Characterizing the Relationship Between Western Corn Rootworm (Coleoptera: Chrysomelidae) Larval Survival on Cry3Bb1-Expressing Corn and Larval Development Metrics

Effects of SmartStax® and SmartStax® PRO maize on western corn rootworm (Diabrotica virgifera virgifera LeConte) larval feeding injury and adult life history parameters

Field-evolved resistance of the western corn rootworm (WCR), Diabrotica virgifera virgifera LeConte, to Bacillus thuringiensis Berliner (Bt) proteins Cry3Bb1 and Cry34/35Ab1 (now classified as Gpp34Ab1/Tpp35Ab1) expressed in the pyramid SmartStax® has been documented in areas of the United States (U.S.) Corn Belt. SmartStax® PRO is a recently registered rootworm-active pyramid containing the same Bt proteins expressed in SmartStax® plus DvSnf7 dsRNA. Little to no published data is available comparing efficacy of the technologies or potential effects of dietary exposure on adult WCR fitness. Therefore, experiments were conducted to compare effects of adult WCR dietary exposure to SmartStax® and SmartStax® PRO on life history parameters and efficacy of the technologies in the field with both Bt-susceptible and Bt-resistant WCR populations. WCR life history parameters evaluated included adult longevity, head capsule width, egg production, and egg viability. Results of small-plot field trials indicated that both technologies provided a high level of root protection when a Bt-susceptible WCR population was present. Root protection was reduced on SmartStax® but maintained on SmartStax® PRO when WCR Bt resistance occurred. Lifetime egg production was the key life history parameter that was significantly reduced when either Bt-susceptible or Bt-resistant adult WCR were fed SmartStax® or SmartStax® PRO diet. A potential fitness advantage was apparent as egg production was significantly higher in the Bt-resistant than Bt-susceptible population. The similar response by the Bt-susceptible WCR population to SmartStax® and SmartStax® PRO indicates that results were caused by sublethal dietary exposure to Bt proteins. Adult size (males < females) and egg viability (high: >95%) were not significantly different among treatments but longevity results were inconsistent between years. Collectively, the field efficacy and life history parameter data expand existing knowledge of SmartStax® and SmartStax® PRO technologies, which will inform practical WCR resistance management programs.

Reduced susceptibility of western corn rootworm (Diabrotica virgifera virgifera LeConte) populations to Cry34/35Ab1-expressing maize in northeast Nebraska

The western corn rootworm (WCR; Diabrotica virgifera virgifera LeConte) is a significant pest of maize (Zea mays L.) across the United States Corn Belt. Transgenic maize hybrids expressing insecticidal proteins derived from Bacillus thuringiensis (Bt) have been used to manage WCR since 2003. Widespread resistance to Cry3Bb1 (and associated cross-resistance to mCry3A and eCry3.1Ab) has placed increased selection pressure on Cry34/35Ab1 in single-protein and pyramided transgenic maize hybrids. Data on the susceptibility of Nebraska WCR populations to Cry34/35Ab1 has not been published since 2015 and plant-based bioassays conducted in 2017-2018 confirmed resistance to Cry3Bb1 + Cry34/35Ab1 maize, suggesting resistance to Cry34/35Ab1 has evolved in the Nebraska landscape. Therefore, plant-based bioassays were conducted on F₁ progeny of WCR populations collected from northeast Nebraska in 2018 and 2019. Larval survival and development were used to classify resistance to Cry34/35Ab1 in each WCR population. Bioassays confirmed incomplete resistance to Cry34/35Ab1 maize in 21 of 30 WCR populations; 9 of 30 WCR populations remained susceptible to Cry34/35Ab1. Collectively, results indicate that northeast Nebraska WCR populations were in the initial stages of resistance evolution to Cry34/35Ab1 during 2018-2019. Appropriate resistance management strategies are needed to mitigate resistance and preserve efficacy of rootworm-active products containing Cry34/35Ab1.

Evidence of western corn rootworm (Diabrotica virgifera virgifera LeConte) field-evolved resistance to Cry3Bb1 + Cry34/35Ab1 maize in Nebraska

BACKGROUND

Western corn rootworm (WCR; Diabrotica virgifera virgifera) field-evolved resistance to transgenic maize expressing the Cry3Bb1 protein derived from Bacillus thuringiensis (Bt) has been confirmed across the United States Corn Belt. Although use of pyramided hybrids expressing Cry3Bb1 + Cry34/35Ab1 has increased in recent years to mitigate existing WCR Bt resistance, susceptibility of Nebraska WCR populations to this rootworm-Bt pyramid has not been assessed. Plant-based bioassays were used to characterize the susceptibility of WCR populations to Cry3Bb1 and Cry3Bb1 + Cry34/35Ab1 maize. Populations were collected from areas of northeastern Nebraska with a history of planting Bt maize that expressed Cry3Bb1 and Cry34/35Ab1.

RESULTS

Significant differences in mean corrected survival among populations within Bt hybrids indicated a mosaic of WCR susceptibility to Cry3Bb1 + Cry34/35Ab1 and Cry3Bb1 maize occurred in the landscape. All field populations exhibited some level of resistance to one or both Bt hybrids when compared to susceptible laboratory control populations in bioassays. Most WCR populations exhibited incomplete resistance to Cry3Bb1 + Cry34/35Ab1 maize (92%) and complete resistance to Cry3Bb1 maize (79%).

CONCLUSION

The present study confirms the first cases of field-evolved resistance to Cry3Bb1 + Cry34/35Ab1 maize in Nebraska and documents a landscape-wide WCR Cry3Bb1 resistance pattern in areas characterized by long-term continuous maize production and associated planting of Cry3Bb1 hybrids. Use of a multi-tactic integrated pest management approach is needed in areas of continuous maize production to slow or mitigate resistance evolution to Bt maize. © 2021 Society of Chemical Industry.