410 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1958 
posited in a compact mass on wounds of these animals. The larvae 
hatch in less than 24 hours. They begin feeding on the tissues and 
reach maturity in about 5 days (pl. 2, fig.1). The larvae then leave 
the wound, drop to the ground, and enter the soil for pupation (pl. 2, 
fig. 2), emerging as adults in about 8 days during warm weather. 
Larvae cause severe destruction of tissues, producing a straw-colored 
and often a bloody discharge from the wound. An infestation attracts 
more screwworm flies, resulting in multiple infestations consisting of 
several hundred to several thousand larvae of all sizes (pl. 6). Feed- 
ing by the large number of larvae will kill the host unless the infesta- 
tion is destroyed and the wound is treated with protective remedies. 
Several screwworm smears and other preparations are in use to treat 
or prevent infestations. These remedies and good livestock-manage- 
ment practices which reduce the number of susceptible wounds mini- 
mize losses. In spite of these control efforts, the insect causes losses 
in the United States which are estimated to vary from $20 million to 
$40 million annually. An additional loss of game animals, particu- 
larly deer, cannot be measured in terms of dollars. 
The screwworm is largely a subtropical insect. Cold winter weather 
pushes the infestations into warmer areas. In the Southwest the 
winter survival area in south Texas usually does not exceed 30,000 to 
40,000 square miles. In the Southeast, during normal winters, the 
survival line is about as far north as Gainesville, Fla. In the spring 
there is an upsurge of screwworm flies, and the pest spreads rapidly. 
The rate of natural spread northward may be as much as 25 to 35 
miles per week. In addition to natural spread the insect often be- 
comes established early in the season outside of its normal range 
through shipment of infested animals and increases to damaging 
levels before frost. 
DEVELOPMENT OF CONCEPTS AND RESEARCH LEADING TO THE 
STERILE-MALE TECHNIQUE FOR CONTROL 
A. W. Lindquist (unpublished reports, 1935) obtained data which 
showed that the total number of screwworm flies in the environment 
is low in comparison with most insects. This significant feature in 
screwworm population dynamics has an important bearing on the 
use of sterile males for their control. Melvin and Bushland (1936) 
developed a method of rearing the insect on an artificial medium in 
the laboratory, which provided a vital link in the chain of events 
leading to the sterile-male technique for control. 
The possibilities of screwworm control by utilizing some natural 
or induced mutant characteristic that would not be detrimental to 
the insect when reared in the laboratory but would place it at a 
disadvantage in nature were first considered by the writer in 1937. 
Of the possible genetic approaches, the one that seemed to warrant 
