The comparative toxicity of DDT and analogues to susceptible and resistant houseflies and mosquitos

Studies of the comparative toxicity of more than 120 DDT analogues to susceptible and insecticide-resistant houseflies (Musca domestica L.) and mosquitos (Culex fatigans Wiedemann and Anopheles albimanus Wiedemann) have shown that the relative effectiveness of these compounds against DDT-resistant insects is correlated with the susceptibility of the molecule to attack by DDT-ase at the benzylic hydrogen. Compounds highly effective against DDT-resistant flies and mosquitos are produced by blocking this detoxication mechanism by o-chlorination, α-fluorination, and by altering the aliphatic portion of the molecule as in the nitropropyl, neopentyl, dichlorocyclopropyl, and trichlorobenzanilide derivatives. These compounds offer practical possibilities for the control of DDT-resistant insects. The correlation of structure, DDT-like activity, and resistance ratios gives new insight into the mode of action of DDT and the nature of DDT resistance.     

Effectiveness of organo-phosphorus insecticides against houseflies and mosquitos

The paper describes the research being undertaken on organo-phosphorus insecticides for the control of houseflies and mosquitos. The information obtained from laboratory and field tests indicates that these insecticides are at present effective substitutes for DDT and other chlorinated-hydrocarbon insecticides for use against resistant houseflies and culicine mosquitos, but the residual applications are not as long lasting as those of DDT and therefore will probably not be as efficient in anopheline control.     

Dehydrochlorination and DDT-resistance in Culex mosquitos

The mechanism of resistance to DDT was investigated in Culex mosquitos to ascertain whether it was associated with detoxication to DDE as in houseflies and Aedes aegypti. Resistant larvae of C. tarsalis were found to effect this dehydrochlorination, whereas the susceptible larvae did not; resistant larvae of C. fatigans completely converted all the absorbed DDT to the metabolite DDE. Enzyme assays in vitro showed that the resistant C. fatigans had 10 times the dehydrochlorinating activity, and resistant C. tarsalis four times the dehydrochlorinating activity, of their susceptible counterparts. The DDT-dehydrochlorinase enzyme of C. fatigans resembled that of Aedes aegypti more than that of the housefly, though differing from it in at least one respect.     

The conversion of DDT to DDE by some anophelines

The metabolism of DDT has been followed in pure lines of laboratory-reared resistant and susceptible anophelines using gas-liquid chromatography. Relatively large amounts of DDE were formed in vivo by susceptible strains of Anopheles stephensi, A. quadrimaculatus and A. gambiae, and by resistant strains of A. stephensi and A. quadrimaculatus. Resistant and susceptible A. albimanus showed no difference in the rate of DDE production. Volatile metabolites of DDT other than DDE were not observed in most of the strains chromatographed. A procedure for the measurement of DDT-dehydrochlorinase in small numbers of mosquitos was devised based on conditions developed for the glutathione-dependent DDT-dehydrochlorinase present in resistant houseflies. This method has shown the enzyme to occur in highest titre in DDT-susceptible A. albimanus and A. stephensi, indicating little or no correlation with resistance to DDT. Major differences in dehydro-chlorination rates and in patterns of resistance between anopheline species have been observed. In A. stephensi, these differences extend to the level of strains.     

Insecticide-resistance in Culex pipiens fatigans

The three main subspecies of the Culex pipiens complex—pipiens, fatigans and molestus—have similar “normal” levels of susceptibility to insecticides. C. p. fatigans larvae resemble those of anopheline mosquitos in sensitivity to dieldrin, but are more resistant to DDT; the adults are unusually resistant to chlorinated insecticides, but do not differ greatly from other species of mosquito in susceptibility to organophosphorus compounds.     

