R. M. Bohart and W. D. Murray reported that unsatisfactory results in the control of Aedes nigromaculis was experienced in Tulare and Merced Counties in 1949. To confirm the field observations, laboratory tests were made with larvae of the species collected in three pastures, which had previously received repeated DDT larvicide applications, in the mosquito-abatement district in Tulare County. They compared the larvae with larvae from three pastures in Kings County not known to have been previously treated with DDT. Based on the dosages required to cause 50 percent mortality, the average for the larvae from Tulare County was more than 10 times that for the control larvae. The least resistant of the larval lots from the treated fields required about three times as much DDT as the most resistant lot from the control area. In comparative tests between DDT and toxaphene, the latter was considerably the more toxic to the DDT-resistant larvae but less toxic to the control lots.

DDT residues applied to walls of living quarters and other buildings have been widely used in different countries to control carriers of malaria. Tests to determine whether an increase in tolerance had occurred were carried out in two areas where this method of control had been in operation for several years. The results were negative.

In the Mexican village of Temixco, DDT sprays were applied to the interior wall surfaces of all houses and other buildings once in early spring each year from 1945 to 1948. The sprays reduced markedly the numbers of Anopheles pseudopunctipennis in the village and in the surrounding rice fields. Laboratory tests were run in 1948 under the direction of J. B. Gahan and Wilbur G. Downs to determine the relative susceptibility of adults that had been collected in the village and the untreated village of San Jose, about 10 miles away. In June and July 96 tests were conducted with about 2,000 adult mosquitoes from each village. The insects were reared from gravid females collected in the two places and were tested by exposure to cloth panels impregnated with DDT. The average mortality was somewhat higher for the mosquitoes from the treated village than it was for those from the untreated village (56 percent versus 43 percent for the two sexes combined). The finding seemed to demonstrate that no loss of susceptibility had occurred.

G. F. Ludvik and others reported in 1950 on the first year of a study of DDT resistance in Anopheles quadrimaculatus in the Tennessee River Valley after 5 years of routine treatment, in which were used DDT residues against larvae and adults. They subjected specimen material to a variety of tests in comparison with similar material from untreated areas. The comparisons consisted of laboratory larvicidal tests in suspensions of DDT, exposures of larvae in pans to aerial DDT sprays, exposures of adults to DDT-treated panels, and release of adults in residue-treated rooms. The mosquitoes from treated areas showed slightly greater tolerance to DDT in some of the tests, but the workers concluded from their preliminary studies that they had not developed an outstanding resistance.

R. W. Fay and others have reported the results of preliminary experiments to determine the possible development of a resistant strain of Anopheles quadrimaculatus. Adults of an insectary-reared colony of the species were exposed for four successive generations to DDT-treated panels for enough time to give mortalities of about 66 percent. Eggs from the surviving females then were obtained for rearing. In tests of susceptibility to DDT of the exposed strains, the mean mortality showed a slight but statistically significant drop in the first generation. No change occurred during the next three generations but was followed by an increase to the original level in the first generation after discontinuance of exposure to DDT. In tests against other insecticides, a similar loss of susceptibility was shown to methoxychlor but not to chlordane, benzene hexachloride, aldrin, or TDE (DDD). Because the pattern of increase and decrease in resistance was basically different in these tests from that reported for house flies (in which the changes in each direction were much more gradual) further confirmation of these results seems necessary before conclusions can be drawn.

W. V. KING is a technical consultant in the Orlando laboratory of the division of insects affecting man and animals, Bureau of Entomology and Plant Quarantine. The work at the laboratory is supported by funds allotted by the Secretary for Defense for investigations of entomological problems of medical importance to the military forces and the development of methods of control of the insects involved. Dr. King has been with the Bureau most of the time since 1912 and his work has been chiefly on insects affecting man.

As a special agent of the Rockefeller Foundation, he spent 3 years in the Philippines on investigations of malaria mosquitoes. On active duty in the Sanitary Corps of the Army during the Second World War, he spent nearly 3 years in New Guinea and other parts of the western Pacific on malaria control and mosquito investigations. He was in charge of the Orlando laboratory until 1951, when he relinquished his administrative duties.