SOME NEWIZR IDEAS IN MOSQUITO CONTROL By HAROLD FARNSWORTH GRAY, Dr. P. H. Lectwer En Public Health, University
of
California, Berkeley YPrior to about 1943, mosquito control practices appeared to have be- come rather static. Most of the effort of men engaged in the work ap- peared to be concentrated on refinements of details as to equipment and methods, and there was little to indicate that any basic changes in either programs or methods were in the offing. There were still many prob- i lems to be solved, and there were abundant opportunities for the de- velopment of new ideas, and indeed there were still many opportunities for more effective exploitation and use of some old ideas which had not been fully utilised, particularly in the field of naturalistic control meas- ures.
The only apparent major argument lay between those groups who were proceeding along the Enes of mosquito control, or abatement, and a limited group who believed that mosquito erao?ica¿ion, or at least the
extirpation of a single species in a particular geographic area, was a practicable goal. The arguments on this phase of the work would prob- ably have been confined to the fíelds of practicability and economy, the advocates of species extirpation believing from experience with AnopheZees gambiae and Aties aegypti in Brasil that species extirpation is actually practica1 and ultimately economical, though requiring a relatively large initial outlay.
There were also some apparent major arguments between those groups &ho were concerned with mosquitoes as vectors of specific diseases only, and those whose concern was principally with mosquitoes as pests causing economic damage to urban and agricultura1 areas and intense discomfort to people. The first group was concerned solely with particular vector ’ species, and to some degree was satisfied with only such measures as
would reduce the incidence of a disease to tolerabIe limits, provided this could be accomplished at a very low cost. In some cases their controlling factor has been the low income of the group involved, as was the situa- tion in India where P. F. Russell and others were endeavoring to reduce malaria incidence with weekly pyrethrum sprays in native huts. Ob- viously among such unfortunate groups the bitter struggle for mere existence precludes any attention to human comfort; when one must eat but sparingly even disease prevention of necessity can cost but little.
Conversely, among population groups of a relatively high economic status based on large scale production of economic goods, disease pre- vention alone is not enough. Not only is it required that disease vectors be controlled, but demands for control of pest mosquitoes also become
322 PAN AhlERICAN SANJTARY BUREAU [April urgent, not only for human comfort (which is a legitimate demand) but also to minimize economic loases of many types, among which may be mentioned real estate values, butter and egg production, crep losses due to farm labor inefficiency, and a multitude of other ascertainable and ( measurable economic losses in agriculture, manufacturing and merchan- dising.
To a considerable degree in societies in which ecomonic productivity has reached a reasonably high leve1 the trend has been to utilize those methods of mosquito control which are effective in reducing the pro- duction, at the breeding source, of at least the more numerous species of , mosquitoes, irrespective of their disease vector potentialities. The costs of such abatement measures have been low relatively to the economic gains resulting. But in societies having a low leve1 of economic pro- ’ ductivity the trend has been toward minimum control measures directed at vector species and to a certain extent toward attacking the adult insect rather than attempting larval control, if the latter is comparatively expensive .
With the advent of DDT the situation has defmitely changed in some respects, and it now appears to be necessary to change our ideas in par%, and to re-orient our mosquito control programs. The one outstanding thing about DDT is that it gives us a wholly new weapon in mosquito control, that is, a residual effect material applied to surfaces in dwell- ings, animal quarters and other places to which adult mosquitoes may resort . As a larvicide, or as a space spray, DDT is just another insec- ticide, more potent in some respects than those which we have possessed, but essentially not changing our basic methods. But as a residual ap- plication to surfaces, DDT is entirely new in concept and effect. It is this effect which makes it necessary to re-orient our thinking in regard to control measures, and to modify our programs and techniques so as to use the residual effect to best advantage.
It is possible to make a single application of DDT residual spray to the interiors of dwellings, animal quarters and other structures which will be effective for monthsincuttingdowntoverylownumbersseveral species of mosquitoes which attack man and animals, and by so doing reduce greatly the incidence of diseases such as malaria, urban yellow fever, probably epidemic virus encephalitis, probably also filariasis, and dengue (where Aëdes aegypti is the vector-where Aëdes albopictus is the vector there may be some doubt).
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prevention of mosquito production. Here, however, the use of DDT as an application to ground surfaces prior to hatching gives promise of an effective new technique in control of these species. But we are still a long way from utilizing adequately the naturalistic measures of con- trolled re-flooding and re-draining, and of constant leve1 flooding supple- mented by fish control, as effective and economical measures for the prevention of production of these species of mosquitoes.
Except for the “field” species of mosquitoes, to what extent can we expect this new technique of DDT residual spraying to modify our pur- poses and change our practices in mosquito control? 1 do not pretend to know all the answers to this question, but some answers appear to be rather obvious.
Among societies of low economic status it appears probable that we have in DDT residual spray a new technique which can be used effec- tively to reduce disease incidence among these people, by only a few sprayings per year, at an expense presumably within their very slender means of paying for the service. We can perhaps also expect within a few years of such a program that the economic productivity of such so- cieties may rise sufficiently to justify further expenditures for control of mosquito breeding, which in turn should result in greater productivity, better health and higher standards of living.