Assessment of the residual toxicity to Anopheles gambiae of the organophosphorus insecticides malathion and Baytex

As part of a WHO programme to evaluate possible substitutes for the chlorinated hydrocarbons to which anopheline mosquitos in many countries have become resistant, two organophosphorus insecticides, malathion and Baytex, were tested as residual sprays on various types of surface against Anopheles gambiae adults in experimental huts at Magugu in Tanganyika. The long-lasting toxicity of both insecticides on relatively impervious surfaces was confirmed, but—as is the case with the chlorinated hydrocarbons—shorter persistence was obtained on sorptive surfaces. As most of the mosquitos rested on the roof, its surface was of much greater importance than the wall surface in determining kill; trials with naturally entering mosquitos indicated that where the roof was of grass malathion was to be preferred to Baytex. Observations confirmed that control huts attracted far more mosquitos than the treated huts and that the insecticide vapour apparently masked the human odour.     

Inheritance of DDT-resistance in a European strain of Blattella germanica (L.)

A strain of Blattella germanica has recently been obtained from Europe which has proved to be highly resistant to DDT. This paper reports on genetical studies designed to show the mechanism of inheritance of the trait. Various crosses involving this strain and strains susceptible to DDT or possessing genetic markers were made. Progeny were tested for resistance to DDT and were examined for markers. The results show that DDT resistance is inherited as a simple Mendelian trait, with the hybrid being incompletely susceptible. Resistance appears to be on the same linkage group as ”balloon-wing”, but is independent of ”orange-body”.     

Age determination of some flies and mosquitos by daily growth layers of skeletal apodemes

A new method of determining age in certain Diptera, including Culex and Aedes mosquitos, involves counting—sometimes with the aid of staining techniques—the layers of cuticle growth that form daily at different points (inner apodemes) of the skeleton. If developed, the method would be of great importance for eradication and control programmes. The authors recommend further studies, in particular on the application of the technique to Anopheles and certain flies such as Calliphora.     

Exposure time versus concentration in the WHO standard test for mosquito resistance to chlorohydrocarbon insecticides

In order that the WHO standard test for mosquitos may yield a median lethal dose value (LC₅₀) in the case of highly tolerant species and resistant strains, it is necessary to increase the exposure period. The experiments described in this paper, done with a DDT-tolerant strain of Culex pipiens fatigans, show that doubling the exposure period has the same effect as doubling the DDT concentration, since it doubles the uptake of DDT into the mosquitos, and has the result of halving the LC₅₀ obtained.     

Tests for the existence of genetic variability in the tendency of Anopheles culicifacies species B to rest in houses and to bite man

Experiments were carried out in Sri Lanka on the malaria vector Anopheles culicifacies using the mark—release—recapture technique. Collections were made in cattle-baited huts fitted with exit traps, and in nightbiting catches on human subjects. The same individual mosquitos were caught biting cattle and man on different occasions and the numbers caught demonstrated an overall preference for the cattle. The mean interval between successive blood meals in the field was estimated to be 2.3 days.     

Studies of the genetics of resistance to parathion and malathion in the housefly

Although considerable research has been made into the genetics of resistance to DDT in the housefly—often with conflicting conclusions being drawn from the results—little has been done on the mode of inheritance of resistance to organophosphorus compounds. The experiments described in this paper were designed to fill this gap.     

Partial suppression of malaria parasites, and of the transmission of malaria, in Aedes aegypti (L.) doubly-infected with Semliki Forest virus and Plasmodium gallinaceum Brumpt

Laboratory experiments were undertaken with Aedes aegypti infected with both Semliki Forest virus (SFV) from infant mice and Plasmodium gallinaceum from fowls to determine if such double infection of mosquitos suppressed their ability to transmit the malaria parasite, a possibility suggested to explain reduction in malaria transmission in Uganda in 1960 when Anopheles funestus and Anopheles gambiae were transmitting both malaria and o''nyongnyong virus to the African population. In general, transmission of fowl malaria was not prevented by SFV infection in Aedes, although some malariometric indices, and consistently the mean oocyst count, were slightly lower in doubly-infected mosquitos than in controls. In one experiment, however, 60% of the Aedes infected 8 days previously with SFV died within 48 hours of ingesting a malarious meal. Mortality was selectively in favour of the survival of SFV-infected mosquitos negative for, or least heavily infected with, malaria; depression in the presence of the virus of the intensity of malaria infection in the individual Aedes also occurred. Some physiological factor—a stress in adult life or, possibly more important, suboptimal larval nutrition—appears to have been crucial to eliciting the adverse effect on the mosquitos themselves and on their malaria infections. Suppression of the development of a malaria parasite in a mosquito, and of malaria transmission, by concurrent infection of the vector with an arbovirus can happen, but is by no means inevitable.     

