Characterization of field evolved resistance to transgenic Cry1Fa maize in Spodoptera frugiperda (J. E. Smith)

Transgenic Bt crops expressing Cry and Vip toxins from Bacillus thuringiensis (Bt) have been increasingly planted to manage insect pest damage on agricultural crops. The high adoption of Bt-based insecticidal technologies suggests an increase selection pressure for the evolution of resistance in insect populations. So far, nine insect species have developed field evolved resistance to Bt crops, yet the mechanisms involved in field evolved resistance are unknown. In the present study, the resistance mechanism in field evolved resistance to maize producing Cry1Fa in Spodoptera frugiperda collected in fields from Puerto Rico was characterized. High levels of resistance to Cry1Fa have been observed in S. frugiperda with recessive and autosomal mode inheritance. Binding experiments showed the reduced binding of Cry1Fa toxin to brush border membranes of resistant (456) larvae compared to susceptible (Benzon) larvae. The same binding reduction was observed for Cry1A toxins, but not for Cry1Ca toxin. This reduced binding signifies the modification of a common Cry1Fa-Cry1A toxin binding site. Comparison of receptor protein levels revealed reduced alkaline phosphatase (ALP) levels in resistant compared to susceptible larvae. This reduced expression of ALP phenotype was linked to Cry1Fa resistance in S. frugiperda.

In cross-resistance studies using bioassays, reduced susceptibility to Cry1Ab and Cry1Ac toxins was detected and no differences in susceptibility to purified Cry1Bb, Cry1Ca, and Cry1Da toxins or Xentari WG and Dipel ES pesticidal formulations compared to susceptible larvae was detected. The cross-resistance patterns observed in these bioassays are in agreement with data from competition experiments indicating an altered binding site for Cry1A and Cry1Fa toxins in 456 larvae. The only difference detected in fitness cost studies was a significant increase in the larval developmental time in resistant insects, which could result in emergence asynchrony between susceptible and resistant moths. The lack of fitness costs was also supported by stable resistance after 12 generations of rearing in the absence of a selective (transgenic maize) agent. This work is the first study on field level resistance to a Bt crop. Results from this study will help to understand resistance mechanisms responsible for field-level resistance and formulate improved resistance management practices.