Recent developments in methods of mosquito control*

Bull. Org. mond. Sante) 1974, 50, 323-328 Bull. Wld Hlth Org. Recent developments in methods of mosquito control* CLIFFORD S. LOFGREN 1 Since residual insecticide spraying in domiciles does not sufficiently control some species of anophelines to halt malaria transmission, alternate methods of control have been investigated. These include ultra-low-volume (UL V) sprays or aerosols, the release ofsterile males to suppress or eradicate populations, and the use of diseases or parasites to interfere with transmission or to reduce populations of mosquitos. The ULV aerial sprays gave practical control of Anopheles albimanus in Haiti and interrupted malaria transmission. The release of sterile males almost eliminated an isolated population of A. albimanus in a small valley in El Salvador. Studies are in progress to evaluate the effect on fieldpopulations of anophelines of a protozoan disease and two nematode parasites. During the past 24 years, the control of anopheline vectors of malaria has been accomplished primarily by the spraying of interior walls of domiciles with insecticides having residual activity, particularly DDT or dieldrin. This method has usually been extremely effective, although problems have arisen when the insects have developed resistance to the chemicals or when their behaviour is such that they do not come into contact with the deposits. Obvi- ously, other methods of control are needed in such situations or in others where there is a need for a diversity of approaches to anopheline control. This paper reviews the status of new mosquito control techniques with emphasis on methods that may be useful in the control of malaria and its vectors. The voluminous literature on the evaluation of residual insecticides and on field evaluation, sanitation, and water management is not reviewed, though these must form an integral part of any programme of mosquito control. The alternate methods discussed are the use of ultra-low-volume (ULV) insecticide sprays or aerosols, the sterile-male method of popu- lation control, and biological control with parasites or diseases. While none of these methods may be really new, their application to the control of anopheline vectors of malaria is relatively new. * Presented at the Symposium on Malaria Research, Rabat, Morocco, 1-5 April 1974. This paper reflects the results of research only. The mention of a pesticide does not constitute a recommendation of the product by the US Department of Agriculture. 'Entomologist and Research Leader, Insects Affecting Man Research Laboratory, Agricultural Research Service, US Department of Agriculture, Gainesville, Fla. 32604, USA. ULTRA-LOW VOLUME METHODS The most significant development in the chemical control of mosquitos during the past decade has been the introduction of the ULV method of applying insecticides. The principles of the method and its advantages have been described in detail by Lof- gren (1, 3). This technique usually involves the direct dispersal of the technical insecticide or, in the case of solid chemicals, of concentrated liquid formulations. The first ULV applications against mosquitos were made by aircraft (1); in the USA, however, the technique is now used extensively with ground equip- ment. The amounts of insecticide applied by aircraft seldom exceed 440 ml/ha and are usually less than 220 ml/ha. If 4.5-9 litres/ha is considered to be a standard high-volume rate of application (18), the reduction in amount of insecticide applied is at least 10-20-fold. The same or greater decreases in volume occur with ground applications for mosquito control (< 4 litres/h instead of 160-480 litres/h). The interest in the technique is easily understood when one considers the economics. For example, the Mosquito Control District of Chatham County, Georgia, re- ports that the cost of ULV ground application of malathion is about 74% less than that of high-volume thermal fogging of fuel oil solutions of the same insecticide (2). This saving results from a combina- tion of factors including reduced amount of insecti- cide per unit area, elimination of the diluent, and greater operating efficiency of the equipment. The 3195 - 323 - C. S. LOFGREN questions that have arisen about ULV applications are related to the ability to atomize the insecticide in such a way as to produce adequate distribution and to the efficiency of different sizes of droplet. At present, atomization with available aircraft nozzles is rela- tively inefficient, and atomization from ground equip- ment is highly efficient (3), which undoubtedly ac- counts for the greater amounts of insecticide (about 5-10-fold) required if aerial treatments are to produce equivalent control. However, the small volumes ap- plied by either route do not present a problem in the coverage of the treatment area if atomization is sufficient. For example, 89 ml of malathion (a standard dose for ULV aerial application of mala- thion), if atomized into droplets 25 ,um in diameter, would yield a total of 1.13 x 1010 droplets; if the droplets were 10 ,tm in diameter, the total would be 1.76 x 1011 droplets (4). Mount (5) reviewed the literature and concluded that the most efficient sizes for ground and aerial applications were in the range of 5-10 ,um and 10-25 ,um, respectively. Scientists at the Insects Affecting Man Research Laboratory recently confirmed those conclusions with the aid of a scanning electron microscope (6). When mosquitos were exposed to aerosols of soybean oil (insecticides could not be used because they volatilized in the microscope) and then photographed, 93% of the droplets deposited on the wings in one test series ranged from 2 to 10 ,um in diameter even though the aerosol contained droplets ranging from < 2 ,um to 32 ,m. Fine atomization is therefore essential for efficient ULV applications. No data are available concerning actual control with ground equipment though several papers have been published that demonstrate the kill of anophe- lines held in small cages outdoors. It is of particular interest that anophelines show extreme sensitivity to some of the new pyrethroid insecticides. For example, Mount & Pierce (7) showed that the toxicity of some such compounds to A. albimanus Wiedemann is 24-80 times that of malathion. Also, field tests of the control of anophelines with ULV aerial applications are limited. However, several evaluations of the control of A. albimanus have been made. For exam- ple, in 1967, malathion and fenthion were applied to jungle areas in the Panama Canal Zone from helicop- ters (8), and control was obtained with rates 3 times those used for conventional mosquito control. Sub- sequently, control in a larger area of the jungle was obtained with fenthion applied at the conventional rate (30 ml/ha) because better atomization of the insecticide was obtained (9). In the later trials, furthermore, more rapid treatment of the larger area was achieved by flying wide swath widths (0.64 km) at higher than normal altitudes (91 m instead of 46 m) in crosswinds in order to take optimum advantage of the drift of the droplets. Thus, 8 100 ha were treated in less than 1 hour. In this test, two applications of fenthion gave better than 85% control of adult A. albimanus for 31 days at distances of 1.6 km or more from the treatment border and > 75% control at 0.16 to 0.80 km from the border. Another, even more successful, demonstration of the practical application of the ULV technique was made in 1972 in Haiti by the Center for Disease Control, US Public Health Service (10-12). The test area of about 8 100 ha was located on the southern peninsula of Haiti in an isolated valley surrounded on three sides by mountains. Falciparum malaria was highly endemic in the area: less than 1 % of the total population of Haiti lived in the valley, but this proportion accounted for over 20% of the malaria in Haiti between 1969 and 1971. The main objective was to determine whether sufficient control of A. albi- manus could be obtained with ULV malathion to affect the transmission of malaria. In all, 6 aerial applications were made from 27 October to 16 November with a twin-engine aircraft fitted with flat fan spray nozzles, at a rate of 439 ml of malathion per ha for the first application and 329 ml/ha thereafter. All applications were made in the morning when meteorological conditions were favourable (wind velocity <4.8 km/h and air temperature <27°C); 4-5 days were required to complete each treatment. The treatments caused a sharp decrease in the biting rates of A. albimanus, from about 27 to less than 3 per man-hour, and they remained low for a full month after completion of the treatments. More important, 4 weeks after the spraying began, the incidence of malaria cases showed a significant de- crease in comparison with the incidence during the previous 6 years. Indeed, an analysis of the cases that did occur revealed that they were concentrated in areas of the valley that had not been sprayed. The data for the test in Haiti suggest a cause and effect relationship between the spraying and the incidence of malaria and appear to prove that aerial ULV spraying