412 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1958 
gens from X-rays. Sterility in the female resulted at a dosage of 5,000 
roentgens. 
Cage tests, in which irradiated males were in direct competition 
with normal fertile males, showed that the mating habits of the ster- 
ile males were essentially the same as for normal males. There was 
no evidence of discrimination on the part of females. However, fe- 
males that had mated with sterile males would not mate subsequently 
with fertile males. When sterile and fertile males in ratios ranging 
from 1:1 to 9:1 were present in a cage with normal virgin females, 
the ratio of sterile to fertile egg masses produced by the females was 
essentially the same as the ratio of sterile to fertile males. The pres- 
ence of sterile females in the cages did not seem to alter the results. 
Thus numerous laboratory tests by these investigators, using X-ray 
equipment furnished by the Army at Fort Sam Houston, San Antonio, 
Tex., and a cobalt-60 irradiation unit provided by the Oak Ridge Na- 
tional Laboratory, indicated promise for screwworm population con- 
trol through the use of sterile males. 
The next stage in the investigations involved field experiments. 
Would the method reduce the biotic potential of natural screwworm 
populations? The answer to this question was not easily obtained. 
How many wild flies were emerging in an area during various seasons 
of the year? How far and how soon after release would the released 
insects migrate, and would they disperse uniformly among the natural 
population? What ratio of sterile to fertile insects would be re- 
quired to start a downward trend in the natural population ? 
These were some of the many unknowns. Because of strong migra- 
tion tendencies of the flies, it proved futile to attempt to obtain reliable 
information in small to moderate-size areas surrounded by heavily 
infested territory. It was concluded that small isolated islands offered 
the best opportunities for valid tests with the research funds available. 
Consequently, one of the islands off the west coast of Florida, called 
Sanibel, having an area of 15 square miles, was selected for tests. Fer- 
tile flies heavily charged with P*? were first released on the island at 
the rate of 100 females and 100 males per square mile to get some idea 
of the natural population. Eggs deposited by the released flies on 
host animals could be distinguished from those deposited by natural 
wild flies because of the radioactivity. The ratio of radioactive to 
nonradioactive egg masses obtained suggested that the release of 100 
reared males per week provided more released than wild flies in the 
population. A subsequent program in which sterile males were re- 
leased at the rate of 100 per square mile per week vesulted in virtual 
elimination of a natural population within a few months, but eradica- 
tion could not be demonstrated because a few fertile flies were migrat- 
ing onto the island from the mainland or nearby infested islands. 
