Indirect root defenses cause induced fitness costs in Bt-resistant western corn rootworm

Authors: HILTPOLD, IVAN - Western Sydney University; Hibbard, Bruce

Submitted to: Journal of Economic Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/6/2018
Publication Date: 8/2/2018
Citation: Hiltpold, I., Hibbard, B.E. 2018. Indirect root defenses cause induced fitness costs in Bt-resistant western corn rootworm. Journal of Economic Entomology. 111(5):2349-2358. https://doi.org/10.1093/jee/toy220.
DOI: https://doi.org/10.1093/jee/toy220

Interpretive Summary: Plants genetically modified to produce insecticidal toxins from the bacterium Bacillus thuringiensis (Bt) have been extensively used to manage the western corn rootworm. Evolution of resistance to Bt toxins in this pest has forced the consideration of alternative pest control and improved insect resistance management. Entomopathogenic nematodes (EPN - nematodes that feed on insects), which are obligate insect parasites, are attracted towards volatile compounds emitted in response to rootworm feeding damage. The quantity of these volatile compounds produced by two Bt maize hybrids and non-Bt hybrids in the same maize genetic background (isolines) was determined in the laboratory after induction with Bt susceptible and Bt resistant rooworm larvae. The attraction of EPNs towards the Bt hybrids was tested in the laboratory and in the field. Volatile compounds were produced when Bt resistant insects induced Bt hybrid corn whereas induction by Bt susceptible rootworm larvae did not trigger a strong plant response. In the laboratory and the field, roots of one hybrid emitted volatile compounds attracting EPNs, when attacked by Bt resistant rootworm larvae. In the field, those plants experienced less feeding damage than the other hybrid which produce a less EPN attractive blend in response to Bt resistant rootworm damage. Survival of Bt resistant rootworm larvae was lower on the hybrid attracting EPNs and comparable to the survival of Bt susceptible rootworm without EPNs. This trade-off of Bt resistance is defined here as an induced fitness cost, and offers a viable tool to management of Bt resistance in the western corn rootworm.

Technical Abstract: Plants genetically modified to produce insecticidal toxins from the bacterium Bacillus thuringiensis (Bt) have been extensively used to manage the western corn rootworm (WCR), Diabortica virgifera virgifera, in the USA. Evolution of WCR resistance to Bt toxins has forced the consideration of alternative pest control and improved insect resistance management. Entomopathogenic nematodes (EPNs), obligate insect parasites, are attracted towards root volatile organic compounds (VOCs) emitted in response to WCR herbivory. The production of VOCs of two Bt maize hybrids and their isolines was evaluated in the laboratory after induction with Bt susceptible and resistant WCR. The attraction of EPNs towards the Bt hybrids was tested in the laboratory and in the field. VOCs were produced when Bt resistant insects induced Bt hybrids whereas induction by Bt susceptible WCR did not trigger a strong plant response. In the laboratory and the field, roots of one hybrid emitted VOCs attracting EPNs, when attacked by Bt resistant WCR. In the field, those plants experienced less herbivory than the other hybrid which produce a less attractive blend in response to Bt resistant WCR herbivory. Survival of Bt resistant WCR was lower on the hybrid attracting EPNs and comparable to the survival of Bt susceptible WCR without EPNs. This trade-off of Bt resistance is defined here as an induced fitness cost, and offers a viable tool to management of Bt resistance in WCR.