Chemosterilants as an Approach to Rodent Control

Capitalizing on research directed toward oral contraceptives for humans, a wide variety of compounds are now under study for their practical value in i n h i b i t i n g rodent reproduction to suppress detrimental populations. This paper discusses the specifications of ideal rodent chemosterilants and the advantages of chemosterilants over other methods of control, and compares the potential values of chemosterilants acting on females, males, and both sexes. Specific situations are detailed where chemosterilants w i l l be most valuable in rodent control, together with proposed methods of application. Chemosterilants are not expected to become a panacea for control, but since they are based on sound biological principles they should be a safe and effective approach to regulation of rodent populations. Recent years have seen considerable enthusiasm over the possible use of chemosterilants for suppressing population levels of troublesome rodents. Interest in this approach has existed for a long time, but a major surge of interest has developed in the United States during the last few years. This interest has been stimulated in part by chemical and pharmaceutical companies, which have recently expanded their research on potential chemosterilants. Since a variety of specific descriptive terms are sometimes used — such as antiferti1ity agents, spermatocides, embryocides, and gametocides — confusion might be avoided if we adopt the all-inclusive term "chemosteri1 ant." A chemosteri1 ant can be defined as a chemical that can cause permanent or temporary s t e r i l i t y in either or both sexes or, through some other physiological aspect, reduce the number of offspring or alter the fecundity of the offspring produced. This definition, thus, does not define the mechanism or the specific phase in which the compound operates; it simply classifies the compound on the basis of its biological effect. Researchers in the United States and elsewhere are currently evaluating chemosterilants in both laboratory and field. There is l i t t l e indication, however, that chemosterilants are currently being used in normal or routine rodent control practices. There is a multitude of information on compounds that affect various aspects of mammalian reproduction, and in many of these studies the test animals were laboratory rodents. The purposes of most of these studies range from contributing to basic knowledge of physiological mechanisms to studying the effects of drugs destined for human contraceptives; but, even though they are not oriented toward regulating naturally occurring rodent populations, they do provide information of enormous value in the selection of candidate chemosterilants for rodent control purposes. In this respect, vertebrate pest control has and w i l l benefit immensely from the intensive search now under way for contraceptives for humans. Credit for the concept of sterilization of insects goes to Dr. E. F. Knipling, who proposed it in 1938 (Smith, 1966). About two decades later, after many years of research, the concept was applied to eradication of the screwworm, Cochliomyia hominivorax (Cqrl.), a serious pest of livestock in southeastern United States. We would hope that rodent chemosteri lants w i l l become a practicality much more quickly, although, realistically, they w i l l not be found and proven effective without considerable effort and expense. The concept of regulating reproduction in rodents and other w i l d mammals has been discussed by Knipling (1959) and Davis (1961). In the United States, Davis conducted one of the earliest studies of t his approach to rodent control-in 1957-58 on a rat population in the c i t y of Baltimore. The results, though promising, were inconclusive, and the studies were terminated for other reasons (Davis, 1961). This approach to rodent control has received greater attention more recently (Balser, 1964b; Brooks and Bowerman, 1969; Howard, 1967a, 1967b, 1968; Howard and Marsh, 1969; Marsh and Howard, 1969; Pingale et al., 1967; Skinner, 1968; Srivastava, I966; Wetherbee, 1964). Research on chemosterilants is further along for the suppression of pest birds than for rodent control. This approach to regulating pest species of birds has been reported on by Davis (1959), Elder (1964), Vandenbergh and Davis (1962), Wetherbee (1964, I967), and others. Balser (1964a, 1964b), Linhart (1963, 1964), Linhart et al., (I968), and Linhart and Enders (1964) studied the possib ilitie s of chemosterilants for several predator species, but the technique is not yet in general use.