the insecticides, when treatments have been discontinued and residues on walls and in the soil disappear, then the chlorinated hydrocarbons will Mosquitoes again become useful as chemicals to reduce fly populations. and DDT 4. Still greater emphasis should be placed on sanitation. Insecticides W. V. KinP should be used only to augment the fly control obtained by strict sanita- tion. There is no substitute for sani- Increased resistance to DDT has tation. been recorded for several species of mosquitoes in widely separated parts of W. N. BRUCE, a native of Nebraska the world. Included are the house mos- and a graduate of the University of quitoes, Culex pipiens in Italy and C. Nebraska, is associate entomologist for quinquéfasciatus in India; two salt- the Illinois Natural History Survey. marsh species, Aedes taeniorhynchus He worked at Iowa State College for and A, sollicitansj in Florida; and two 2 years as ifistructor and research as- floodwater species, Aedes nigromaculis sociate and has conducted research on and A. dor salis ^ as well as Culex tar salis insects affecting man and animals for in California. An encouraging fact is the Illinois Natural History Survey that two species of Anopheles failed to since 1945. show increased resistance in areas where they had been exposed for sev- For further reading on resistant flies, Mr. eral years to DDT residual treatment Bruce suggests his articleSj Latest Report on in buildings. Fly Control, Pests, volume ly, number 6, E. Mosna was apparently the first pages y y 28 (1949)3 and House Fly Toler- ance for Insecticides, with G. C. Decker, to report increased resistance in a Soap and Sanitary Chemicals, Volume 26, species of mosquito, Culex pipiens number 3, pages 122-125, 145-^47 i^QSo), autogenicus (molestus) from the Ton- and articles by— tine marshes in Italy. He found many W. V. King and /. B. Gahan: Failure of DDT to Control House Flies, Journal of live specimens of the species in bed- Economic Entomology, volume 42, pages rooms of houses in May 1947, where 405-409. 1949. for the second year 5 percent DDT in Arthur W. Lindquist and H, 0. Wilson: kerosene had been applied as a residue Development of a Strain of Houseflies Re- sistant to DDT, Science, volume loy, page for the control of Anopheles. Speci- 2y6. 1948. mens he collected from the interiors Ralph B. March and Robert L, Metcalf: were exposed to the treated walls and Insecticide-Resistant Flies, Soap and Sani- were alive after 48 to 72 hours, but tary Chemicals, number y, pages 121, 123, specimens from a laboratory strain died ^35> ^39' 1950' A. Missiroli: Riduzione o cradicazione within 3 to 5 hours. He thought it pos- degli anofcli? Rivista di Parassitologia, vol- sible that two races of this variety of ume 8, number 2/3, pages 141-169. i94y. mosquito might exist, distinguished K. D. Quarterrnan: The Status of Fly Resistance to Insecticides in the Savannah basically by the difí'crent grade of re- Area and Its Implications in the General sistance to DDT. Laboratory tests with Problems of Fly Control, C. D. C. Bulletin, the eighth generation reared from re- volume 9, number 11, pages 3-y. 1950, sistant material showed that the resist- Giuseppe Saccà: Sull'esistenza di mosche ance was transmitted through eight domestiche rcsistenti al DDT, Rivista di Parassitologia, volume 8, number 2/3, pages generations without marked diminu- i2y-i28. i94y. tion. From preliminary laboratory and James Sternburg, C. W. Kearns, and W. field tests with chlordanc and benzene N. Bruce: Absorption and Metabolism of hexachloride, Mosna learned that the DDT by Resistant and Susceptible House Flics, Journal of Economic Entomology, insecticides had residual action lasting volume 43, pages 214-219. 1950. more than 4 months and were thcre- 328 Yearbook of Agriculture 1952 fore suited to practical control of Culex the untreated areas at the time the col- that are resistant to DDT. lections were made for the tests. In India, from experiments con- The evidence from the laboratory ducted for 10 months, J. F. Newman tests demonstrated the increased toler- and others learned that successive gen- ance of the specimens from the treated erations of the southern house mos- areas. In the larvicide tests, the mortal- quito exposed in the laboratory to DDT ity of fourth-stage larvae averaged residues showed a marked increase in about 16 percent, compared with an resistance to DDT. A 20-minute expo- average of nearly 90 percent for the sure caused 100 percent mortality of control larvae. Similarly, in space-spray females originally, but no mortality re- tests with I percent DDT solutions sulted from 30-minute exposures a few against reared females, the comparable months later. A similar resistance to figures were 18 percent and 83 per- benzene hexachloride also was shown. cent. The results indicated a fourfold The failure of DDT sprays to give increase in tolerance or more. Larvae satisfactory control of the common of Aedes taeniorhynchus collected in salt-marsh mosquito and another salt- 1949 from a treated area in Sarasota marsh species, Aedes taeniorhynchus^ County on the Gulf Coast also showed in Broward County in Florida, was first increased resistance. noticed in 1947 in Hollywood, where In tests on mosquito specimens from miuch DDT had been applied in pre- Brevard