can be used to control anophelines and the transmission of malaria. However, the technique must be adjusted in terms of the practical and economic aspects of each situation since large-scale applications of insecticide can be quite expensive. For example, in the USA the cost of malathion at a rate of 329 ml/ha is at present about $0.50 to $0.60/ha, and 324 MOSQUITO CONTROL 325 the cost of the multi-engine aircraft may range from $100 to $200 per hour of operation. The total cost will therefore be related to the number of applications of insecticide and the time required to make each application. Nevertheless, the results of the studies in the Panama Canal Zone suggest that two applications of an insecticide with adulticidal and larvicidal prop- erties might give sufficient initial control so that no more than one application per month would be needed thereafter. Indeed, in areas with both wet and dry seasons, treatments may be necessary only when large populations of mosquitos are present. Also, the time required for application can be reduced if the swath spacings are the maximum that will provide good coverage. For example, in the Panama Canal Zone, control was obtained with swath spacings of 0.64 km. We can conclude that ULV aerial applications would be useful in areas where residual sprays are ineffective in controlling anopheline populations. Also, the use of ground ULV aerosols should be further investigated since they are more economical. In particular, portable equipment may be useful in some situations for both domiciliary and peridomici- liary treatments and would provide a means of controlling anophelines that bite outdoors and in- doors. STERILE MALE TECHNIQUE The sterile male technique is one of the most intriguing methods of insect control developed during the past several decades. Its practicality was demon- strated in the 1950s in the USA against the screw- worm fly, Cochliomyia hominivorax (Coquerel), and it has since been evaluated against numerous other insects. However, it has only been in the past few years that an application to mosquito control has been successfully demonstrated. The largest research effort at the present time is being made by the World Health Organization's Research Unit on Genetic Control in New Delhi, India, which is investigating the effectiveness ofmale Culexpipiensfatigans Wiede- mann sterilized by irradiation, chemicals, cytoplasmic incompatibility, or genetic translocations. Although the Gainesville laboratory of the US Department of Agriculture has been working on the application of the technique to anophelines since the late 1950s, its first successful demonstration of the technique was with the southern house mosquito, C. p. fatigans Wiedemann. In studies conducted in 1969 on Seahorse Key, a small island off the Gulf Coast of Florida, a small population of this mosquito was controlled and almost completely eliminated by continual releases of males sterilized by thiotepa a for 10 weeks (13). The principle of the sterile-male technique is not complicated, but its execution can be difficult. Obvi- ously, thorough knowledge of the population dy- namics and ecology of the insect is essential. Also, the insect must be adaptable to laboratory colonization and rearing, and methods of sterilizing the males and distributing them in the field must be evolved. However, if all these problems can be solved, the technique is very effective. Indeed, it is more effective and less expensive than insecticidal control when population densities of the insect are low. After the Seahorse Key test against C. p. fatigans, investigators at the Gainesville laboratory began stu- dies to adapt the sterile-male method to the control of A. albimanus. The studies were made in El Salvador in cooperation with the Central America Research Station (CARS) of the Center for Disease Control. The test site was a small isolated farming area of about 12 km2 that surrounded a small lake, Lake Apastepeque (14). Normally, anophelines bred pri- marily in the aquatic vegetation around the margin of this lake, but during the wet season they also bred in a low marshy area adjacent to, and nearly the same size as, the lake. This area of El Salvador has definite wet and dry seasons. Populations of the mosquito reach a maximum during September to November of each year and are at their lowest levels from February to April. The test was therefore planned so that releases would begin at the low point of the population cycle when minimum numbers of sterile males would be required. Then, if no fertile insects migrated into the