Assessing fitness costs of the resistance of Spodoptera frugiperda (Lepidoptera: Noctuidae) to pyramided Cry1 and Cry2 insecticidal proteins on different host plants

Fall armyworm (FAW), Spodoptera frugiperda (Smith), is one of the major pests targeted by transgenic crops expressing insecticidal proteins from Bacillus thuringiensis (Bt) Berliner. However, FAW presents a high capacity to develop resistance to Bt protein-expressing crop lines, as reported in Brazil, Argentina, Puerto Rico and the southeastern U.S. Here, FAW genotypes resistant to pyramided maize events expressing Cry1F/Cry1A.105/Cry2Ab2 (P-R genotype) and Cry1A.105/Cry2Ab2 (Y-R genotype) from Brazil were used to investigate the interactions between non-Bt hosts (non-Bt maize, non-Bt cotton, millet and sorghum) and fitness costs. We also tested a FAW genotype susceptible to Bt maize and F₁ hybrids of the resistant and susceptible genotypes (heterozygotes). Recessive fitness costs (i.e., costs affecting the resistant insects) were observed for pupal and neonate to adult survival of the P-R genotype on non-Bt cotton; larval developmental time of the P-R genotype on millet and sorghum; larval and neonate-to-adult developmental time of the Y-R genotype on non-Bt cotton and sorghum; the fecundity of the Y-R genotype on non-Bt cotton; and mean generation time of both resistant genotypes. However, on non-Bt cotton and non-Bt maize, the P-R genotype had a higher fitness (i.e., fitness benefits), displaying greater fecundity and rates of population increases than the Sus genotype. Non-recessive fitness costs (i.e., costs affecting heterozygotes) were found for fecundity and population increases on millet and sorghum. These findings suggest that, regardless of the disadvantages of the resistant genotypes in some hosts, the resistance of FAW to Cry1 and Cry2 Bt proteins is not linked with substantial fitness costs, and may persist in field conditions once present.

Performance of Seed Treatments Applied on Bt and Non-Bt Maize Against Fall Armyworm (Lepidoptera: Noctuidae)

Fall armyworm (FAW), Spodoptera frugiperda (J. E. Smith), is the main pest of maize in Brazil, attacking plants from emergence to reproductive stages. Here, we conducted studies to evaluate the efficacy of two seed treatments (chlorantraniliprole alone and imidacloprid combined with thiodicarb) on Bt and non-Bt maize in laboratory bioassays with distinct FAW strains that are susceptible, selected for resistance to Bt-maize single (Cry1F) or pyramided (Cry1A.105 + Cry2Ab2) events and F1 hybrids of the selected and susceptible strains (heterozygotes), and in the field against a natural infestation. In the laboratory, leaf-discs from seed treated Bt-maize plants at 7 d after emergence (DAE) increased the mortality of FAW resistant, heterozygote, and susceptible strains up to 24.8%, when compared with the respective maize grown without a seed treatment. In the field against natural infestations of FAW, Bt maize with a seed treatment had ~30% less FAW damage than non-Bt maize with the same seed treatment at 7 and 14 DAE. No differences in FAW damage was observed between Bt and non-Bt maize grown with and without a seed treatment at 21 DAE. Maize seeds treated with chlorantraniliprole alone or imidacloprid and thiodicarb combined presented limited protection against early infestations of FAW strains under laboratory and field studies.

Laboratory and field survival of Spodoptera frugiperda (Lepidoptera: Noctuidae) on Bt and non-Bt maize and its susceptibility to insecticides

BACKGROUND

Field-evolved resistance of fall armyworm (FAW), Spodoptera frugiperda (Smith), has been reported to Bt maize technologies in Brazil. The control failures of FAW by Bt maize increased the use of insecticides for their control. However, no information is available on the interaction between resistant FAW and their response to insecticides. Here, we evaluated the survival of FAW strains on Bt and non-Bt maize in laboratory and field conditions, and their susceptibility to insecticides.

RESULTS

In the laboratory, resistant FAW larvae reared on Bt and non-Bt maize showed a similar susceptibility to spinetoram (LC₅₀  = 0.16-0.18 µg a.i. cm^-2 ) and chlorfenapyr (LC₅₀  = 0.17-0.20 µg a.i. cm^-2 ). However, their susceptibility was lower than that of the susceptible strain reared on non-Bt maize: LC₅₀  = 0.05 (spinetoram) and 0.08 (chlorfenapyr) µg a.i. cm^-2 . In contrast, heterozygous strains had similar susceptibility to the susceptible strain. In field trials, no differences in FAW survival were detected between strains when the commercial dose of two insecticides was applied in Bt and non-Bt maize.

CONCLUSION

FAW strains surviving on Bt and non-Bt maize, at the same development stage, have similar susceptibility to insecticides. The integrated pest management practices and insect resistance management importance of these results are discussed. © 2019 Society of Chemical Industry.