Mosquitoes and DDT

W. V. King.

Increased resistance to DDT has been recorded for several species of mosquitoes in widely separated parts of the world. Included are the house mosquitoes, Culex pipiens in Italy and C. quinquefasciatus in India; two salt-marsh species, Aedes taeniorhynchus and A. sollicitans, in Florida; and two floodwater species, Aedes nigromaculis and A. dorsalis, as well as Culex tarsalis in California. An encouraging fact is that two species of Anopheles failed to show increased resistance in areas where they had been exposed for several years to DDT residual treatment in buildings.

E. Mosna was apparently the first to report increased resistance in a species of mosquito, Culex pipiens autogenicus (molestus) from the Pontine marshes in Italy. He found many live specimens of the species in bedrooms of houses in May 1947, where for the second year 5 percent DDT in kerosene had been applied as a residue for the control of Anopheles. Specimens he collected from the interiors were exposed to the treated walls and were alive after 48 to 72 hours, but specimens from a laboratory strain died within 3 to 5 hours. He thought it possible that two races of this variety of mosquito might exist, distinguished basically by the different grade of resistance to DDT. Laboratory tests with the eighth generation reared from resistant material showed that the resistance was transmitted through eight generations without marked diminution. From preliminary laboratory and field tests with chlordane and benzene hexachloride, Mosna learned that the insecticides had residual action lasting more than 4 months and were therefore suited to practical control of Culex that are resistant to DDT.

In India, from experiments conducted for 10 months, J. F. Newman and others learned that successive generations of the southern house mosquito exposed in the laboratory to DDT residues showed a marked increase in resistance to DDT. A 20-minute exposure caused 100 percent mortality of females originally, but no mortality resulted from 30-minute exposures a few months later. A similar resistance to benzene hexachloride also was shown.

The failure of DDT sprays to give satisfactory control of the common salt-marsh mosquito and another salt-marsh species, Aedes taeniorhynchus, in Broward County in Florida, was first noticed in 1947 in Hollywood, where much DDT had been applied in previous years to control heavy infestations.

The failure was observed again in 1948 and 1949, when similar difficulty was experienced in Brevard County near Cocoa Beach and the Banana River Airbase, where an extensive salt marsh had been treated repeatedly with DDT sprays the previous 4 years. In June 1949 the results of aerial spraying operations in the area were checked by members of the Orlando laboratory of the Bureau of Entomology and Plant Quarantine. It became evident that satisfactory reduction of adults of the two salt-marsh species A. taeniorhynchus and A. sollicitans was not obtained with the standard dosage of 0.2 pound of DDT per acre. Even twice that dosage failed to give as good control as had been obtained with the standard dosage. This indication of increased resistance was confirmed by laboratory tests in which larvae and reared adults of A. taeniorhynchus and A. sollicitans were compared for susceptibility to DDT with similar specimen material of A. taeniorhynchus from other areas in the same county that were not known to have received DDT applications previously or only an occasional treatment for adult control. A. sollicitans were not present in the untreated areas at the time the collections were made for the tests.

The evidence from the laboratory tests demonstrated the increased tolerance of the specimens from the treated areas. In the larvicide tests, the mortality of fourth-stage larvae averaged about 16 percent, compared with an average of nearly go percent for the control larvae. Similarly, in space-spray tests with 1 percent DDT solutions against reared females, the comparable figures were 18 percent and 83 percent. The results indicated a fourfold increase in tolerance or more. Larvae of Aedes taeniorhynchus collected in 1949 from a treated area in Sarasota County on the Gulf Coast also showed increased resistance.

In tests on mosquito specimens from Brevard County, chlordane and benzene hexachloride, both technical and refined ( lindane), produced about the same mortalities of larvae and adults from the treated areas as from untreated areas at similar dosages. That was true also of parathion in larvicide tests. Parathion was not included against the adults. TDE, like DDT, was much less toxic to the specimens from the treated area. Toxaphene was somewhat less toxic. Lindane was by far the most toxic compound to adults, and parathion to the larvae from all areas.

Aerial spray tests with several insecticides were also carried out against the DDT-resistant mosquitoes. Lindane, the most effective of the insecticides tested, gave good control of adults at dosages of 0.05 and 0.1 pound per acre. Technical benzene hexachloride (12 percent gamma) at 0.2 and 0-4 pound and dieldrin and parathion at 0.05 and 0.1 pound gave results nearly equal to lindane. Chlordane and DDT at 0.2 and 0.3 pound per acre and toxaphene at 0.2 pound were not highly effective in most tests.

Larvicidal tests on small plots were conducted with several insecticides applied as emulsions. In the Cocoa Beach area DDT was much less effective than in untreated areas, but the other materials dieldrin, parathion, lindane, technical benzene hexachloride, and toxaphene all gave good and approximately similar results in both the treated and untreated marshes. Dieldrin and parathion were the most effective at dosages of 0.025 and 0.05 pound per acre, closely followed by lindane and toxaphene.