Insecticide Resistance of A. Quadrimaculatus in Bolivar County, Miss.

Abstract

D ESPITE the widespread use of DDT residual applications for the eradication of malaria in the southeastern United States during the period 1946 to 1950, there has been no indication that Anopheles quadrimaculatus (Say) has developed resistance to this compound. Kruse and associates (1) reported that A. quadrimaculatus became less susceptible to larvicidal treatments of DDT in the Tennessee Valley Authority malaria control program, but later studies (2) indicated that factors other than resistance were responsible for the apparent lack of effectiveness of the control operations. However, field studies in Bolivar County, Miss., now demonstrate that, while A. quadrimaculatus in that area is still susceptible to DDT, it is highly resistant to dieldrin, BHC, and chlordane. The first indication of resistance in this species was noted in field tests, employing the method of Fay and associates (3). In these tests an average 24-hour mortality of 15 percent was obtained when 62 adult mosquitoes (3 replicate tests) were exposed to paper treated by immersion in a 0.25 percent dieldrin-xylene solution. To substantiate this apparent loss of susceptibility to dieldrin, further tests were conducted with paper treated by immersion in 0.25 percent and in 1.0 percent dieldrin solutions. DDTtreated surfaces also were assayed. The female mosquitoes used were collected from natural resting places at five widely separated locations in Bolivar County during June and July 1955 and held overnight with food and water before their exposure to chemical residues. The results of these tests clearly indicated that A. qutadrirnaculatw9 was highly resistant to dieldrin residues although quite susceptible to deposits of DDT (table 1). Since the dosage of the toxicants per square foot was unknown in the foregoing tests, additional tests were run using the technique of Fay and associates (4) in which plywood panels were treated with known dosages of the toxicant. Four chemicals were evaluated: DDT at 200 mg./sq. ft., dieldrin at 25, 50, 100, and 200 mg./sq. ft., BHC at 50 mg./sq. ft., and chlordane at 100 mg./sq. ft. The exposure period was 30 minutes, except for the DDT tests in which 30-, 60-, and 120-minute periods were employed. To compare the susceptibility of the Bolivar County A. quadrimaculatus with that of nonresistant A. quadrim1aculatus, parallel tests were conducted at the Public Health Service Technical Development Laboratories, Savannah, Ga., with a laboratory-reared strain (TDL). The adult mosquitoes of the TDL strain were processed in the same manner as the Bolivar County adults. The plywood panels used in the tests were sprayed in quadruplicate sets at Savannah. After the treatment, two sets of panels of each dosage were tested at Savannah against the laboratory strain. Two sets of panels (one The authors are all with the Communicable Disease Center of the Public Health Service. They are stationed in Savannah, Ga., with the Technical Development Laboratories where Mr. Quarterman is chief. Mr. Mathis and Dr. Fay are entomologists, and Dr. Schoof is chief of the Biology Section.