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USE OF IN VECTOR CONTROL 595 evidence of benefit in African children from the actual pyrethrum is less expensive than might be reduced parasite rates in a small sample. imagined (even allowing for considerable increases since the trials described were done; in some cases Trials in Nigeria: A. gambiae, A. melas economies could be made by using locally grown pyrethrum). In a paper on malaria in West Africa, Muirhead- o The main objection to pyrethrum sprays is the Thomson remarks that, in villages near Lagos, frequency of application. Apart from labour pyrethrum spraying, even at four or six days a in some could be alleviated week, failed to bring about a substantial reduction in costs (which countries by village self-help schemes), the difficulty of orga- either the number of anophelines or their infectivity. nizing such regular treatments on a national scale would seem to preclude this method as a way of General conclusions achieving malaria eradication. 1. The use of pyrethrum sprays can substantially 3. What, then, are the future possibilities of reduce malaria transmission, at least in some parts pyrethrum for malaria control? There are three of the world. The use of 0.1 0% in kerosene possible uses, all connected with resis- sprayed at about 20 ml per 1000 cubic feet, prefe- tance: rably twice a week, is necessary. The effectiveness (a) If vectors become resistant to both types of of the method could probably be greatly improved chlorinated residual insecticide, pyrethrum sprays by the use of modern synergists and better sprayers. might perhaps be considered as alternatives to It is not at present clear to what extent the bene- organo-phosphorus compounds. On existing ficial effects of pyrethrum spraying depend on (a) results (mainly over 15 years old) they do not look reduction of numbers of vectors; (b) shortening of as effective; but further investigation with modern the life expectation of the vectors; (c) repelling them synergists and equipment are required. from entering houses (See White et al.1). (b) Admixture of pyrethrins with residual sprays 2. While residual synthetic are effect- would destroy resistant bed bugs and (for a short ive, they are not likely to be challenged by pyrethrum. time) houseflies. This would greatly assist public This is not so much on grounds of cost, because the relations in antimalaria spray campaigns. (c) It is conceivable that such a mixture might of o Muirhead-Thomson, R. C. (1947) Trans. roy. Soc. help to suppress emergence of resistant strains trop. Med. Hyg., 40, 511 mosquito.

Use of Pyrethrum in Vector Control * by Dr RAJINDAR PAL, Deputy Director (NMEP), Malaria Institute of India, Delhi, India

Since the advent of organic synthetic insecticides, combined with non-toxicity to man. In view of pyrethrum has been pushed to the background. these properties, pyrethrum may be an effective However, as with many new products, expectations insecticide for vector control and it would seem have tended to outrun performance and not all that insufficient attention has been paid to it. authorities have subscribed wholeheartedly to Pal (1953) has discussed the use of pyrethrum in the optimistic views concerning the organic synthetic malaria control, and the present paper covers other products (Feuell, 1957). For example, it was pointed possible use of this insecticide in vector control. out that pyrethrum possessed certain useful proper- ties not shared by chlorinated hydrocarbons, such Bed-bugs as a quick knock-down effect, and toxicity to The addition of pyrethrins to residual insecticides resistant to organic synthetic insecticides has been found to be effective in the control of which tend to hide in * Note submitted to the WHO Expert Committee on insects such as bed-bugs Insecticides, September 1959. crevices during the day. The pyrethrins tend to 886J 596 NOTES