Among societies of large economic income and high rates of produc- tivity it appears probable that DDT residual sprays will be used prin- cipally as supplements to existing mosquito control measures, resulting in more effective mosquito control with greater satisfaction in public comfort. However, 1 do not expect to see DDT residual spray used to an appreciable extent in dwellings in cities in temperate climates, as it does not appear to be necessary or even advantageous in such situations. Its use is more apt to occur in rural houses, particularly in the southern states of the United States.
It appears probable that DDT will be used extensively as a residual spray in houses in tropical and semi-tropical climates, irrespective of other mosquito control measures, for its effect upon insects such as flies, fleas, bed bugs, etc., partly as a disease preventive measure, and partly as a means of increasing comfort. As a practica1 matter, in tropical regions it will probably be found by the groups of good economic status that it will be to their advantage to make substantial payments toward defraying the cost of residual spraying programs in the houses of the people of low economic status, because by so doing they will increase their own comfort and minimize their own chances of infection.
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in a geographical area of limited extent to be within the bounds of prac- ticability. Examples which come to mind are isolated valleys in Peru and islands such as Corsica and Sardinia. Undoubtedly the gambiae extermination campaign in northern Brazil in 193840 could have been accomplished more quickly and at less cost if DDT had then been avail- able.
THERMAL AEROSOLS
The general idea of thermal aerosols as a technique for applying insec- ticidal materials is far from new. However, with the advent of DDT it has become possible to apply clouds of a small-particle-sized insecticide of high toxicity for use as an adulticide against mosquitoes, ílies and many other insects. By “fogging” extensive areas with such thermal aero- sols, migrating swarms of mosquitoes can be killed and public annoyance from their attacks greatly reduced. Unfortunately, adverse conditions of wind, temperature, humidity, rain, etc., may make it difficult, often at critica1 times, to apply thermal aerosols effectively when they are most needed. With good luck as to weather conditions, they can be a helpful adjunct for mitigating annoyance, or perhaps even disease trans- mission, when because of failure in basic control measures there has been an extensive hatching out, and/or migration, of either vector or pest mosquitoes.
In a number of cases extensive applications of DDT thermal aerosols have been made in cities, and surprisingly enough the residents have remarked, “New the mosquito people are really doing something.” No one tells them that al1 this obvious smoke is simply a confession of fail- ure on the part of the mosquito abatement agency-it had failed to con- trol that crep of mosquitoes before it hatched out and migratedinto town; or adequate measures for the control of breeding had been ne- glected in town until the population of vectors or pests had built up to a dangerous point, and it had become necessary to apply emergency meas- ures to make up for past negligente.
There are legitimate uses for DDT thermal aerosols in mosquito con- trol work, as well as in the control of other insects. Unfortunately, it has been used in some situations for the avowed control of diseases which either are not transmitted by insects, or in which insect trans- mission evidently can play only a minor role at best. A combination of public ignorance of epidemiology, of public hysteria, and perhaps an excessive salesmanship on the part of salesmen of thermal aerosol gen- erating equipment, may be the reasons for this misdirected effort and useless squandering of public funds.
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mate, effective and safe limits of action. It is particularly important at present that the public be neither misinformed, nor allowed to draw erroneous conclusions from careless or irresponsible statements con- cerning this insecticide. A deíinite degree of intellectual honesty and plain truth telling is necessary. Used intelligently and ín accordance with ita limitations, DDT is an effective adjunct in the mosquito con-
trol campaign. Let us use it as such, and tell only the truth about it, without exaggeration.
NUEVAS IDEAS SOBRE EL CONTROL DE MOSQUITOS (Sumario) Los problemas en ambos lados de la frontera son iguales entre sí y no son muy diferentes de los de otras partes, constituyéndolos el uso del agua, del hielo y lo árido de la regiõn. Hasta hace poco los métodos de control se hallaban en una fase relativamente estacionaria, con s610 pequeños perfeccionamientos, especial- mente en los procedimientos “naturalistas.” Los conceptos son: (1) Control 0 disminuci6n hasta límites tolerables, para la protección contra enfermedades, malestar y p6rdidas econ6micas, y (2) Control seleccionado, de cierta especie, en una zona determinada, de lo cual constituye un buen ejemplo el control del temido
A. gambiae, en el Brasil, hace algunos años. En muchos países ha sido necesario limitar el control de modo que armonice con el simple sostenimiento de la vida humana, situación que ha sufrido un cambio radical debido a los nuevos procedi- mientos en el control de mosquitos, surgidos hacia 1943. El nuevo insecticida DDT se puede fácil y económicamente aplicar a las superficies en que se posan los mosquitos, destruyendo la mayoría aunque no todas las especies, y siendo efectivo durante meses. Aun cuando por razones de economia haya que emplearlo en cantidades limitadas, resulta muy eficaz en lae viviendas humanas. Puede apli- carse en forma de pulverización o de “neblina” de aerosol, esta ultima forma es bastante popular aunque quizas no tan eficaz si se tiene en cuenta que los mosqui- tos generalmente se hallan confinados a sitios aislados. Puede considerarse prudente el consejo de emplear DDT tomando en cuenta sus limitaciones.
