POTENTIAL FOR GENETIC SUPPRESSION OF INSECT POPULATIONS 



tion. If 500 insects were released per acre to 

 give a release ratio of 50:1 and if the total 

 population (native plus released) was held static 

 thereafter, four insects would be capable of 

 surviving when the conditional lethal trait 

 would be expressed in the F 2 or subsequent 

 generations. Whether this level of survival would 

 result in eradication would depend on the species 

 concerned, but in all likelihood so few insects on 

 1,000 acres would represent eradication. In ac- 

 tual practice a judgment would have to be made 

 concerning the minimum density required for 

 survival. After this judgment is made, the re- 

 lease ratio required for complete suppression 

 can be obtained from table 29. 



Examination of cases 2-6 in table 41 shows 

 that when a conditional lethal trait is deter- 

 mined by more than one gene, the trait may be 

 expressed in a progressively higher proportion 

 of individuals from generation to generation. 

 Cases 7-15 show that when the release strain 

 possesses more than one conditional lethal trait, 

 the efficiency of suppression is progressively 

 augmented in succeeding generations. If the 

 conditional lethal traits were expressed in the 

 last generation of the season, release ratios 

 ranging from 10:1 to 50:1 would be adequate to 

 decimate most populations with four genera- 

 tions per year. This would require releases 

 ranging from 100 to 500 insects per acre when 

 the natural population consists of 10 insects per 

 acre. Higher ratios would be required if the 

 species had fewer than four generations per 

 year. In most instances these ratios would have 

 to be two to five times higher to achieve the same 

 suppression in the F 2 as in the F 4 . 



If a conditional lethal trait is under the con- 

 trol of more than one gene in a species with two 

 generations per year, conditional lethal genes 

 would be about one to 10 times more efficient 

 than dominant lethal mutations in suppressing 

 insect populations. If the species has three gen- 

 erations per year, conditional lethal genes 

 would be up to 100 times more efficient than 

 dominant lethal mutations. Since the release of 

 insects carrying dominant lethal mutations is 

 considered feasible for eradicating a number of 

 economic insects existing at low population 

 levels, the practical significance of the condi- 



tional lethal genes, if such can be found and 

 used, is readily apparent. 



We calculated the effects of overflooding a 

 native population once every generation with a 

 strain homozygous for one, two, or three con- 

 ditional lethal traits. We assumed that appro- 

 priate measures would be taken to hold the 

 population constant in all generations. We cal- 

 culated the relative frequencies of each geno- 

 type in the Fi, F 2 , F 3 , and F.j generations, and 

 these values are shown in tables 35-40. 



To understand the significance of these fig- 

 ures, we will again consider a field of 1,000 

 acres with 10 insects per acre, or a total of 

 10,000 insects. This population is held static 

 throughout the season with insecticide applica- 

 tions or other suitable means. We calculated the 

 effect on this population when the conditional 

 lethal traits are expressed. The number of sur- 

 vivors is 10,000 times the relative frequency of 

 the genotypes capable of surviving. The latter 

 data were obtained from tables 35-40. To dem- 

 onstrate the potential of conditional lethal 

 traits for population suppression and to illus- 

 trate the use of tables 35-40, we then calculated 

 cases 16a-30b, as shown in table 41. 



Case 16a in table 41 was calculated by using 

 table 35. If 100 insects are released per acre 

 during each generation (release ratios of 10:1), 

 the frequency of viable genotypes would rapidly 

 decline from 83 in the Fi to in the F 4 . Com- 

 parison of case 16a with cases 17-21 shows that 

 the efficiency of suppression may be significantly 

 greater when a conditional lethal trait is under 

 the control of several genes than when the trait 

 is determined by a single fully dominant factor. 

 Cases 22-30 show that when the release strain 

 possesses more than one conditional lethal trait, 

 the release ratio required to destroy the popula- 

 tion is much smaller than when the release 

 strain possesses only one conditional lethal 

 trait. 



Release ratios ranging from 1:1 to 10:1 would 

 be adequate to decimate most populations with 

 four generations per year if the conditional 

 lethal traits are expressed in the last genera- 

 tion. In many instances such low ratios would 

 be adequate for species with two generations 

 per year but not for those with one. To sup- 



