358 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1947 



DDT. Field tests by J. B. Gahan (5) in that year against Anopheles 

 quadrimaculatus Say at Tallahassee, Fla., and Stuttgart, Ark., dem- 

 onstrated reduction of adults by at least 91 to 99 percent and of larvae 

 by at least 57 to 63 percent. The desirability of treating all the dwell- 

 ings in a highly malarious district became evident. A satisfactory 

 location near Cuernavaca, Mexico, was suggested by George C. Payne, 

 of the International Health Division of the Rockefeller Foundation. 

 We liked Dr. Payne's suggestion because the malaria vector in that 

 locality was a different species of Anopheles and we were eager to 

 learn whether all species were equally vulnerable to DDT residual 

 or surface sprays. In the Cuernavaca area this anopheline breeds 

 almost exclusively in the rice j&elds, therefore the larvae are restricted 

 to well-defined areas. Several small Mexican villages a few miles 

 west of Cuernavaca seemed to offer just the combination of conditions 

 we were looking for — a new species of anopheline with restricted 

 breeding habits, an impoverished community, and lots of malaria. 



Early in February 1945 J. B. Gahan (3) was detailed to go to 

 Mexico and, with the cooperation of the Rockefeller Foundation and 

 the Mexican Department of Health, treat two villages with a DDT 

 spray. Two nearby villages were to be left untreated as checks. 



In April and May a 5-percent DDT water emulsion prepared from 

 a concentrate containing 25 percent of DDT, 68 percent of xylene, 

 and 7 percent of Triton X-100 was applied to dwellings at a rate of 

 about 1 gallon to each 1,000 square feet (about 200 mg. of DDT per 

 square foot) . This work was well done by two trained and responsible 

 Mexican technicians under the daily guidance of Gahan. The houses 

 sprayed had walls of adobe, stone, wood, brick, straw, and cane. The 

 relative abundance of ATwpheles larvae in the treated areas and in 

 comparable untreated areas was determined from samples of water 

 dipped in the adjoining rice fields. 



The numbers of adult mosquitoes found in the treated villages after 

 May were never more than 2 percent of the numbers found in the un- 

 treated communities and were usually less than 1 percent, indicating 

 reductions of more than 98 percent. Assuming that a treated village 

 and an untreated one used for comparative purposes would have had 

 about the same mosquito populations had no sprays been applied, we 

 can say that the DDT residues reduced the number of larvae at least 

 85 percent throughout a 4-month period. However, if an adjustment 

 is made for a difference actually found between the two villages before 

 the spraying was started, the decrease is calculated as 94 percent. 



I flew to Mexico City in September 1945 (15) and spent a month 

 observing this project just before the work was closed down for the 

 season, Mr. Gahan and his Mexican assistants easily proved to me 

 the striking scarcity of adult mosquitoes in the villages that had been 

 treated and of anopheline larvae in the adjoining rice fields, in com- 