area, the released sterile males would cause an additional reduction, continual releases of males would prevent any buildup, and the population would decline to a negligible level. The releases of sterile males took place between 19 April and 15 September 1972. The total for the period was 4.3 million; the average per day ranged from 15 000 to 40 000. The total estimated population at the beginning of the test was 22 000 each of females and males; the daily emergence of each sex was about 7 000. The released insects were reared at CARS in San Salvador and sterilized in the pupal stage by exposure for 1 h to a 1 % water solution of P,P-bis (1- aziridinyl)-N-methylphosphinothioic amide (15). They were then placed on wet filter paper on the bottom of aluminium pans, covered, and held over- a 1,1',1"-phosphinothioylidynetrisaziridine. C. S. LOFGREN night in foam plastic chests. The next morning, the chests were transported by car 60 km to the test area, and the pans were placed at release stations around the lake or marsh. The males were allowed to disperse at will. The results were evaluated by making twice- weekly biting collections of females near the lake, by collecting resting adults from stables, and by dipping larvae from the lake. From 10 August to 1 November the nightly number of females collected in the stables ranged from 0 to 8 compared with 288 to 1 800 at the same time the previous year (16). Furthermore, no native females were obtained in biting collections during September, and the numbers remained low until the end of November though they had averaged 103 females per man-hour in the same period in 1971. Finally, the density of 3rd- and 4th-instar larvae declined from a high of 44.64/M2 to a low of 0-3.72/M2 from 24 Au- gust to 1 November, though the larval populations of A. pseudopunctipennis Theobald, which also bred in the lake, remained normal throughout the test (the elimination of A. albimanus thus had no effect on this species). The study therefore clearly demonstrated the potential of the sterile-male technique. The sterile-male technique has obvious limitations and it cannot be considered to be a complete replacement for residual insecticides. Any suggested use will have to be evaluated. For example, the fact that only the one species is controlled may be a great disadvantage if more than one anopheline vector of malaria is involved. The technique may have its greatest potential in relatively isolated areas where control of a single species is failing because of resistance to insecticides or because the species is strongly exophilic. Plans are in preparation for a large-scale trial of the technique in an area where an epidemiological assessment of the effect on malaria transmission can be made. The sterile-male technique will be integrated with insecticidal and sanitation control by using the two latter procedures to help reduce the population to a very low level; the sterile- male technique will be used to maintain the popula- tion at a low level. BIOLOGICAL CONTROL WITH PATHOGENS The increased interest in diseases of mosquitos in recent years has resulted in the discovery of a surprisingly large number of new pathogens, includ- ing viruses, protozoa, fungi, and nematodes. For example, Hazard (18) found that 5% of the A. gam- biae Giles and 16% of the A. funestus Giles in the vicinity of Kaduna, Nigeria, were infected. Pathogens probably play an important role in reducing popula- tions of mosquitos, especially during the immature stages. Service (19) documented average mortalities of immature stages of 95-97% for A. gambiae in Kenya and stated that larval nematodes, Coelomo- myces fungi, and epibionts made important contribu- tions. Diseases may eventually be used for anopheline control; however, methods of mass producing, stor- ing, formulating, and dispersing the pathogens are not at present available, which emphasizes the need for increased effort. At present, three pathogens of anophelines are being considered for field tests-the nematodes Reesimermis nielseni Tsai & Grundmann and Dixi- mermis peterseni Nickle and the protozoan Nosema stegomyiae Marchoux, Salembeni & Simond (which is probably the same as N. algerae Vavra & Undeen). Nosema stegomyiae was first reported as a pathogen of anophelines by Fox & Weiser (20), who found it in colonies of A. gambiae in Liberia. Hazard (unpub- lished report to WHO, 1972) reported its occurrence in the USA in laboratory colonies of A. albimanus, A. balabacensis Baisas, A. gambiae, A. quadrimacula- tus Say, and A. stephensi Liston. All the available data indicate that