Additional studies with new insecticides as residual sprays in buildings naturally infested with Anopheles quadrimaculatus

Because some species of Anopheles mosquitos have been developing resistance to DDT and dieldrin, field studies have been conducted to evaluate the effectiveness of recently developed insecticides as residual sprays in buildings naturally infested with adults of Anopheles quadrimaculatus. As a deposit at 2 g/m², o-isopropoxyphenyl methylcarbamate remained highly effective for more than one year. This compound and 6-chloro-3,4-xylyl methylcarbamate were still producing excellent control 11-12 weeks after application at 1 g/m². Other highly effective materials were m-isopropylphenyl methylcarbamate and 4-(methylthio)-3,5-xylyl methylcarbamate at 2 g/m². At the present time, o-isopropoxyphenyl methylcarbamate appears to offer the best prospects for use in residual spray programmes because it has long residual action, affects mosquitos rapidly, has only a slight odour and appears to be safe to use if spraymen take reasonable precautions.     

Toxicity of some newer insecticides to larvae and pupae of Culex fatigans Weidemann

The toxicity of DDT, dieldrin, endrin, malathion, parathion, Systox and P4 to larvae and pupae of Culex fatigans has been studied. Two colonies of mosquitos were maintained for the purpose—one at 88°±2°F (31.1°±1.1°C) and 70%-80% relative humidity, and the other at 72°±3°F (22.2°±1.7°C) and 45%-55% relative humidity.     

The practical implications of resistance of malaria vectors to insecticides

Insecticide resistance is an inherent characteristic dependent on relatively simple genetic mechanisms. This seems to be especially true of dieldrin resistance in anopheline mosquitos, though less obvious in DDT resistance among these species; little is known as yet about the inheritance of organophosphate and carbamate resistance as it occurs in Anopheles albimanus. The speed of selection of resistance depends on the original frequency of the gene or genes involved, the nature of the resistance imparted, and the selection pressure of the insecticide. This in turn depends on the inherent toxicity of the chemical, the efficiency with which it is applied, the proportion of the mosquito population coming under its influence, and the behaviour of the mosquito. In the past, too much reliance has been placed on the determination of the LD₅₀ in assessing the presence or absence of insecticide resistance. Quite high incidences of resistant individuals can result in such small changes in the LD₅₀ that resistance may be overlooked. The use of single discriminating dosages is advocated, based on concentrations of insecticides that normally kill all susceptible individuals. The authors discuss such dosages in respect of dieldrin and DDT, and put forward newly-established tentative discriminating dosages for organophosphorus and carbamate insecticides, which await field confirmation.     

Studies on aircraft disinsection at ��blocks away��

With a view to expediting as much as possible the disinsection of aircraft required under the International Sanitary Convention for Aerial Navigation, experiments were performed under operating conditions on the disinsection of passenger cabins after the closing of the aircraft doors following embarkation but before take-off (designated ”blocks away” disinsection) with single-use, disposable aerosol dispensers. A formulation containing 1.6% by weight pyrethrum extract (25% pyrethrins) and 3% DDT at a dosage of 10 g per 1000 cubic feet (35 g/100 m³) gave satisfactory control of non-resistant mosquitos and created no passenger reaction. A formulation containing 3.40% pyrethrum extract (20% pyrethrins) and 1.17% DDT at a dosage of 14-19 g per 1000 cubic feet (48-64 g/100 m³) was biologically effective for both resistant and non-resistant mosquitos but was markedly irritant to some passengers. The authors suggest lines along which further research might be conducted.     

