Ecological Factors 



The behavior of the insect with respect to dispersion, as previously pointed out, 

 could play a vital role in determining the effectiveness and practicability of using sterile 

 insects. Although emphasis in the discussion has been placed on average population 

 density per unit area and total population density in the entire area to be treated, the need 

 for information on the distribution of insects within the area under control is of vital 

 importance. It is known, for example, that the number of insects within the environment 

 varies greatly for such insects as the screw- worm fly and tsetse flies. In the current 

 screw-worm program, the rate of liberation may exceed 1,000 flies per square mile in 

 known heavily infested environments, and as few as 200 per square mile in areas of low 

 density. 



The chances for effective utilization of sterile insects probably are best for those 

 species that tend to disperse to a considerable extent. Insects that have a, greatly 

 restricted range of movement may pose problems of achieving adequate distribution and 

 placement of sterile insects in the required number to satisfactorily compete with the 

 number of normal insects in different parts of the total area. For example, in attempting 

 to control or eradicate scale insects by releasing sterile insects, we could anticipate 

 difficulty in achieving adequate distribution of the released insects even on a single tree. 

 In order to achieve complete population control of an insect, it is important that all 

 segments of the population are adequately overflooded with sterile insects. 



In experiments to test the ability of sterile insects to achieve complete population 

 control, it is essential that the population be completely isolated from the insect in 

 surrounding areas. If the test area is within flight range of adult females that may have 

 mated with fertile males outside of the release area, complete elimination cannot be 

 demonstrated. Moreover, the released insects that move out of the area will reduce the 

 impact on the natural population within the area. Failure to demonstrate eradication of 

 the screw-worm on the Island of Sanibel, 2 miles off the west coast of Florida, even 

 though the density of the natural population declined rapidly after sterile-male releases 

 were started, led to the decision to conduct further pilot eradication tests on the Island 

 of Curapao, to which, it was believed, screw-worm flies could not migrate from the 

 South American mainland — a distance of about 50 miles. 



Tropical fruit flies are known to be long-range migrants. Fruit fly investigations 

 by the Hawaii fruit fly laboratory on the Island of Rota, which is about 35 miles from 

 Guam, made it possible with available research resources to conduct valid eradication 

 tests involving both the melon fly and the oriental fruit fly, but even here there is 

 evidence that some flies will succeed in crossing the 35-mile water barrier. 



On the basis of current knowledge of the flight range of the screw- worm fly, a 

 valid eradication experiment could not have been conducted in an area of less than 

 40,000 square miles, and it probably would have required an area of 100,000 square miles, 



- 23 - 