County, chlordane and ben- vious years to control heavy infesta- zene hexachloride, both technical and tions. refined (lindane), produced about the The failure was observed again in same mortalities of larvae and adults 1948 and 1949, when similar difficulty from the treated areas as from un- was experienced in Brcvard County treated areas at similar dosages. That near Cocoa Beach and the Banana was true also of parathion in larvicide River Airbase, where an extensive salt tests. Parathion was not included marsh had been treated repeatedly against the adults. TDE, like DDT, with DDT sprays the previous 4 years. was much less toxic to the specimens In June 1949 the results of aerial spray- from the treated area. Toxaphene was ing operations in the area were checked somewhat less toxic. Lindane was by by members of the Orlando laboratory far the most toxic compound to adults, of the Bureau of Entomology and Plant and parathion to the larvae from all Quarantine. It became evident that areas. satisfactory reduction of adults of the Aerial spray tests with several in- two salt-marsh species A. taeniorhyn- secticides were also carried out against chus and A. sollicitans was not ob- the DDT-resistant mosquitoes. Lin- tained with the standard dosage of 0.2 dane, the most efTective of the insecti- pound of DDT per acre. Even twice cides tested, gave good control of adults that dosage failed to give as good con- at dosages of 0.05 and o.i pound per trol as had been obtained with the acre. Technical benzene hexachloride standard dosage. This indication of in- (12 percent gamma) at 0.2 and 0.4 creased resistance was confirmed by pound and dieldrin and parathion at laboratory tests in which larvae and 0.05 and o. I pound gave results nearly reared adults of A. taeniorhynchus and equal to lindane. Chlordane and DDT A. sollicitans were compared for sus- at 0.2 and 0.3 pound per acre and ceptibility to DDT with similar speci- toxaphene at 0.2 pound were not men material of A. taeniorhynchus highly efiPectivc in most tests. from other areas in the same county Larvicidal tests on small plots were that w^ere not known to have received conducted with several insecticides ap- DDT applications previously or only plied as emulsions. In the Cocoa an occasional treatment for adult con- Beach area DDT was much less effec- trol. A. sollicitans were not present in tive than in untreated areas, but the Mosquitoes and DDT other materials—dicldrin, parathion^ parative tests between DDT and tox- lindanCj technical benzene hexachlo- aphene^ the latter was considerably the ridcj and toxaphene—all gave good more toxic to the DDT-resistant larvae and approximately similar results in but less toxic to the control lots. both the treated and untreated DDT residues applied to walls of marshes. Dieldrin and parathion were living quarters and other buildings the most effective at dosages of 0.025 have been widely used in different and 0.05 pound per acre, closely fol- countries to control carriers of malaria. lowed by lindane and toxaphene. Tests to determine whether an increase Indications of increased resistance to in tolerance had occurred were carried the effects of DDT in larvae of Aedes out in two areas where this method of nigromaculis and A. dor salis in Kern control had been in operation for sev- County, Calif., were noticed in the fall eral years. The results were negative. of 1947 and early in 1948 on a large In the Mexican village of Temixco, ranch that had been regularly treated DDT sprays were applied to the inte- by truck and plane and had also been rior wall surfaces of all houses and used for experiments on the applica- other buildings once in early spring tions of DDT emulsion siphoned into each year from 1945 to 1948. The the irrigation water. The dosage was sprays reduced markedly the numbers increased from 0.15 to 0.25 parts per of Anopheles pseudopunctipennis in million with continued failure. Later a the village and in the surrounding rice part of the fields was treated with DDT fields. Laboratory tests were run in by plane at the rate of 0.4 pound per 1948 under the direction of J. B. acre and part with toxaphene at 0.3 Gahan and Wilbur G. Downs to de- pound. The toxaphene killed all stages termine the relative susceptibility of of larvae, but the DDT failed to kill adults that had been collected in the even the first stages. Complaints that village and the untreated village of DDT was not giving good control of San Jose, about 10 miles away. In the larvae of Culex tar salis were also June and July 96 tests were conducted received in the district at about the v/ith about 2,000 adult mosquitoes same time. from each village. The insects were R. M. Bohart and W. D. Murray reared from gravid females collected reported that unsatisfactory results in in the two places and were tested by the control of Aedes nigromaculis was exposure to cloth panels impregnated experienced in Tulare and Merced with DDT. The average mortality was Counties in 1949. To confirm the field somewhat higher for the mosquitoes observations, laboratory tests were from the treated village than it was for made with larvae of the species col- those from the untreated village (56 lected in three pastures, which had percent versus 43 percent for the two previously received repeated DDT lar- sexes