8 



MISC. PUB. 1178, U.S. DEPT. OF AGRICULTURE 



(4) The number of generations per year may 

 profoundly affect the likelihood of complete 

 suppression. When matings between partially 

 sterile insects produce no offspring, the more 

 generations per year (hence the more releases 

 made), the greater is the likelihood for com- 

 plete suppression. Generally this is also true 

 when matings between partially sterile insects 

 yield a number of offspring proportionate to 

 their levels of sterility. However, when the re- 

 lease strain has one conditional lethal trait 

 inherited as two autosomal genes (four native 

 alleles required for survival) or three autosomal 

 genes (five native alleles required for sur- 

 vival), then complete suppression would be 

 achieved if the trait was expressed in the F3 

 generation but not if the expression was de- 

 layed to the F4 when 396,000 insects are 

 released per generation that are 90 percent 

 sterile and when the rate of increase is tenfold 

 (case 7). 



(5) When one of the traits is determined by a 

 dominant gene on the sex chromosomes, the 



number of male survivors may differ widely 

 from the number of female survivors. Doubtless 

 the low probability of females finding a mate 

 when the density of males is low would have an 

 important role in complete suppression. 



If matings between partially sterile insects 

 yield offspring partially or fully proportionate 

 to the level of sterility, the population may be- 

 come large and threaten damage to the econom- 

 ic host. This consideration, along with adverse 

 effects of sterilization procedures and economic 

 factors, should be taken into account in deciding 

 whether the population to be completely sup- 

 pressed should be held in check by partial ste- 

 rility or by conventional means. In lepidopter- 

 ous species partial sterility is inherited (8-10). 

 In such species the use of partial sterility would 

 be much more effective than indicated in our 

 calculations in facilitating the introduction of 

 genes for conditional lethal traits. Similar con- 

 siderations would apply if the release insects 

 differed from the native insects by one or more 

 reciprocal chromosomal translocations. 



Suppression of Population Held Static by Conventional Means 

 With Conditional Lethal Traits 



By suitable means such as application of 

 insecticides, a population of insects may be pre- 

 vented from increasing. For example, a popula- 

 tion that increases fivefold between genera- 

 tions may be held static by destroying 80 

 percent of each generation. We calculated 

 the effects of overflooding only the parental 

 (spring) generation with a strain homozygous 

 for one, two, or three conditional lethal traits. 

 The population is assumed to remain static in 

 all subsequent generations. We calculated the 

 relative frequencies of each genotype in the Fi, 

 F 2 , F3, and F 4 generations by using the general 

 methods already outlined. However, the number 

 of insects with each genotype was divided by 

 the total number of insects. For purposes of 

 computation, we assumed that each mated fe- 

 male produced one pair of offspring and the 

 insects were fully fertile. These values are 

 shown in tables 29-34. 



To understand the significance of these fig- 

 ures, consider a field of 1,000 acres with 10 

 insects per acre, or a total of 10,000 insects. This 



low population for a number of economic pests 

 such as the boll weevil (Anthonomus grandis 

 Boheman), pink bollworm (Pectinophora gos- 

 sypiella (Saunders)), and perhaps others is 

 feasible and is held static, or at least is not per- 

 mitted to reach economic density levels, through- 

 out the season by using insecticide applications 

 or other available control measures. We can 

 calculate the effect on this population when the 

 conditional lethal traits are expressed. The num- 

 ber of survivors is the product of the number of 

 insects times the relative frequency of genotypes 

 capable of surviving. In this example we obtain 

 the frequency of viable genotypes from tables 

 29-34 and multiply this frequency by 10,000. 

 We calculated cases la-15b in table 41 in order 

 to illustrate the use of tables 29-34 and to dem- 

 onstrate the potential of conditional lethal 

 traits for population suppression. The method 

 used to derive table 41 is shown in table 15. 



Case 1 in table 41 was calculated by using 

 table 29. With one gene the genotypic frequen- 

 cies do not change from generation to genera- 



