Restoration of susceptibility following removal of selection for Cry34/35Ab1 resistance documents fitness costs in resistant population of western corn rootworm, diabrotica virgifera virgifera

Authors: PADDOCK, KYLE - University Of Missouri; Hibbard, Bruce; Barry, Julie; SETHI, AMIT - Corteva Agriscience; MUELLER, ASHLEY - Corteva Agriscience; Shelby, Kent; PEREIRA, ADRIANO - University Of Missouri

Submitted to: Pest Management Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/4/2021
Publication Date: 4/8/2021
Citation: Paddock, K., Hibbard, B.E., Barry, J.M., Sethi, A., Mueller, A.L., Shelby, K., Pereira, A. 2021. Restoration of susceptibility following removal of selection for Cry34/35Ab1 resistance documents fitness costs in resistant population of western corn rootworm, diabrotica virgifera virgifera. Pest Management Science. 77(5):2385-2394. https://doi.org/10.1002/ps.6266.
DOI: https://doi.org/10.1002/ps.6266

Interpretive Summary: Corn rootworms are the most devastating maize pests in the United States. Management costs plus yield losses caused by larvae feeding on the corn roots are estimated to be over $2 billion annually. Rootworms have evolved resistance multiple chemical insecticides, crop rotation, and transgenic corn. When selection has been removed to other management techniques, resistance has continued. In this study, we wanted to evaluate if a laboratory strain of the western corn rootworm that was resistant to transgenic corn expressing insecticidal toxins, would become susceptible to the toxin after removal from selection to resistance. We also evaluated fitness parameters such as female fecundity, adult lifespan, larval development, and adult emergence between resistant and susceptible strains. The susceptibility to the toxin was restored after at least six generations of removal from selection. In addition, beetles from susceptible colonies lived longer than resistant beetles which resulted in females from susceptible colonies producing significantly more eggs than resistant colonies. For the first time, major fitness costs due to resistance were documented, which may mitigate resistance spread in the field to some extent.

Technical Abstract: BACKGROUND: Management of the corn pest, western corn rootworm (WCR), Diabrotica virgifera virgifera (LeConte) (Coleoptera: Chrysomelidae), relies heavily on the planting of transgenic corn expressing toxins produced by the bacterium Bacillus thuringiensis (Bt). This has resulted in the evolution of resistance to all of the four commercially available Bt toxins targeting coleopteran insects. In this study, we evaluated the susceptibility of a Cry34/35Ab1-resistant WCR colony in seedling and diet toxicity assays after removal from selection for six and nine generations. In addition, female fecundity, egg fertility, adult lifespan, larval development, and adult emergence were evaluated in two Cry34/35Ab1-resistant and two susceptible WCR colonies to assess fitness costs. RESULTS: Susceptibility to Cry34/35Ab1 was restored in a colony removed from selection after six and nine generations based on diet toxicity assays and comparisons of relative survival, head capsule width, and dry weight in plant assays. Thus, pronounced fitness costs associated with resistance to Cry34/35Ab1 were documented by susceptibility being restored within six generations. In separate studies evaluating specific fitness costs, larval fitness when reared on isoline corn did not differ between resistant and susceptible colonies. However, beetles from susceptible colonies lived longer than resistant beetles which resulted in females from susceptible colonies producing significantly more eggs than resistant colonies, with no differences in egg fertility. CONCLUSIONS: The presence of a fitness cost that may contribute to the restoration of susceptibility to Bt has not been documented in other Cry3-resistant WCR populations and could have significant impact on the deployment of resistance management practices.