Demographic Responses of Two House Mouse Populations to Moderate Suppression Measures with 1080 Rodenticide

Abstract

Poisoning procedures used in mouse control operations often suppress an infestation without eliminating it; after an initial decline, the population apparently becomes more or less stabilized at a lower level. The mechanisms which underly this adjustment hold considerable interest for both the population ecologist and the pest control operator. During 1948 and 1949 an attempt was made to analyze the responses of a natural infestation of house mice (Mus i'nuschtds) to the withdrawal of poison after a period of 33 months in which professional control operations with 1080 (sodium fluoracetate) rodenticide had consistently failed to provide more than what was considered moderate control. This study (population A) was supplemented with another study (population B), made concurrently, in which population changes were analyzed as poisoning procedures were instituted. In both cases measurements were made of population size, mortality rates, life expectancy, population composition, breeding activity, local movements, trap susceptibility and poison susceptibility. Reports on the latter three aspects of the study have already been published (Young, Strecker and Emlen, 1950; Young, Neess and Emlen, 1952; Emlen and Crow, 1951). Data were collected over a period of eight months from September 28, 1948 to June 1, 1949. The first four mi-onths provided information on conditions before poison was removed in population A and before it was introduced in population B. The second four months provided 'the data for evaluation of the changes induced by poison withdrawal and poison introduction. The studies were made in two old vivarium buildings at the University of Wisconsin in Madi3on. Both buildings contained much natural mouse marborage and an abundant supply of food, provided for the experimental animals such as rabbits, doves, and chickens, but constantly available to the wild mice. Both buildings were maintained at room temperatures and were dark at night. Both were reasonably isolated from other mouse infested buildings, and their populations were thus essentially unaffected by ingress and egress. Traps set in large numbers in neighboring buildings captured over 500 mice, of which only 2 were marked animals from the populations under study. Sodium fluoracetate is a highly toxic and essentially tasteless rodenticide giving excellent results in laboratory tests (Anon., 1952). Field tests on house mice may give complete kills under optimum conditions; but populuations often persist, especially where bait are inadequately distributed as in the present instance (EEmlen, 1950). The poisoning procedure followed in this study used a one percent water solution distributed in one pint chicken drinking-wells, permanently maintained and refilled once each month. There were 18 wells or stations in building A, an average of one every 722 square feet of floor, and 8 in building B, an average of one every 622 square feet. Stations were distributed primarily in accordance with evidence of mouse activity, but no point in either building was more than 25 feet from a poison source. Population data were obtained by three sampling techniques: (1) live traps were maintained in both buildings and operated three consecutive nights each week throughout the eight months of the study. A total of 1781 mice was thus captured, toe-clipped for subsequent identification, and released. By using combinations of toe-clippings on both front feet and hind feet each mouse received a distinctive marking usable for individual identification on recapture or recovery. (2) Dead mice (eventually totalling 514) found near the poison and elsewhere were collected and examined at each visit. (3) In each of the two buildings, the study was terminated with an intensive snap-trapping campaign carried on over a fifteen day period until capture rates had declined to near zero. A total of nearly 900 mice from this source was available for autopsy. These sampling techniques rarely took any nestlings or young weanlings. The figures and indices to be presented are thus applicable only to the adult segment of the populations. ' Contribution from the University of Wisconsin, Department of Zoology. This work was supported in part by the University of Wisconsin Research Committee from funds supplied by the Wisconsin Alumni Research Foundation, and in part by a grant-in-aid from the National Institutes of Health. 2 Zoology Department, University of Wisconsin, Madison, Wisconsin. 3 Biology Department, Wisconsin State College, LaCrosse, Wisconsin. 'Zoology Department, Miami University, Oxford, Ohio.