74 CIRCULAR 9 7 7, U. ft. DEPARTMENT OF AGRICULTURE 



and elsewhere. They are also the vectors of scrub typhus, or tsu- 

 tsugamushi disease, in the Oriental and Australasian regions. 



Chiggers are commonly found on damp ground protected by 

 vegetation or leaf mold, such as margins of lakes and streams, shady 

 woods, high grass or weeds, and briar patches. They also are picked 

 up at times on lawns, golf courses, and in parks. Decaying logs and 

 branches are often heavily infested. Because of their small size 

 chiggers are not easily seen except against a shiny black background. 

 To locate infested areas a piece. of black cardboard may be placed 

 edgewise on the ground in places that appear favorable for the pests, 

 or black oilcloth may be spread over stumps, logs, or dead twigs. 

 After a few moments the chiggers may be seen moving rapidly over 

 the surface and accumulating on the upper edge. They can also be 

 detected on polished black shoes, and these are sometimes used for a 

 rapid survey. 



DDT is not very effective against chiggers, but dieldrin, chlordane, 

 toxaphene, and BHC have all given good control when sprayed on 

 infested ground at rates of 1 to 2 pounds per acre. Emulsion sprays 

 gave the best results, but dusts were also effective in some tests. 

 Dieldrin at 1 pound and toxaphene at 2 pounds per acre maintained 

 a high degree of control for 1 to 2 months or more. The other mate- 

 rials were slightly less effective and less consistent. 



The treatment of clothing as protection against chiggers is discussed 

 on pages 75-80. 



REPELLENTS 



More than 10,000 chemicals were given screening tests as mosquito 

 repellents at the Orlando laboratory. Many of them were synthesized 

 especially for this purpose during the war by university groups under 

 contracts with the Office of Scientific Research and Development. 

 Smaller numbers were tested against other insects, usually after their 

 repellency to mosquitoes had been demonstrated. In the laboratory 

 tests several hundred of the liquid compounds, when applied to the 

 skin, gave complete protection against the yellow-fever mosquito for 

 3 hours or more, and were placed in the most effective category 

 (class 4). However, most of them were eliminated from further 

 consideration because of skin irritation or sensitization, disagreeable 

 odor, or staining properties. Others were not available in sufficient 

 quantity for further testing. The remainder were subjected to 

 extensive evaluation in laboratory and field tests under various 

 conditions and against different species of mosquitoes. 



REPELLENTS APPLIED TO THE SKIN 



It was shown in these studies that a repellent may differ greatly in 

 effectiveness against different species. Therefore an investigation 

 was made of mixtures as a means of providing protection against a 

 wider range of species. In this way the mixture known as 6-2-2 was 

 developed and adopted as standard issue. It contained 6 parts of 

 dimethyl phthalate and 2 parts each of 2-ethyl-l,3-hexanediol and 

 I nd alone. Indalone was included because of its effectiveness par- 

 ticularly against the biting stable fly, but as this species is seldom 

 important as a pest of persons, it was later eliminated and other 



