, killed all lice present at the time of treatment and killed any lice that hatched from eggs or that crawled on the person as long as a month after treatment.

Even though DDT in powder form was very good, research was continued in an effort to find even more effective ways to use the material. When clothing was dipped into a dry-cleaning solution (Stoddard solvent) containing from 1 to 2 percent of DDT, and all excess liquid wrung from the garments, Howard A. Jones, chemist, found that the treatment would prevent lice from living in the clothes for many months and it even continued to kill lice after four to six washings. A water-emulsion preparation containing 1 to 2 percent of DDT was also developed. This was made from a concentrate consisting of 25 percent of DDT, 65 percent of xylene, and 10 percent of Triton X-100.

After the value of DDT for louse control was fully established, the chemical was recommended for use by the armed services. The Army then developed simple methods of treating large numbers of people by using ordinary dust guns. The chemical has since been used in many parts of the world. The Army in Naples and the Rockefeller Foundation in Mexico demonstrated that typhus epidemics can be stopped almost immediately after the people are deloused. As a result, louse-borne typhus is now one of the diseases of man that can be easily and quickly brought under control; now we can hope that this disease, which has killed more people than all wars, will eventually be eradicated from the earth.

The head louse (Pediculus humanus humanus L.) is closely related to the body louse. Its habits are different, however, in that the head louse lives in the hair of the head instead of in the clothing. It is common throughout the world and is a problem of considerable importance in this country, especially among school children.

DDT powder dusted in the hair is effective in controlling the insects, but a powder is somewhat objectionable. Consequently, several liquid preparations containing DDT were developed by Dr. Eddy. The formula most widely used by the Army, which was known as the NBIN concentrate, consisted of 68 percent of benzyl benzoate, 6 percent of DDT, 12 percent of benzocaine ( ethyl p-amirtobenzoate), and 14 percent of a suitable wetting agent that makes the solution mix with water. One part of the concentrate must be diluted with 5 parts of water before it is applied. The resulting solution, which then contains 1 percent of DDT, kills the lice and prevents infestations for at least 2 weeks. The eggs of the lice are not affected by DDT but are killed by the benzocaine present in the formula. The benzyl benzoate, which I shall mention again later, is used because it will control the human itch mite and is a solvent for DDT and benzocaine.

The crab louse (Phthirus pubis (L.) ) is not a disease carrier, but is one of the annoying parasites that must be dealt with. It can also be readily brought under control with either the 10-percent DDT powder or with the NBIN solution. The powder must be dusted thoroughly on all hairy portions of the body, and on individuals having much body hair it should be dusted over the entire body. Because DDT will not kill the eggs or nits, a second treatment must be given from 7 to 10 days after the first. The NBIN concentrate must be diluted with 5 parts of water, as described for use against the head louse, before it is applied. The liquid formula, well applied, will control crab lice in one treatment.

The development of DDT for the control of mosquitoes is one of the most important of all advances in the field of medical entomology. The insecticide is effective against larvae and adults of both the Anopheles (malaria carriers) and culicines (pest mosquitoes and transmitters of other diseases).

In carrying out the extensive investigations that were necessary to study the various aspects of mosquito control, a large colony of a malaria mosquito (Anopheles quadrimaculatus Say) was reared under laboratory conditions in order that insects would be available for test purposes. Several thousand new materials and special preparations of promising chemicals have been tried against the larvae and adults in the laboratory, and the more effective compounds and formulas were tested in natural breeding places.