Purchased by U. S. Department of
Agriculture for official use

Exploratory Studies on Gamma Radiation for the Sterilization
and Control of *Anopheles quadrimaculatus*¹

A. N. DAVIS, J. B. GAHAN, DONALD E. WEIDHAAS, and CARROLL N. SMITH, *Entomology Research Division, Agric. Res. Serv., U.S.D.A.*

ABSTRACT

In laboratory studies on the effect of gamma radiation on *Anopheles quadrimaculatus* Say, it was found that dosages of 8,865 to 12,900 r applied in the pupal or adult stage were required to cause complete sterility. Irradiated females mated to unirradiated males produced no eggs, whereas unirradiated females mated to irradiated males produced a normal number of eggs but none hatched. When irradiated males were introduced into caged populations of normal males and females at ratios of 4:1:1 or less, usually no reduction in the total number of viable eggs was produced, but at ratios of 6:1:1 and 10:1:1 there was a reduction of about 80%. The dosages (r) required to cause 50% and 100% mortality in 48 hours were: for eggs, 2,600 and >11,000; for larvae, 32,000 and 120,000; for pupae, 22,000 and 40,000.

The eradication of the screw-worm, *Callitroga homini-vorax* (Cqrl.), from the island of Curaçao by the release of sterilized flies (Bushland *et al.* 1955, Baumhover *et al.* 1955) led to consideration of other pests that might be susceptible to eradication by this method. The method appeared to hold some promise for use against certain

anophelines. These mosquitoes do not pass the winter in the egg stage, and consequently spring populations are usually low. Even if not suitable for use alone, the method might be useful in combination with others. Considerable effort is devoted to antimalaria campaigns, and sterilized insects might be released to apply the coup de grâce to a population already depleted by other control measures, the survivors of which might not be reached by any other means. Laboratory rearing procedures are available and it seems probable that mass rearing and irradiation techniques could be developed. Preliminary laboratory studies to evaluate the method against a representative anopheline appeared to be justified.

Laboratory studies were conducted to determine the effect of gamma radiation on *Anopheles quadrimaculatus* Say in various stages of development, and the results of adding sterilized males to caged normal populations. Two cobalt-60 radiation sources were used. One gave an aver-

¹ Accepted for publication March 20, 1959.

# Exploratory Studies on Gamma Radiation for the Sterilization and Control of *Anopheles quadrimaculatus*¹

## ABSTRACT

Reprinted from SCIENCE, October 9, 1959, Vol. 130, No. 3380, pages 902-904

# Sterile-Male Method of Population Control

Successful with some insects, the method may also be effective when applied to other noxious animals.

E. F. Knipling

Laboratory investigations (1) followed by the successful eradication of the screwworm (Callitroga hominivora) from the island of Curaçao (2) have established the value of utilizing induced sexual sterility as a means of regulating insect populations. Progress in the eradication program currently under way in Florida and other southeastern states under the sponsorship of the U.S. Department of Agriculture and the Florida Livestock Board is further confirmation of the soundness of this approach for the control of this pest of livestock.

I have previously discussed the principle involved in control or elimination of an insect species through the release of a dominant population of sexually sterile males (3). The purpose of this article is to consider further possibilities of controlling insect and other animal populations through application of the same principle. It is hoped that such discussion will stimulate research leading to broader application of this technique for use against insects and possibly for regulating or for eliminating from problem areas undesirable populations of higher animals, such as rodents, predators, and other wild animals.

These possibilities are based on the following biological principle: The introduction of sexually sterile but otherwise sexually vigorous males, and to a lesser extent females, into the natural population of an animal species will have greater influence in reducing the biotic potential of the population than elimination of the same number of individuals from the population by destruction or removal.

The author is director of the Entomology Research Division of the Agricultural Research Service, U.S. Department of Agriculture, Beltsville, Md.

If this is a valid biological principle, then biologists concerned with the regulation of animal populations might find it profitable to devote considerable research effort to the development of practical procedures for introducing substantial numbers of sexually sterile males of a species into the population rather than to devote all efforts to the development of ways to kill the organisms. The relative effectiveness of the two procedures will depend on a number of factors, including the mating and dispersal habits of the species. Many obstacles to practical application of the sterile-male technique may limit its usefulness. However, full exploration of the method may lead to broader application than is apparent with current knowledge.

Theoretically, the maximum degree of reduction in the biotic potential of the total population of an organism is directly proportional to the ratio of sterile to fertile males. The degree of control that can actually be achieved probably will vary with the species and may fall well below the theoretical maximum because of the many factors involved. However, the potential effect, provided that normal sexual behavior is retained in the males, is much greater than when the conventional procedure of animal population control is followed. This can be shown to be the case even when the effectiveness of the sterile males is well below the theoretical maximum.

## Insect Populations

To illustrate the possibilities of the sterile-male method I have established certain hypothetical conditions. On the basis of these conditions, I have calculated the theoretical maximum effect on the populations of insect and other ani-

mal species when they are treated in different ways for the introduction of the sterility factor. Research to develop effective and economical ways of achieving a high level of male sterility in natural populations would greatly extend the application of the procedure for pest control. Thus far, the rearing and release of a dominant number of irradiated insects is the only method that has been employed. If a chemical or other satisfactory method of causing high sexual sterility could be developed which could be applied to the natural populations of certain insects, it would not be necessary to rear and release insects, as is now done for screwworm eradication.

The effect of procedures that cause the desired type of sexual sterility in an insect population, in comparison with those that kill the same number of individuals, is indicated in Table 1. The following assumptions are made in this hypothetical model. (i) The female is monogamous in mating habit, and each sterile male in the population is fully competitive with normal fertile males in mating with the normal females. (ii) The sterilizing agent or procedure will induce sexual sterility in 90 percent of the males and females of each generation, and the killing agent will kill 90 percent of both sexes in each generation. (iii) The biotic rate of increase in an untreated population is fivefold in each generation, and the survivors in the two treated populations increase at the same rate. The potentially greater effect of the sterility approach is readily apparent from the figures in the table.

As shown in Table 2, inducing sexual sterility in the natural population by chemicals or other means is even more effective than rearing the insect and releasing a dominant number of gamma-irradiated males, the procedure used for screwworm eradication.

Theoretically, the effects of a treatment that induces sterility in 90 percent of the natural population is much more marked in the first generation than the effect of releasing enough sterile males

Table 1. Theoretical population trends of an insect species subjected to no treatment and to a chemical or other treatment that affects 90 percent of the population (i) by killing and (ii) by sexual sterilization.

<table><thead><tr><th>Generation</th><th>No treatment</th><th>Treatment that kills</th><th>Treatment that induces sterility</th></tr></thead><tbody><tr><td>Parent</td><td>1,000,000</td><td>1,000,000</td><td>1,000,000</td></tr><tr><td>F<sub>1</sub></td><td>5,000,000</td><td>500,000</td><td>50,000</td></tr><tr><td>F<sub>2</sub></td><td>25,000,000</td><td>250,000</td><td>2,500</td></tr><tr><td>F<sub>3</sub></td><td>125,000,000</td><td>125,000</td><td>125</td></tr></tbody></table>