Dieldrin resistance in Culex pipiens fatigans in Malaya

Resistance to insecticides in Culex pipiens fatigans has already been reported from two areas in Malaya. In Penang two years'' use of BHC as a larvicide resulted in the development of a strain which was found to have acquired a tenfold resistance to BHC, and also to dieldrin to which it had not been exposed. In Singapore, when larval control became unsatisfactory after 6 months'' use of a dieldrin emulsion, laboratory experiments confirmed that active resistance to dieldrin had developed. The present observations report the finding of two further dieldrin-BHC resistant strains of C. p. fatigans in Malaya, but differ from the previous reports in that resistance, in one strain at least, was developed as a result of house-spraying with dieldrin against adult mosquitos. In this strain resistance to dieldrin was about 100 times in both adults and larvae, resistance to gamma-BHC in larvae was about 20 times, while resistance to DDT was slight.     

Development of organophosphorus resistance in Indian strains of Aedes aegypti (L.)

Populations of the yellow-fever mosquito, Aedes aegypti, have developed resistance to cholorinated hydrocarbons in many parts of the world, but not to organophosphorus insecticides. Seven Indian strains of Ae. aegypti were found to be tolerant to DDT and highly susceptible to certain organophosphorus compounds such as Abate, Dursban, fenthion and fenitrothion. Hence selection studies were started with these organophosphorus compounds. Laboratory selections on these strains for 20 generations with Abate, Dursban, malathion, fenthion and fenitrothion increased the tolerance of the F₂₀ larvae to these insecticides by 2.4 times, 3.7 times, 3 times, 5.6 times and 2 times, respectively. The dosage—mortality lines of the successive generations were steep and parallel, suggesting these were instances of tolerance and not of resistance. In contrast, DDT selection showed rapid changes in dosage—mortality lines, indicating the development of resistance. The organophosphorus selected strains generally showed only a 2-3-fold increase in cross-tolerance to other organophosphorus compounds.     

Studies on the Mechanism of DDT Resistance in Culex pipiens fatigans: Lipids of Susceptible and Resistant Strains

The mechanism of DDT resistance in Culex pipiens fatigans is poorly understood. Earlier studies indicated that the dehydrochlorination of DDT does not explain resistance in this species. Studies on the role of lipids as a mechanism of resistance included the estimation of lipid content and the determination of the proportions of different classes of lipids in the larvae of susceptible and resistant strains. There was no evidence of any correlation between the lipid content and DDT resistance in this species and the proportions of neutral lipids, phospholipids and fatty acids of different strains did not indicate any consistent correlation with DDT resistance.     

Evidence of the development of resistance to DDT by Anopheles sacharovi in the Levant

Resting A. sacharovi reappeared in the autumn of 1953 on some walls in certain localities of north Lebanon which had been treated with DDT during the preceding 18 months. Tests were made on the spot with female mosquitos having a known minimum contact of 5 to 15 minutes with DDT residues, chemically estimated at 0.1-2.0 g/m(2). The resulting mortality within 48 hours (corrected to allow for deaths observed in the control runs) was 60.1% and 45.1% according to the technique used, compared with 100% to 83% within 24 hours in similar tests on the same strains some months earlier. The specimens which died had contacted a lower average residue than those which survived, and the latter included some exposed for 15 minutes to 1.8 g/m(2).The theory is put forward that selection for DDT resistance (a phenomenon already known in A. sacharovi elsewhere) is probably occurring in these Lebanese strains after only two seasons of regular spraying. The prevalence of sooty surfaces, which absorb DDT, is considered to be a factor contributing to selection by reducing residues to a level within the "critical range" for the non-resistant mosquitos. Attention is drawn to work which indicates a similar response to DDT in strains of A. gambiae and A. funestus in Africa.The importance is stressed of planning antimalaria campaigns in a way that will either prevent selection for resistance or will permit counteraction of it before it can lead to large wastage of public money.