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Effect of Two Bacillus thuringiensis¹ Proteins on Development of the Fall Armyworm² after Seven-Day Exposure

Portilla, Maribel, Blanco, Carlos A., Arias, Reneé, Zhu, Yu Cheng
TheSouthwestern entomologist 2020 v.45 no.2 pp. 389-404
Bacillus thuringiensis, Spodoptera frugiperda, adults, backcrossing, bacteria, bacterial proteins, bacterial toxins, bioassays, corn, crystal proteins, insect development, insecticide resistance, lethal concentration 50, pupae, transgenic plants, Puerto Rico
Field-evolved resistance of fall armyworm, Spodoptera frugiperda (J.E. Smith) (Lepidoptera: Noctuidae), to the bacterium Bacillus thuringiensis (Bt) has been reported worldwide as one of the most serious threats to the sustainability of Bt maize crop. Therefore, it is important to assess the magnitude of adult survival and the possibility of cross-resistance of fall armyworm neonates exposed to Bt proteins. In this study, bioassays were used to examine susceptibility of two field-collected Cry1Fa-resistant strains of fall armyworm from Puerto Rico (456RR, 512RR) and their crosses with a susceptible strain (Monsanto SS) (456SR-RS, 512SR-RS). LC₅₀ values varied in both Cry1Fa-resistant strains and in their backcrosses with the susceptible strain. The two RR strains were more tolerant to Cry1Fa and Cry1Ac proteins in earlier instars of development than were their crosses. Greater survival to the adult stage was obtained in the 512 RR strain and their RS-SR crosses when exposed to all concentrations of Cry1Ac and 1Fa. Survival to adult in the 456 RR was much greater when exposed to Cry1Fa than to Cry1Ac. Adults of 456 RR and their crosses survived only when exposed to the lowest concentrations of Cry1Ac. Our data confirmed great resistance to Cry1Fa and Cry1Ac in S. frugiperda larvae from Puerto Rico. However, based on the larvae that survived the 7-day diet bioassay and developed to pupae and adult maturity on regular diet, their LC₅₀ values were less for both resistant strains and their crosses.