combined). The finding seemed vicide applications, in the mosquito- to demonstrate that no loss of suscep- abatement district in Tulare County. tibility had occurred. They compared the larvae with larvae G. F. Ludvik and others reported in from three pastures in Kings County 1950 on the first year of a study of not known to have been previously DDT resistance in Anopheles quad- treated with DDT. Based on the dos- rimaculatus in the Tennessee River ages required to cause 50 percent mor- Valley after 5 years of routine treat- tality, the average for the larvae from ment, in which were used DDT resi- Tulare County was more than i o times dues against larvae and adults. They that for the control larvae. The least subjected specimen material to a va- resistant of the larval lots from the riety of tests in comparison with sim- treated fields required about three ilar material from untreated areas. times as much DDT as the most resist- The comparisons consisted of labora- ant lot from the control area. In com- tory larvicidal tests in suspensions of 330 Yearbook of Agriculture 1952 DDT^ exposures of larvae in pans to the time since igi2 and his work has aerial DDT sprays, exposures of adults been chiefly on insects affecting man. to DDT-treated panels, and release of As a special agent of the Rockefeller adults in residue-treated rooms. The FoundatioUy he spe7it 3 years in the mosquitoes from treated areas showed Philippines on investigations of malaria slightly greater tolerance to DDT in mosquitoes. On active duty in the Sani- some of the tests, but the workers con- tary Corps of the Army during the Sec- cluded from their preliminary studies ond World War, he spent nearly 5 years that they had not developed an out- in New Guinea and other parts of the standing resistance. western Pacific on malaria control and R. W. Fay and others have reported mosquito investigations. He was in the results of preliminary experiments charge of the Orlando laboratory until to determine the possible development ig^i, when he relinquished his admin- of a resistant strain of Anopheles quad- istrative duties. rimaculatus. Adults of an insectary- reared colony of the species were ex- Dr. King cites the following articles for posed for four successive generations to some of the information in his article and suggests them for further reading: DDT-treated panels for enough time }. H. Bertholf: DDT Resistant Mosqui- to give mortalities of about 66 percent. toes in Broward County, Fla., Florida Anti- Eggs from the surviving females then Mosquito Association Proceedings^ pages were obtained for rearing. In tests of 80-83. 1950. susceptibility to DDT of the exposed R. M, Bohart and W. D. Murray: DDT Resistance in Aedes nigromaculis Larvae, strains, the mean mortality showed a Proceedings of the i8th Annual Conference slight but statistically significant drop of the California Mosquito Control Associa- in the first generation. No change oc- tionj pages 20-21. 1950. curred during the next three genera- Thomas L. Cain, Jr.: Observations on tions but was followed by an increase DDT-resistant Species of Mosquitoes Found in Brevard County, Florida Anti- to the original level in the first genera- Mosquito Association Proceedings^ pages tion after discontinuance of exposure 84-85. 1950. to DDT. In tests against other insecti- C. C. Deonier: Aerial Spray Tests on cides, a similar loss of susceptibility was Adult Salt-Marsh Mosquitoes Resistant to shown to methoxychlor but not to DDT, with T. L. Cain, Jr., and W. C. Mc- Duffie, Journal of Economic Entomology, chlordane, benzene hexachloride, al- volume 43, pages 506-51-0, 1950; Resistance drin, or TDE (DDD). Because the of Salt-Marsh Mosquitoes to DDT and pattern of increase and decrease in re- Other Insecticides, with I. H. Gilbert, Mos- sistance was basically different in these quito News, volume 10, pages 138-143. 1950. tests from that reported for house flies R. W. Fay, W. C. Baker, and M. M. (in which the changes in each direc- Grainger: Laboratory Studies of the Re- tion were much more gradual) further sistance of Anopheles quadrimaculatus to confirmation of these results seems nec- DDT and Other Insecticides, National essary before conclusions can be drawn. Malaria Society Journal, volume 8, pages 137-^4^' 1949- James B. Gahan, Wilbur G. Downs, and W. V. KING is a technical consultant Heliodoro Celis S.: Control of Anopheles in the Orlando laboratory of the divi- pseudopunctipennis in Mexico with DDT sion of insects aßeding man and ani- Residual Sprays Applied in Buildings. Part mals^ Bureau of Entomology and Plant II, American Journal of Hygiene, volume 49, pages 285-289. 1949- Quarantine. The work at the labora- W. V. King: DDT-Resistant House Flies tory is supported by funds allotted by and Mosquitoes, Journal of Economic Ento- the Secretary for Defense for investi- mology, volume 43, pages 52/^-532. 1950. gations of entomological problems of G. F. Ludvik, W. E. Snow, and W. B. medical importance to the military Hawkins: The Susceptibility of Anopheles quadrimaculatus to DDT after Five Years forces and the development of methods of Routine Treatment in the Tennessee of control of the insects involved. Dr. River Valley, National Malaria Society King has been with the Bureau most of Journal, volume 10, pages 23-34. 1951.