the natural population. For tsetse flies, because of their low reproductive potential 

 under natural conditions, as well as under laboratory conditions, an initial matching 

 population of sterile males may be adequate to start a downward trend in the population. 



In model 11, the initial ratio was established at 3:1 to provide a safety factor 

 that in all probability would be necessary. Several students of tsetse fly population 

 dynamics have pointed out, however, that these insects are generally highly stable in 

 terms of population densities so that there is relatively little fluctuation in average 

 abundance from year to year. For an insect having such a low net increase potential, 

 irrespective of population density, a matching population of sterile males may be equally 

 as drastic in affecting reproduction as a 10 to 1 ratio would be. for such insects as the 

 screw- worm fly, fruit flies, and other species. If this should prove to be a valid 

 assumption, then we have, in relation to other insects, a factor of 10 in favor of the 

 method for tsetse flies, which would offset to a great extent the inherently low reproduc- 

 tive capacity of these insects. 



Another factor that could be of major significance in the mass -production of 

 tsetse flies would be the feasibility of utilizing all of the reared females for reproduction 

 purposes. The cost of manually separating males from females probably would not 

 exceed 0. 1 cent per male. A cost factor of this magnitude for each individual insect is 

 not now regarded practical in connection with the sterility program for such insects as 

 the screw- worm and tropical fruit flies. For tsetse flies, however, this cost factor 

 would be minor in relation to the total production cost that may be practical. Moreover, 

 it is possible, if not probable, that the release of males only would result in greater 

 efficiency of the sterile insects than the release of both sexes. It may also be desirable, 

 or even necessary, to release males only in certain situations so as to minimize the 

 extra hazard that large releases of sterile flies may create. 



Thus, in a mass tsetse fly rearing program, it is proposed that all of the females 

 produced be used for reproduction, and most of the males harvested be used for releases. 

 Since the males of most insects have the capacity to inseminate many females, it seems 

 probable that a low male-to-female ratio in the brood colony would be satisfactory. If 

 each female only averaged slightly more than two progeny during its life, the number of 

 males required for release, and the number of females required to maintain adequate 

 production, automatically would become properly balanced, provided a low ratio of 

 males would insure fertilization of the females. Therefore, the objective in research 

 should be to develop the most economical way to maintain an adult colony on essentially 

 a self-sustaining basis, with just enough surplus males to insure adequate fertilization 

 of the females. The writer has communicated with Mr. J. Ford of the Department of 

 Veterinary Services, Salisbury, Southern Rhodesia, to obtain his views regarding the 

 feasibility of rearing tsetse flies in large outdoor cages or in a natural high-population 

 environment suitably modified for maximum fly production. Research along these lines 

 is underway. 



Production costs for tsetse flies at a level of about 5 cents per male, or $50, 000 

 per million, should make the use of sterile tsetse flies well within the range of practica- 

 bility under a wide range of circumstances. 



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