9 « 
Psyche 
[June-September 
with the USDA applied the best known animal husbandry practices 
and tried larvicides and repellents to treat and protect livestock 
wounds directly. While these expensive measures did help to cut 
livestock losses, enough larvae survived in neglected livestock and wild 
animals to keep the infestation alive and dangerous. By 1957, the 
State of Florida and the Federal Government were ready to support 
the then new technique of eradication based on male sterilization, and 
funds were appropriated to begin the campaign against the screw- 
worm. 
Biology and Nature of Damage 
The screwworm is an obligatory feeder in the flesh of living 
mammals. Each female fly lays her eggs in a mass of about 200 on 
scratches or near exposed wounds on the animals, and the eggs take 
12-24 hours to hatch. The larvae then enter the wound and feed 
extensively on the muscle tissue. As tissue decomposition advances, 
more and more female flies are attracted to infested wound areas, and 
the maggot populations at such sites increase correspondingly. The 
larvae burrow in the tissues for five to seven days, after which they 
leave the wound and drop to the ground, where they burrow into the 
soil to pupate. The pupal stage lasts a week or more, depending upon 
the temperature. The pupa is vulnerable to low temperatures, and 
freezing soil or prolonged cold kills it. After eclosing from the 
puparium, the adult flies disperse and seek food. Flies have been 
found to disperse to distances as great as 35 miles in one week. In the 
summer, mating begins two days after eclosion, and four to six days 
later the females have been mated and have laid fertile eggs. The 
sexes reach adulthood in about equal numbers, and the females mate 
only a single time, although the males normally mate several times. 
(Some attention has been given to breeding males that will mate 
a greater number of times.) Females segregated from males in the 
laboratory to prevent fecundation oviposit as readily as do mated 
females. In summer conditions, females live two to four weeks as 
adults, and may deposit three, four or more egg masses during this 
span. 
Because oviposition is triggered only by the presence of a wound on 
a suitable host animal, and because of predation of mature larvae by 
insects, especially by ants, the number of adults produced is rarely 
high. Uvalde County, Texas, has had the heaviest infestations in the 
United States, with 100-500 flies produced per square mile per week, 
but infestations south of the border may be even heavier. 
Massive infestations of screwworm can quickly weaken and kill 
even full-grown cattle, and very small animals often succumb before 