stimulate movement and thus result in better contact found that the lice developed less than twofold with residual insecticides (Davidson, 1955). Pyre- resistance to this insecticide. thrum and was found to be It was found by Cole et al. (1958) that pyrethrum more toxic to bed-bugs than many of the chlorinated 0.20% plus sulfoxide 2.0% resulted in 920% mortality hydrocarbon and organophosphorus insecticides. against DDT-resistant body lice, even after 20-22 In residual toxicity tests, also, under laboratory days of application. Pyrethrins (0.5%) had more conditions, pyrethrum and piperonyl butoxide gave lasting effect than against resistant lice. 1000% kill within 24 hours even after 24 weeks and the only other insecticides giving this much effect were malathion and , whereas DDT tested Pyrethrum spray has an immediate effect on under similar conditions never gave more than 80 % cockroaches, as on bed-bugs, because it tends to kill after the first week of treatment. At the end stimulate their movement from the hiding places of 28 weeks the kill was only 150%. In dwellings and rapidly knocks them down. On the basis of treated with pyrethrum and piperonyl butoxide, active toxicant, pyrethrum is several times as toxic newly hatched bugs were found only during the as DDT, and as toxic as gamma-BHC, to cock- second week after treatment. Synergized pyrethrum roaches (McGovran & Piquett, 1945). A powder was also found to be toxic to bed-bugs resistant to containing sodium fluoride and pyrethrum has been chlorinated hydrocarbons (Logfren et al., 1958). successfully used for the control of cockroaches. Furthermore, the toxicity of pyrethrum can be Body lice greatly increased against cockroaches by the addi- tion of synergists (Piquett et al., 1947). Sprays American Myl powder containing pyrethrum, containing 0.050% pyrethrum plus 1 % piperonyl phenol-S as an anti-oxidant, N-isobutyl-undecyl- butoxide, or dusts containing 0.1 % pyrethrum plus enamide as a synergist, and 2 : 4 dinitroanisole as 1 % piperonyl cyclonene, were found to produce an an ovicide has proved quite efficient for the control immediate flush, quick knock-down, and complete of body lice and does not cause irritation of the mortality (Donohue, 1948). Gould (1948) has also skin (Knipling & Dove, 1944; Bushland et al., reported that three commercial mixtures containing 1945). Because of their oily ingredients, the powders piperonyl cyclonene and pyrethrum extract gave tend to clog the duster and do not sift throughout excellent results against the German but the entire clothing. Such powders have, therefore, lacked residual effect. been found satisfactory for individual use but less Nash (1950) observed that allethrin appeared to suitable than DDT or gamma-BHC for mass be approximately as toxic as pyrethrum in the treatment. knock-down and kill of the German cockroach at Oils containing pyrethrum have been used against a concentration of 0.20% and less toxic than pyre- head lice and preferred to DDT, as they are not thrum against the American cockroach. When the noticeable in the hair (Twinn & MacNay, 1948). toxicity was determined by the topical application Busvine (1946) observed that pyrethrum, with or method, allethrin was found to be less toxic than without synergist, was more effective than thio- pyrethrum against both German and American cyanates and , and less effective cockroaches (Mitlin & Babers, 1955). than DDT and BHC, against the lice. Garson & The development of resistance to various Eddy (1949) observed that d-camphoric acid, chlorinated hydrocarbon insecticides in German piperonyl cyclonene, and many other compounds cockroaches stimulated investigations to find other have a synergistic effect on pyrethrins when used insecticides which would give satisfactory control against body lice. Furthermore, these compounds and, at the same time, be safe enough for use in enhance the lasting effect of pyrethrins. barracks, houses, halls, restaurants, etc. Dust Allethrin and cyclethrin with synergists have containing 98.9 % sodium fluoride, 0.1 0% pyrethrum, also been found effective against body lice; but and 1 % piperonyl butoxide was found to be quite pyrethrum with sulfoxide was found to be more effective against resistant cockroaches (Keller et al., effective, with a more pronounced residual effect 1956). Jarvis & Grayson (1957) tested malathion (Burden & Cole, 1956). Cole et al. (1957) selected with and without allethrin against both susceptible lice for 30-35 generations on cloth treated with low and resistant German cockroaches and found that concentrations of pyrethrum and sulfoxide, and these formulations gave good control. Keller et al. USE OF PYRETHRUM IN VECTOR CONTROL 597