N. stegomyiae is not highly pathogenic. However, other subtle effects of the disease can influence the ability of a mosquito to transmit malaria so this pathogen should be of interest to malariolo- gists. For example, Anthony et al. (22) showed that both the longevity and the egg production of adult A. albimanus were reduced by the nosematosis. Savage et al. (23) reported a pronounced decrease in the longevity of female A. quadrimaculatus infected with both N. stegomyiae and P. gallinaceum and about a 4-fold decrease in the number of surviving females with sporozoites. These findings were con- firmed by Hulls (24) and Ward & Savage (25). The effect of any or all of these factors on the capability of an anopheline population to transmit malaria could be quite dramatic. For example, the hypothetical population model developed by Anthony et al. (22) for A. albimanus was used to demonstrate that a reduction in adult female longevity of one-half could cause an 85-97 % reduction in the number of females capable of transmitting malaria. No field trials have yet been undertaken to assess the effect of dispersing N. stegomyiae against natural anopheline populations. However, the Insects Affect- ing Man Laboratory will make small preliminary ttials in the Panama Canal Zone in 1974 with A. albimanus. The preliminary work now in progress 326 MOSQUITO CONTROL 327 at the laboratory to support these trials includes methods of mass production and studies of methods for the storage of spores, for formulation, and for field application. Research with the nematodes D. peterseni and R. nielseni is being conducted at the Agricultural Research Service Gulf Coast Mosquito Research Laboratory, US Department of Agriculture, at Lake Charles, La. Diximermis peterseni is specific to anophelines, while R. nielseni will infect a wide variety of mosquitos. Methods of mass producing R. nielseni in C. p. fatigans have been developed, and field trials with inundative releases of about 1 000-2 000 pre- parasitic larvae per m2 of water surface have pro- duced a parasitism rate of 76-85% in anopheline larvae (J. J. Petersen, personal communication). Field tests with D. peterseni suggest that it may be an effective biological control agent against anophe- lines because of its persistence in the breeding areas. Petersen (personal communication) reported that when an uninfected breeding area was infected with these nematodes, 88-90% of all larvae samples taken after 2-3 years were infected, even though the site was completely dry during certain periods. RESUME DEVELOPPEMENTS RECENTS CONCERNANT LES METHODES DE LUTTE CONTRE LES MOUSTIQUES La lutte contre le paludisme a enregistre des echecs dans certaines regions du monde en raison de la resistance des moustiques aux insecticides a effet remanent pulverises dans les habitations, de l'exophagie de certaines especes et du fait que certaines autres sont endophages mais exophiles. Dans de telles situations, l'interruption de la transmission du paludisme peut exiger la destruction com- plete des populations d'anophelines. D'autres methodes de lutte peuvent etre envisagees, notamment les applications d'insecticides en pulverisa- tions ou en aerosols sous ultra-faible volume, par epan- dage aerien ou au sol; la technique des males steriles; et la lutte biologique utilisant certaines maladies des vecteurs. D'exp6riences effectuees recemment a Haiti, il ressort que des applications successives de malathion sous ultra- faible volume, par epandage aerien, permettent de lutter efficacement contre Anopheles albimanus et d'interrompre la transmission du paludisme a Plasmodium falciparum. En El Salvador, des essais faisant appel a la technique des males steriles ont montre que ce procede etait potentielle- ment utilisable pour l'6limination d'A. albimanus. Des etudes pr6liminaires sont envisagees en vue d'evaluer I'action d'un protozooaire pathogene, Nosema stegomyiae, et de deux nematodes parasites, Reesimermis nielseni et Diximermis peterseni, sur les populations d'anophelines. Ces nouvelles methodes doivent cependant faire l'objet de plus amples recherches visant a determiner leur prati- cabilite sur les plans dconomique et operationnel. REFERENCES 1. LOFGREN, C. S., ET AL. Annual review of entomology, 15: 321-342 (1970). 2. FULTZ, F. O., JR., ET AL. Mosquito news, 32 (4): 501- 504 (1972). 3. LOFGREN, C. S. American journal of tropical medicine and hygiene, 21: 819-824 (1972). 4. WEIDHAAS, D. E., ET AL. Mosquito news, 30 (2): 195- 200 (1970). 5. MouNT, G. A., Mosquito news, 30 (1): 70-75 (1970). 