Aside from my personal tests of many dopes, I have had some interesting correspondence on this topic with sportsmen in various parts of the world. I quote from one letter received from Norman Fletcher, of Louisville:
"Upon the swampy trout streams of Michigan on a warm May day . . . when the insects are abundant and vicious . . . pure pine tar is by far the best repellent when properly used. I give two recipes:
Mix cold in a mortar. If you wish, you can add 3 per cent, carbolic acid to above. Sometimes I make it 1 1/2 oz. tar.
Simmer for half an hour, and when cool add Oil pennyroyal........ i ounce.
There are many others of similar nature, but the above are as good as any. . . . Now as to use of above: apply freely and frequently to all exposed parts of person, and do not wash off until leaving the place where the pests abound. You can wash your eyes in the morning, and wash the palms of your hands as often as may be necessary, but if you wish to be immune, don't wash any other exposed parts. . . . When you get accustomed to it you will find some compensating comfort. ... I have had to contend with mosquitoes, deer-flies, black-flies, and midges . . . and have found "dope" with tar in it the best. I know that where mosquitoes are not very bad, oil of citronella, oil of verbena or of lemon-grass or of pennyroyal mixed with vaselin will keep them off, if the mixture is applied frequently. These essential oils are quickly evaporated, however, by the heat of the body. Camphorated oil is also used by some; this is simply sweet oil with gum camphor dissolved in it: the camphor is volatile and soon evaporates. . . . Now I don't much like tar dope because I can not wash my face and hands as often as I could wish; but when it is necessary to get some trout, without being worried too much by the insects, I can stand the tar for a few days".
Doctor L. O. Howard, Chief of the Bureau of Entomology, U. S. Department of Agriculture, recommends the following tar dope:
" Fishermen and hunters in the North Woods will find that a good mixture against mosquitoes and black-flies can be made as follows: Take 2 1/2 pounds of mutton tallow, melt and strain it. While still hot add 1/2 pound black tar (Canadian tar), stir thoroughly, and pour into the receptacle in which it is to be contained When nearly cool stir in 3 ounces of oil of citronella and 1 1/2 ounces of pennyroyal".
It is my own experience that tar glazes do the work when the weather is comparatively cool, but when it is so hot that one perspires freely both by night and day there is no chance for a glaze to be established. The stuff melts and runs in your eves. A hard rain will wash it off. Thick dopes, more or less sticky, are unpleasant at all times, and especially at night. For these reasons, and for appearance sake, most people will prefer to use a fluid or unguent that is less disagreeable, even though it must be renewed every hour or two.
As for protective liquids, it is safe to say that everything in the pharmacopoeia that seemed the least promising has been tried. The oils of pennyroyal, cloves, lavender, citronella, eucalyptus, cedar and sassafras are used singly or in combination. Spirits of camphor is offensive to insects but soon evaporates.
Citronella is the favorite. All insect pests dislike it; but some people, too, find the odor intolerable. The oil of lavender flowers (genuine) has a pleasant odor, and is equally effective, but it is quite expensive. Both of these oils are bland, whereas most of the others are irritant and will make the eyes smart if the least bit comes in contact with them. Artificial oil of lavender is worthless.
The protection afforded by a given oil depends somewhat upon locality (number, species, persistence of insects), and, apparently, the personal equation cuts some figure, for what works satisfactorily with one man affords no immunity to another. Hence the more popular dopes are " shot-gun prescriptions," compounded on the principle that if nne ingredient misses another may hit.
The trouble with all the essential oils is that their protective principles are volatile. To retard evaporation, add double or treble the amount of castor oil, which has a good body and is itself repugnant to the whole created kingdom. After mixing, put up some of this thick liquid in a small capped oil can (bicycle oiler), to carry in the field.
Thicker dopes, which can be put up in collapsible tube^ like artists' colors, are made by mixing the oil with carbolated vaseline, or with borated lanolin. The latter is a particularly good base because it is not only antiseptic but it is also the best preventive of sunburn, excellent for blistered feet, and a particularly good application for slight wounds and abrasions. Add enough oil of lavender flowerr to give it a strong odor, and put it up in tubes to keep out moisture. I know nothing better in the line of " elegant preparations" to keep off mosquitoes.
One of these is creosote. Another is the tincture of ledum palustre (wild rosemary, a European relative of our Labrador tea). Oil of cassia (i.e., oil of cinnamon) is said to be an irritant poison to all kinds of insects, and " its power remains a long time after it has dried".
Another thing that flies of all sorts find bad for their systems is quassia. It is used as an ingredient of fly poisons, as a parasiticide, and in some fly dopes. Either the fluid extract or the solid may be employed, according to the base.
Carbolic acid in sweet oil (1 to 16) is often used where insects are very insistent. It has the obvious advantage of being a good antiseptic as well. On a trip to Hudson Bay, Dr. Robert T. Morris employed a very strong solution, of which he reported: