One method involves rearing, sterilization, and release of insects into 
the natural population so the sterile insects will compete in mating with 
normal ones and thus lower the reproduction rate. The other method, still 
in early stages of investigation, is to treat and sterilize insects in the 
natural population to reduce reproduction, rather than treat to kill. The 
latter plan saves the expense of rearing the pests to be sterilized. In either 
case, the sterile insects mate with fertile ones and the eggs that result are 
sterile. 
By the former plan, enough sterile insects are released to outnumber 
the natural insect population and lower its reproductive capacity sufficiently 
to cause a downward population trend. If such releases of sterile insects 
are continued at the same level for several generations, the natural popula- 
tion will decline more rapidly with each subsequent generation since the 
ratio of sterile to fertile insects will become progressively higher each 
generation. This feature of progressive increase in efficiencyas the natural 
population declines is unique. Allother systems of population control achieve 
about the same degree, or perhaps a lesser degree, of effect as the popula- 
tion declines. 
A satisfactory way of producing sterilityinthe insect is a basic require- 
ment in this type of insect control or eradication. Three ways of producing 
sexual sterility in insects have been used or proposed: (1) Exposure to 
gamma radiation, as employed for sterilizing screwworm flies, (2) use of 
chemicals which sterilize insects, and (3) application of certain hybridiza- 
tion techniques. Research on hybridization as a means of producing sterile 
insects for release has thus far been greatly limited. The method could be 
of great value in meeting a number of our key insect problems. 
Release of Sterile Insects for Mating 
Early experiments in sterilizing insects employed X-rays, but the first 
attempts at sterilizing insects as a control measure employed radiation 
with gamma rays from cobalt-60. The eradication of the screwworm from 
the island of Curacao off the Venezuelan coast used the radiation-sterility 
technique developed by USDA entomologists, Later screwworms were 
eradicated in southeastern U.S. through mass releases of irradiated 
screwworm flies. A similar program is now under way to control the pest 
in the Southwest. Information on the use ofthe sterile insect release method 
against the screwworm is given in ARS 22-79, ''Status of the Screwworm in 
the United States."' 
The success of the male-sterility technique in the eradication of the 
screwworm in the Southeast created much interest and speculation among 
scientists as to the potential usefulness of the method against other insects. 
It should be emphasized that the screwworm is among the most favorable 
species for the practical application of the sterile insect release method 
alone because of two features: (1) The natural population of the insect is 
relatively low, even during periods of high destructiveness and (2) a good 
economical method of mass rearing the insect was available. 
The sterility method alone will not be an economically feasible control 
method for many insects that have reachedtheir normal population densities. 
In all probability the method will have its greatest value as a much-needed 
adjunct to other methods of control for well-established insect populations, 
and for eliminating incipient infestations before the natural population has 
reached a high level. 
