TROPICAL FRUIT-FLY MENACE — CHRISTENSON 447 



airplanes distributing the sterile insects flew several million miles. 

 An even larger screw-worm fly factory has been constructed in Texas. 

 There is hope that this fly can be eliminated from all of the United 

 States and eventually even from the entire North American Continent. 



Soon after the concept of radiation sterilization to control insects 

 received publicity a decade or so ago, work to determine its possible 

 application to tropical fruit-fly problems was initiated in Hawaii by 

 the writer and L. F. Steiner. At first it was believed that single- 

 mating habits were a prerequisite for successful application of 

 the method. Tropical fruit flies mate more than once. The first cage 

 experiments with the oriental fruit fly revealed a strong effect on 

 fertility of eggs when normal flies were overflooded with sterile indi- 

 viduals. Pilot tests on the Mediterranean fruit fly in Hawaii and 

 the Mexican fi-uit fly in Mexico, the latter tests conducted by R. H. 

 Rhode, soon demonstrated that overflooding of semi-isolated tropical 

 fruit-fly populations with sterile flies will inhibit population growth. 

 In an isolated-island oriental fruit fly test on Rota in the Western 

 Pacific about 40 miles north of Guam, sterile flies in small boxes were 

 dropped from a U.S. Navy flying boat, and others were allowed to 

 emerge in special emergence trays suspended from trees. Despite sus- 

 tained releases, sometimes as many as 10 million sterile flies per week, 

 the desired amount of overflooding of the oriental fruit fly population 

 could not be achieved because of the abundance of wild hosts, the 

 strong reproductive potential of the fruit flies, losses during distribu- 

 tion, the effects of predation after release, and natural mortality before 

 the flies attained ability to mate. A definite seasonal upturn in infesta- 

 tion in preferred hosts in the spring and early summer of 1962 was 

 the final signal for termination of the experiment. 



The results of the radiation sterilization test on Rota, the first 

 large-scale test of its kind on an insect that breeds copiously on 

 abundant wild hosts, did not prove that this technique will not work. 

 The test taught us that for some tropical fruit flies it may be necessary 

 to reduce wild populations by other means so as to bring them within 

 reach of the sterile-fly release method. A new experiment on the 

 melon fly is now in progress on Rota, where insecticides are being 

 used to reduce the wild flies to low numbers before the sterile flies are 

 introduced. 



Other research on the radiation sterilization of tropical fruit flies 

 is underway in Central America, Egypt, and Greece. Within a few 

 years we should have a good answer to the question of whether this 

 technique is effective against these insects. A method such as this, 

 which can eliminate an insect species from large areas, or even from 

 entire continents, needs to succeed only occasionally, as with the screw- 

 worm fly, to merit recognition as one of man's greatest 

 accomplishments. 



&72-174— 63 33 



