New Insect Repellents

by BERNARD V. TRAVIS

THE OLD MIXTURES of citronella and pennyroyal that had been used for generations to ward off mosquitoes and punkies and blackflies had little value on Bougainville and Guadalcanal. They gave inadequate protection; they had an objectionable oiliness; they did not last long enough, especially on wet and sweating men. In fact, scientists who undertook to discover new and better repellents for the Army found 500 organic chemicals, of the 7,000 they tested for insect repellency, that are superior to the older repellents containing the aromatic oils. More than comfort was involved for the soldiers.

Early in the war it was recognized that men in the outer perimeter of the combat zones would have to rely on repellents for personal protection from the bites of mosquitoes and other insects; at that time control procedures were not known that would quickly eliminate the disease-carrying insects from the active-combat zones, and repellents offered an important means of protection for isolated troops.

Civilians also need good preparations to keep insects away. In certain areas permanent control measures for mosquitoes, sand flies, punkies, and chiggers may be impossible because of unfavorable terrain. In other places control is possible, but the low value of the land or sparse population make it unwise to levy sufficient taxes for the installation and maintenance of permanent control. Large sections of our finest recreational land are unusable because of the extreme annoyance from biting insects during vacation seasons; besides parks and other playground areas, there are sections that remain undeveloped where insects seriously interfere with agriculture or commerce during at least part of each year. For those areas, where control of the pests is either impossible or impractical, good insect repellents and miticides can give satisfactory protection.

In an effort to find improved repellents and to supply the armed forces with information, a project was initiated by the Bureau of Entomology and Plant Quarantine at Orlando, Fla., under the sponsorship of the Office of Scientific Research and Development. In this project chemicals were tested in the customary manner as a skin application and also as a clothing treatment.

In 1944 A. H. Madden, A. W. Lindquist, and E. F. Knipling reported that when the new organic insect repellents were applied to their clothing they received almost complete protection from chigger or red bug bites. Before that protection from chiggers was obtained largely by dusting sulfur on clothing. The sulfur method gives only partial protection and is objectionable to many persons. The Australians later showed that the repellent materials acted on chiggers largely as toxicants rather than as repellents; therefore, in discussing chiggers, the term "miticide" is used here instead of repellent. Between 1942 and 1946, F. M. Snyder and others of the Orlando laboratory tested about 7,000 materials on cloth in their search for better miticides. Many materials were found to be effective as fresh treatments, but only about 300 were outstanding in their resistance to removal by water.

In the investigations we encountered a number of important problems for which we do not yet have answers. Because most of our work was to test the effectiveness of chemicals as repellents, we had little time to get fundamental information as to how the chemicals repel insects. As basic information is accumulated on the repellent mechanism, it may be possible to develop superior repellents.

Another problem is the variation in effectiveness of individual chemicals when tested against different insects. Certain materials that are satisfactory against some species may fail to repel others. Dimethyl phthalate, for instance, is a good repellent for the common malaria vector in the United States, Anopheles quadrimaculatus Say, but it is almost worthless for one of the South Pacific malaria vectors, Anopheles farauti Laveran. Also, during certain periods, an insect species may bite immediately on skin that has been freshly treated, but at other times it may be repelled completely for several hours.

There is considerable variation in the effectiveness of repellents when used on different individuals. The same material may protect one person for several hours, but on another it may be effective for only a few minutes. The insects will bite readily on any spot either on exposed skin or through clothing where no repellent has been applied or where it has been rubbed off. All repellents are washed from the skin easily with water or excessive perspiration.

Repellents have some rather undesirable characteristics, and efforts are being made to discover new materials that do not have these properties. All of the better liquid repellents now in use are rather viscous and feel oily on the skin. Most of the repellents are plastic solvents and will damage paints, varnishes, and many other plastic materials and synthetic cloth, but they can be applied safely to cotton or wool. Care must be taken in their use, as they cause temporary smarting of tender skin or mucous membranes.