Abstract
Background and MethodologyThere is a continuing need to express new insect control compounds in transgenic maize against western corn rootworm, Diabrotica virgifera virgifera (LeConte) (WCR). In this study three experiments were conducted to determine cross-resistance between the new insecticidal DvSnf7 dsRNA, and Bacillus thuringiensis (Bt) Cry3Bb1; used to control WCR since 2003, with field-evolved resistance being reported. Laboratory susceptible and Cry3Bb1-resistant WCR were evaluated against DvSnf7 dsRNA in larval diet-incorporation bioassays. Additionally, the susceptibility of seven field and one field-derived WCR populations to DvSnf7 (and Cry3Bb1) was assessed in larval diet-overlay bioassays. Finally, beetle emergence of laboratory susceptible and Cry3Bb1-resistant WCR was evaluated with maize plants in the greenhouse expressing Cry3Bb1, Cry34Ab1/Cry35Ab1, or DvSnf7 dsRNA singly, or in combination.Principal Findings and ConclusionsThe Cry3Bb1-resistant colony had slight but significantly (2.7-fold; P<0.05) decreased susceptibility to DvSnf7 compared to the susceptible colony, but when repeated using a field-derived WCR population selected for reduced Cry3Bb1 susceptibility, there was no significant difference (P<0.05) in DvSnf7 susceptibility compared to that same susceptible population. Additionally, this 2.7-fold difference in susceptibility falls within the range of DvSnf7 susceptibility among the seven field populations tested. Additionally, there was no correlation between susceptibility to DvSnf7 and Cry3Bb1 for all populations evaluated. In greenhouse studies, there were no significant differences (P<0.05) between beetle emergence of susceptible and Cry3Bb1-resistant colonies on DvSnf7 and Cry34Ab1/Cry35Ab1, and between DvSnf7 and MON 87411 (DvSnf7 + Cry3Bb1) for the Cry3Bb1-resistant colony. These results demonstrate no cross-resistance between DvSnf7 and Cry3Bb1 against WCR. Therefore, pyramiding DvSnf7 with Bt proteins such as Cry3Bb1 and Cry34Ab1/Cry35Ab1 will provide a valuable IRM tool against WCR that will increase the durability of these Bt proteins. These results also illustrate the importance of using appropriate bioassay methods when characterizing field-evolved resistant WCR populations.
Highlights
Corn rootworm-protected Bacillus thuringiensis (Bt) maize has been rapidly adopted on farms across the mid-western U.S Corn Belt, where corn rootworm (CRW) (Diabrotica spp.) have historically caused devastating damage and yield loss to maize, Zea mays L., estimated at approximately $1 billion [1,2]
With field-evolved resistance in at least some field populations to Cry3Bb1 with partial-full cross resistance to mCry3A and eCry3.1Ab, and field-evolved resistance in at least some field populations to Cry34Ab1/Cry35Ab1, there is a critical need to find and express new insect control compounds in transgenic maize against Western Corn Rootworm (WCR) [9,10,11,12,13,14]
Monsanto is developing a new WCR product based on double-stranded RNA (dsRNA) (DvSnf7), but understanding the potential insect resistant management (IRM) value of this product requires assessing potential cross-resistance with other WCR traits and proteins
Summary
Corn rootworm-protected Bacillus thuringiensis (Bt) maize has been rapidly adopted on farms across the mid-western U.S Corn Belt, where corn rootworm (CRW) (Diabrotica spp.) have historically caused devastating damage and yield loss to maize, Zea mays L., estimated at approximately $1 billion [1,2]. SmartStax maize (MON 89034 × TC1507 × MON 88017 × DAS-59122-7), commercially available since 2009, was the first product to offer CRW protection via a pyramid of two distinct Bt protein mechanisms of action, Cry3Bb1 and the Cry34Ab1/Cry35Ab1 binary toxin, and has been commercialized using either a 5% structured refuge or a 5% seed blend refuge in the Corn Belt. Pyramiding traits in this way has the potential to significantly increase the time until resistance develops [8]. Beetle emergence of laboratory susceptible and Cry3Bb1-resistant WCR was evaluated with maize plants in the greenhouse expressing Cry3Bb1, Cry34Ab1/Cry35Ab1, or DvSnf dsRNA singly, or in combination
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