| Summary: | The fall armyworm (Spodoptera frugiperda (J. E. Smith)) is a major pest targeted by transgenic corn expressing Bacillus thuringiensis (Bt) proteins in both North and South America. The objective of this study was to characterize the inheritance and fitness costs of the Cry1F resistance in two fall armyworm populations collected from Puerto Rico (RR-PR) and Florida (RR-FL). To determine the inheritance of the resistance, besides RR-PR, RR-FL, and a Cry1F-susceptible population (Bt-SS), 14 other populations were developed by reciprocal crosses, F1 by F1 crosses, backcrosses, and crosses between RR-PR and RR-FL. Diet-incorporated bioassays were conducted to determine the Cry1F susceptibility for all 17 populations. To assess the fitness costs of the resistance, seven insect populations were assayed on a non-toxic diet as well as on a combined rearing of non-Bt corn leaf tissue and non-Bt diet. The seven populations were RR-PR, RR-FL, Bt-SS, and four F1 populations that were developed from the reciprocal crosses between Bt-SS and the two resistant populations. Biological parameters measured in the fitness tests were neonate-to-adult survivorship, neonate-to-adult development time, 10-day larval mass on non-Bt corn leaf tissue, pupal mass, and sex ratios. The results showed that there might be a different genetic basis for the Cry1F resistance between the Puerto Rico and Florida populations. The Cry1F resistance in RR-PR was likely inherited in >1 recessive or incompletely recessive genes and the genes associated the resistance were sex-linked to the males of the insect. In contrast, the resistance in RR-FL was dominant and more likely controlled by autosomal genes. Cry1F resistance in both resistant populations was associated with considerable fitness costs, especially for the Florida population. The fitness costs in the Cry1F-resistant fall armyworm were revealed in reduced growth, increased mortality, and delayed development. Data generated from this study should be valuable in understanding the mechanisms of Cry1F resistance in fall armyworm and developing effective strategies for resistance management.
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