Susceptibility of field populations of the fall armyworm (Lepidoptera: Noctuidae) from Florida and Puerto Rico to Purified Cry1f protein and corn leaf tissue containing single and pyramided Bt Genes

Larval survival of Cry1F-susceptible (FL),-resistant (PR and Cry1F-RR), and-heterozygous (FL x PR and Cry1F-RS) populations of the fall armyworm, Spodoptera frugiperda (J. E. Smith) to purified Cry1F protein and corn leaf tissue of 7 Bacillus thuringiensis (Bt) corn hybrids and 5 non-Bt corn hybrids was evaluated in the laboratory. The 7 Bt corn hybrids represent 5 Bt corn traits: Herculex®I, which expresses a single Bt protein (Cry1F), and Genuity® VT Double Pro^™, VT Triple Pro^™, SmartStax^™, and Agrisure^® Viptera ^® 3111, which contain ≥ 2 pyramided Bt genes. The original FL and PR populations were collected from corn fields in 2011 in Florida and Puerto Rico, respectively. Diet-incorporation bioassays showed that FL was susceptible to Cry1F protein with a LC50 value of 0.13-0.23 g/g, while PR was highly resistant to Cry1F protein (> 137-fold). FL was also susceptible to all 7 Bt corn hybrids with a 7-day mortality of > 95%, while PR and a backcrossed and reselected population, Cry1F-RR, were highly resistant to Cry1F corn leaf tissue. The resistance was recessive or incompletely recessive. None of the 5 populations of S. frugiperda could survive on Viptera^™ 3111, suggesting this Bt corn trait can completely overcome the resistance and thus should provide a means of managing Cry1F resistance in S. frugiperda. However, Cry1F-RR exhibited a significant cross-resistance to the leaf tissue of the other 3 pyramided Bt corn traits. The possible cross-resistance between single-gene and pyramided Bt corn products suggest that careful selection of Bt genes is essential in use of gene pyramiding strategy for resistance management.