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PEST CONTROL IN PUBLIC HEALTH Samuel W. Simmons U. S. Public Health Service During the 20th century, remarkable progress has been made in eliminating vector-borne diseases from the United States. Such scourges as yellow fever, urban plague, dengue fever, and malaria are no longer with us. Although the reasons for the disappearance of these diseases are not fully known, it is felt that a combination of many factors was responsible, among which are: general im- provement in individual health and chemotherapeutic control, con- centrated efforts to control vector species in strategic areas with more efficient methods, general public education on hazards asso- ciated with the diseases, and improved sanitation in urban areas. It is of interest to note that the elimination of these diseases was not contingent upon eradication of the arthropod vector, because the yellow fever mosquito, the common malaria mosquito, and the oriental rat flea are still well established in this country. Increasing Importance of Vector Problems In view of the foregoing information, it may seem paradoxical to note that vector control activities throughout the United States have increased considerably during recent years, both in the extent of area covered and the types of problems included in control pro- grams. Today's vector control activities are directed principally against vectors of mosquito-borne viral encephalitis and innumer- able "pests" that interfere with man's physical and mental comfort as well as producing secondary infections and allergic manifesta- tions. In addition to mosquitoes, other pestiferous arthropods in- clude sandflies, dog flies, blind mosquitoes or non-biting midges, blackflies, ticks, fleas, and chiggers. The public health importance of vector problems is being in- tensified due to such factors as: (1) the rapid increase in popula- tion, (2) the development of suburban areas in close proximity to breeding sources, (3) the expansion of man-made aquatic habitats created by the construction of new water resources projects, (4) the 14
increasing exposure of man to insects of public health importance due to expanded public use of water-related recreational areas, (5) the development of insecticide resistance, and (6) the public de- mand for a more healthful environment. In many cases, control programs directed against pests in- jurious to public health are confined to urban and suburban areas and thus generally have minimal adverse effects upon wildlife resources. Vector control programs that present the greatest potential hazard from the wildlife standpoint are those directed at extensive mosquito sources in rural areas, particularly fresh-water and salt marshes. For the purposes of this discussion, emphasis will be given to mos- quito control. It is common knowledge that conflicts sometimes arise be- tween fish and wildlife conservation and mosquito control interests. They are usually related to drainage, filling, and the use of in- secticides. Experience on TVA impoundments has demonstrated that fish and wildlife conservation and mosquito control interests are not always antagonistic and that a cooperative approach to prob- lems often results in the development of mutual interests. In order that these conflicts will be kept to a minimum, the following princi- ples should always be followed: (1) the instigation of mosquito abatement programs should be based upon a demonstrated need established by field investigations; (2) in choosing the method of control, adequate consideration should be given to fish and wild- life resources, i. e. , recognition of the wildlife values associated with the mosquito-breeding habitats; (3) public ownership of wet lands should carry with it responsibility for mosquito control. Mos- quito control and wildlife management programs should be under- taken only when in the public interest. One of the most fundamental items in the development of a sound mosquito control program is an adequate survey to determine the source and extent of the problem. The practice of beginning major control operations with inadequate information is highly wasteful, and may result in complete loss of public confidence in a worthwhile project. Methods of Control The choice of control method may be exceedingly complex. Various factors to be considered, some of which are intangible, in- clude: habits and biology of the species, the physical environment, 15
urgency of control - especially vectors of disease such as ence- phalitis - and permanence of control measures. The three principal methods of mosquito abatement are source elimination (reduction), naturalistic control, and chemical control. Source elimination. Where at all feasible, efforts should be made to eliminate breeding places permanently by filling, drainage, impoundment, sanitation, or other means. These are widely known as permanent or primary control methods. These methods are particularly well adapted for all areas of high economic development. Such areas include those within and close to cities or resorts where a high de- gree of control is sought. In the long run, permanent control measures generally are the most effective, economical, and endur- ing of all control measures. Limitations in the use of permanent control methods may arise where the costs are excessive for the size of the area to be protected, or where valuable wildlife re- sources need to be preserved. Naturalistic control. This may be defined as the willful manipulation by man of one or more natural factors - physical, chemical, or biological - so as to prevent or discourage mosquito breeding. Examples include water-level management of impoundments, salinificationof a body of water, and the use of biological control techniques. The best known of the biological agents are the larva-eating fishes, particularly the top-water minnow (Gambusia). Some observers think that the success of tidal ditches is substantially due to making mosquito larvae accessible to native predaceous fishes. Naturalistic control is largely an unexplored field. A recently developed method that appears very promising is the creation of impoundments on salt marshes where the water level can be manip- ulated and controlled. This method not only suppresses production of Aedes mosquitoes but it actually enhances wildlife values. It would be highly desirable for conservation and wildlife agencies to foster and promote research on naturalistic control methods. 16
Chemical control. Pesticides may be used for destroying both the immature and adult forms of mosquitoes. Chemical control includes temporary larviciding, residual larviciding, residual adulticiding, and space spraying. Control at the source, if at all possible, is the procedure of choice. Prior to the advent of DDT, adulticiding was a "premium" type of control; today it is common. The large-scale global cam- paign against malaria vectors is based on the application of residual toxic coatings of chemicals to surfaces where mosquitoes rest. The use of chemicals is frequently termed a supplemental or secondary control method. Limitations of this method include the temporary effectiveness of the treatments, the high cost of re- peated applications, the toxic hazards involved, and the develop- ment of insecticide-resistant strains of insects. One of the chief advantages of chemical control is the immediate and often spectac- ular relief from annoyance. It is for this reason that chemical con- trol measure splay such an important role in suppressing epidemics of mosquito-borne diseases. Chemical applications are also used where the more permanent methods for mosquito control are not feasible. Since chemical control methods may adversely affect fish and wildlife, they should be planned and executed with full con- sideration given to the protection of fish and wildlife values. The best procedure in vector abatement programs is to uti- lize a combination of source elimination, naturalistic, and chemical control methods. Protection of Wildlife Values Insofar as the public health use of mosquito larvicides and adulticides is concerned, the associated dangers to wildlife are largely potential rather than real. Mosquito control has a good record for over 50 years. It should be emphasized that the amount of aerial spraying for mosquito control comprises only about two per cent of the total acreage (lt)0 million acres) sprayed annually in the United States. Furthermore, the dosages needed for mosquito control are generally lower than for most other insect control operations. Every feasible effort should be made to minimize possible detrimental effects of chemicals to wildlife. The following rules or precautions should be observed in the use of insecticides in mos- quito control: 17
1. Use insecticides that are not unduly harmful to wildlife. 2. Apply the minimum dosage consistent with effective control. 3. Applylarvicides to known or verified potential production sites only. 4. Choose dosages, formulations, manner, and frequency of application that comply strictly with Federal and State recommenda- tions. 5. Execute programs under competent supervision. 6. Time the applications in order to accomplish the greatest good and least harm. In conclusion, the best procedure for solving conflicts of in- terest between mosquito control and wildlife conservation is for a cooperative approach by all. In the future, emphasis must be on a positive approach - the promotion of mutual interests based on a proper understanding of each other's problems. 18
