Fitness costs of Cry1F resistance in fall armyworm, Spodoptera frugiperda, collected from Florida

Resistance development in target pest populations is a great threat to the sustainability of transgenic corn expressing Bacillus thuringiensis (Bt) proteins. Fall armyworm (FAW), Spodoptera frugiperda, is a major target of Bt corn in both North and South America. Field resistance of FAW has been documented in Puerto Rico and the southeast coast area of the U.S. Mainland. The objective of this study was to assess the fitness costs of Cry1F resistance in two populations of FAW collected from Puerto Rico (RR-PR) and Florida (RR-FL). To accomplish the objective, insect survival, growth and development of 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 Cry1F resistance in both resistant FAW populations was associated with considerable fitness costs, especially for the Florida population. Compared to Bt-SS, the Puerto Rico resistant population showed an average of 61.1% reduced larval mass at 10 day feeding non-Bt corn leaf tissue, 20.5% higher neonate-to-adult mortality, and 3.7 days delay in neonate-to-adult developmental time and those value for RR-FL were 66.9%, 31.7%, and 4.4 days, respectively.  Knowledge generated from this study should be useful in understanding the resistance mechanisms, and developing effective monitoring and management strategies for the sustainable use of the Bt corn technology.