6. LOFGREN, C. S., ET AL. Journal of economic entomol- ogy, 33 (5): 187-189 (1973). 7. MouNT, G. A. & PIERCE, N. W. Mosquito news, 33 (3): 368-370 (1973). 8. LOFGREN, C. S., ET AL. Mosquito news, 38 (3): 353- 355 (1968). 9. LOFGREN, C. S., ET AL. Mosquito news, 30 (4): 604- 610 (1970). 10. ELIASON, D. A., ET AL. American journal of tropical medicine and hygiene, 23, in press. 11. TAYLOR, R. T. & SALLS, M. American journal of tropical medicine and hygiene, 23, in press. 12. KRAGSTAD, D. J., ET AL. American journal of tropical medicine and hygiene, 23, in press. 13. PATTERSON, R. S., ET AL. Science, 168: 1368-1370 (1970). 14. BREELAND, S. G., ET AL. American journal of tropical medicine and hygiene, 23: 273-281 (1974). 15. DAME, D. A., ET AL. American journal of tropical medicine and hygiene, 23: 282-287 (1974). 16. LOFGREN, C. S., ET AL. American journal of tropical medicine and hygiene, 23: 288-297 (1974). 17. WEIDHAAS, D. E., ET AL. American journal of tropical medicine and hygiene, 23: 298-308 (1974). 328 C. S. LOFGREN 18. WORLD HEALTH ORGANIZATION. Manual on larval Control operations in malaria programmes. Geneva, 1973, p. 133 (WHO offset publication No. 1). 19. SERVICE, M. W. Bulletin ofentomological research, 62: 359-369 (1973). 20. Fox, R. M. & WEISER, J. Liberia journal of parasi- tology, 45: 21-30 (1959). 21. HAZARD, E. I. Proceedings of the IVth International Colloquium on Insect Pathology, College Park, Md., 25-28 August 1970, pp. 267-271. 22. ANTHONY, D. W., ET AL. Proceedings of the Hel- minthological Society of Washington, 39: 428-433 (1972). 23. SAVAGE, K. E., ET AL. Bulletin of the World Health Organization, 15 (6): 845-847 (1971). 24. HULLS, R. H. Transactions of the Royal Society of Tropical Medicine and Hygiene, 65: 421-422 (1971). 25. WARD, R. A. & SAVAGE, K. E. Proceedings of the Helminthological Society of Washington, 39: 434-438 (1972). DISCUSSION NAJERA: Within the context of recent developments in vector control attention should be paid to prob- lems that present campaigns are facing, including the possible scarcity of DDT and the slow pace in the development of new insecticides. Investigations on the spectra of cross-resistance for candidate com- pounds and the overlapping of such compounds with those used locally in agriculture should guide future choices. DIETZ: If the target population is not or cannot be completely isolated against the immigration of al- ready inseminated female mosquitos, then releases have to be maintained indefinitely. Depending on the rate of immigration and the strength of density- dependent regulation, the density of adult females may be higher in the presence of releases than in their absence because of the higher survival rate from egg to adult. An immigration rate of 1% of the local emergence rate can lead to nearly normal adult densities even in the presence of 100 %-effective genetic control. If the target population is isolated, eradication may be achieved only if sterilization and sexing are perfect. When some partly fertile males and females are released together with sterile males, there may exist a maximum release ratio beyond which the target population would be increased rather than decreased. The evaluation of the relative effect of genetic control measures applied in conjunction with insecticides makes it imperative to have a proper basis of comparison. BRUCE-CHWATT: In Upper Volta, the release of sterile hybrid male A. gambiae failed to reduce the local population as much as expected because of a partial barrier to mating between species and perhaps because the trial area was insufficiently isolated. Such a field trial should be repeated in a suitable area, and large cages should be used to allow for mating of the wild population and the controlled release of sterile males. RAMSDALE: It may be possible to achieve adequate control by one or two ULV applications instead of house spraying, when a period of up to two months has to be covered prior to the beginning of effective insecticidal application in agriculture. WmTE: Males of other species of mosquito could perhaps be released so as to make use of possibly greater mating vigour. HADJINICOLAOU: Gambusia has been used with suc- cess for anopheline control in rice fields and for the control of nuisance mosquitos in various types of breeding place including those in urban areas. CAMBOURNAC: Gambusia showed considerable ability to survive in the Tagus Basin of Portugal following its accidental introduction from Spain decades ago. BENMANSOUR: It is advisable to make a proper comparative evaluation of new methods with a locally established method.