(1958) have observed that some strains of German Allethrin was found to be more toxic than pyre- cockroach developed resistance to pyrethrum and thrum to houseflies when tested by the turntable synergists although the degree ofresistance developed method (Gersdorff & Piquett, 1955) and as an was not more than 13 times. aerosol (Nash, 1950), but was found to be equally toxic when tested by the topical application method Fleas (Mitlin & Babers, 1955) and the Peet-Grady method. Smith (1951) has reported that pyrethrum extract Fales et al. (1951), however, observed allethrin to with piperonyl butoxide was more effective against be less effective. Cyclethrin was 1.5 times as toxic fleas than DDT in the form of dust containing as pyrethrum (Gersdorff & Piquett, 1955), whereas 0.5% of the toxicant. Shawarby (1953) determined furethrin was found to be less toxic against house- the MLC of pyrethrum and allethrin along with flies (Fales et al., 1955). But these synthetic com- other chlorinated hydrocarbon insecticides by the pounds do not combine so well with the synergists Busvine & Nash method and found the MLC for as natural pyrethrins (Brown, 1951; Gersdorff & pyrethrum to be lower than that for allethrin (0.5 % Piquett, 1955). and 2.5% respectively). The experiments of Dove (1947), using houseflies, showed that pyrethrum/piperonyl-butoxide sprays Flies might also have residual action of practical value. The control of houseflies has been accomplished Chamberlain (1950) found evidence that this synergist by finely atomized kerosene sprays containing actually stabilized the films and delayed pyrethrum, which not only gave a better quick their deterioration. The experiment of Hewlett knock-down than both chlorinated hydrocarbons (1951), however, did not confirm this. Chlorinated and organophosphorus insecticides, but also high terphenyls were also found to enhance the residual mortalities after 24 hours (Brown, 1951; Gersdorff, effect of allethrin against houseflies (Tsao et al., Piquett & Nelson, 1955). The quick knock-down 1954). effect combined with non-toxicity to mammals The houseflies resistant to chlorinated hydro- therefore make it an ideal household spray. Combi- carbon insecticides do not show any cross-tolerance nation of the synthetic insecticide with pyrethrum, towards pyrethrum or pyrethrum-synergist mixtures e.g., 0.1 % DDT and 0.03 % pyrethrin, gave com- (Pimentel & Dewey, 1949). March & Metcalf plete control of houseflies when emitted at the rate (1950) have therefore suggested the use of space of 1 ml/m3 from an air-blast atomizer, whereas sprays containing 0.1 % pyrethrum and equivalent 0.07% pyrethrum is required as such for complete of pyrethrum and activator in petroleum distillate knock-down and kill (Craufurd-Benson, 1945). as a satisfactory measure for chemical control of Aerosol bombs containing 0.3% pyrethrum and resistant houseflies. Schroeder & Jones (1957) 3.0% DDT have also been successfully used for fly observed that the aerosols containing pyrethrum control. and piperonyl butoxide gave a good performance The solution of synergist and pyrethrum in against resistant houseflies. The fly population in kerosene oil in the ratio of 10 :1 has also been Sweden, which was highly resistant to chlorinated successfully used for fly control (Fisher, 1955; hydrocarbons, was successfully controlled for about Granett et al., 1955). The intensity of synergism a year with dusts containing synergized pyrethrum with some of the compounds, such as piperonyl (0.075% pyrethrum plus 0.6% piperonyl butoxide) butoxide, sulfoxide and sulfone, was found to be and aerosols but, after the use of pyrethrum, the very high, i.e., about 13-20 times against houseflies fly population of the region became 12 times resis- (Gersdorff, Nelson & Mitlin, 1951, 1952; Gersdorff, tant, and control was no longer possible. Moreover, Mitlin & Nelson, 1955; Gersdorff & Piquett, 1955). the pyrethrum-resistant flies showed resistance to Piperettine, another synergist, was found to enhance and (Davies et al., 1958). the effect to about 13 times (Gersdorff & Piquett, In laboratory studies synergized pyrethrum or 1957). allethrin dry-sugar baits were shown to have promise Compounts of a new type-derivatives of methy- in reducing densities of malathion-resistant house- lene dioxyphenyl acetals synthesized by Beroza flies. Combination of pyrethrum or allethrin and (1956) were found to have synergistic effect greater piperonyl butoxide at a ratio of 1 :10 when tested than any other synergist when tested against house- at concentrations of 0.01 % to 1.0% toxicants flies (Gersdorff et al., 1957). against malathion-resistant and susceptible strains 598 NOTES indicated only minor differences in the level of kill thrum-Vaseline and pyrethrum in vanishing cream against the two strains. Field tests of similar baits base have also been employed (Johnson, 1947). against houseflies in dairies showed the pyrethrum Antimosquito cream containing oil of citronella formulation to give satisfactory kills which were and pyrethrum in a stearic acid base has been found superior to those achieved with the allethrin formula- satisfactory (Pal, 1953). Synergized pyrethrum has tions. been found to have a repellent action against houseflies (Howell, 1949). Paulini & Ricciardi Mites (1954) observed that the fumes of pyrethrum have Infestation with the mite 0. bacoti at Sverdlovsk a repellent action against Culex fatigans. in the USSR was controlled by treating infested flats with a mixture containing pyrethrum three Disinsectization of aircraft times at intervals of 5-7 days (Rotblyut, 1954). Considerable data exist to show the dangers of Mosquitos the possible transfer of insects, including disease vectors, by aircraft. Stringent world regulations Pal (1953) has reviewed the use of pyrethrum in now require the disinsectization of aircraft by malaria control. Since then very little has been insecticides. Such insecticides must be highly published in this respect. Space sprays with pyre- efficient and quick-acting, harmless to passengers thrum extracts have in the past achieved very satis- after spraying and non-inflammable (Dauguet, 1949; factory control of malaria in certain areas where Chandler, 1952). Pyrethrum is an essential consti- the vector species was predominantly endophilic. tuent of the standard formulation now employed. The advent of chlorinated hydrocarbons, however, Pyrethrum extract may be applied in the form pushed pyrethrum spraying to the background of a finely atomized aerosol dispersed by a special because of their residual effectiveness and low cost. aerosol bomb. The active ingredients of the bomb Today, with numbers of vector anophelines having in the early stages were pyrethrum and sesame oil. developed resistance to synthetic organic insecticides, Later on DDT was also and the aerosols once again been included, pyrethrum space spraying has containing DDT and pyrethrum were found to be given serious consideration. In situations where very effective (Fales et al., 1951). Aerosols contain- resistant strains have emerged, where destruction ing 0.4% pyrethrum and 3.0% DDT, operated for of large numbers of adult mosquitos is necessary 10 seconds per 1000 cubic feet in aircraft, gave of the mos- for disease control or where abatement satisfactory control and were adopted as a WHO quito nuisance is desired spraying with pyrethrum requirement for disinfestation (Busvine, 1952). may be the method of choice. Aerosols containing pyrethrum and synergists can Use ofpyrethrum as an repellent also be used as these are effective against insects Some insect-repellent formulations containing resistant to synthetic insecticides (Schroeder & pyrethrum have been found quite useful. MacNay Jones, 1957). (1938) recommended 1/2 ounce of oil of thyme, * * 1 ounce of extract of pyrethrum (0.2 pounds of ) and 2-3 fluid ounces of castor oil; applica- I am much obliged to R. L. Kalra, Assistant tion of this formulation was found to be quite Research Officer, Indian Council of Medical effective for 3-5 hours against Aides species. Pyre- Research, for his help in the preparation of this note.

REFERENCES

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