Of all the thousands of chemicals tested, fewer than 10 percent showed sufficient promise to be of service as insect repellents, and only about 15 percent as miticides. After the toxicologists eliminated those that were too toxic or irritating for frequent use, only a very few were left that can be recommended for use on man.

Because of the extreme variation in effectiveness of individual chemicals as repellents, it is impossible now to name even one that will be effective against all species. For instance, Indalone (n-butyl mesityl oxide oxalate) has been found to be one of the most effective repellents against the stablefly, Stomoxys calcitrans (L.), dimethyl phthalate is the best for our malaria mosquito, Anopheles quadrimaculatus Say, and such materials as dimethyl carbate and Rutgers 612 (2-ethyl-1,3-hexanediol) are excellent repellents for various pest mosquitoes of the Aedes group. Selected mixtures of repellents are effective against a wider range of species than any one of the individual chemicals. The most satisfactory mixtures now known are composed of 60 percent dimethyl phthalate and 20 percent Indalone; the other 20 percent may be any of the chemicals that are particularly effective against Aedes mosquitoes. The famous 6-2-2 insect repellent developed during the war is composed of 6 parts dimethyl phthalate, 2 parts Rutgers 612, and 2 parts Indalone. Mixtures containing only two repellents have proved better than single chemicals, but they are generally not so effective as the mixtures containing three selected chemicals.

Any of the repellents I have named may be used for protection from chigger bites if used as a clothing treatment. Dimethyl phthalate is the preferred repellent to use because of its effectiveness, availability, and low cost. Its chief weakness is that it is quickly removed by washing. Benzyl benzoate is much superior to dimethyl phthalate because it is quite resistant to water. However, it is a poor mosquito and fly repellent, and the cost is rather high. If a person wishes protection from chiggers for only short periods and does not expect to get wet, dimethyl phthalate or any of the other repellents may be used with excellent results. If it is desired to use a chemical that is resistant to water and that will remain effective even after the clothes are laundered, benzyl benzoate is by far the most effective available chemical.

Simple water emulsions can be made with either dimethyl phthalate or benzyl benzoate by using emulsifiers such as Stearate 60-C-2280, Tween 60, Tween 80, or a polymerized glycol monostearate. These emulsions are useful for impregnating clothing with miticides or repellents.

Insect repellents must be applied in a thin, uniform film to hands, faces, arms, or other places where insects bite. They must be reapplied as often as needed. This may be every few minutes or every few hours, depending on the species of insects and the conditions of use. Usually the repellent is applied to the skin by pouring a few drops into the palms of the hands and rubbing the hands where a film of the repellent is desired. The repellents should be applied with caution to tender places, such as eyelids and lips, because they will cause a temporary smarting. This reaction is more pronounced on sweaty skin. The same procedure may be used for applying the repellents to clothing, such as socks, shirts, or trousers, where bites occur. Care should be taken not to apply the repellents to clothing that may be damaged by the chemical, such as synthetic cloth. If the numbers of insects are unusually high, it may be desirable to spray the repellent on the clothing or use the water emulsion method described in the next paragraph, although the dilution of the repellent-emulsion mixture with water should be less-1 quart of the mixture to 1 gallon of water.

The principal use of miticides is on clothing, because the chemicals are not so long lasting as skin applications. The materials may be applied by rubbing the chemicals on the clothing by hand or with a sprayer. If these methods are used, special attention should be given to apply the chemical heavily along the openings to the clothing and on the socks both above and below the shoe tops. When complete protection from chiggers is desired, and the exposure to their bites is severe, clothing may be treated with water emulsions of the miticides. The most practical way to use the emulsions is to dissolve 10 parts of the emulsifier in 90 parts of the dimethyl phthalate or benzyl benzoate by weight. Use one-half pint of this mixture to 1 gallon of water. It is best to agitate vigorously 1 part of the concentrate in 2 or 3 parts of water to form a creamy emulsion and then dilute with the remainder of the water. Agitate the emulsion while using to assure a uniform emulsion. Repellents applied to the suit, or shirt and trousers, will greatly reduce the bites from ticks and fleas.

THE AUTHOR

Bernard V. Travis is an entomologist in the Division of Insects Affecting Man and Animals, Bureau of Entomology and Plant Quarantine. He has conducted research in wildlife parasites with special reference to fire ants; screwworm biology; saltmrash mosquito biology and control; and repellents and insecticides for the armed forces, as a member of the Department and in the Navy.