Submitted to: Journal of Applied Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/26/2016
Publication Date: 1/28/2017
Publication URL: https://handle.nal.usda.gov/10113/5695419
Citation: Ludwick, D.C., Meihls, L.N., Ostlie, K.R., Potter, B.D., French, L., Hibbard, B.E. 2017. Minnesota field population of western corn rootworm (Coleoptera: Chrysomelidae) shows incomplete resistance to Cry34Ab1/Cry35Ab1 and Cry3Bb1. Journal of Applied Entomology. 141:28-40. doi: 10.1111/jen.12377.

Interpretive Summary: The western corn rootworm is a major insect pest and is commonly managed with corn expressing insecticidal proteins originally from the bacteria Bacillus thuringiensis (Bt). A Minnesota population of western corn rootworm caused higher than expected levels of damage to transgenic corn in 2013. Beetles were collected from the field and allowed to lay eggs in dishes with soil. Eggs from the beetles were divided into separate portions to be tested in experiments with plants and artificial diets. Results indicated that the larvae showed signs of elevated survival to two Bt proteins. Despite the increased survival, the larvae were slower to grow and gain weight on one of the proteins (Cry34/35Ab1). These data suggest that resistance to a third Bt protein is starting to occur, but is not yet fully developed. Monitoring of populations like this allow researchers to better understand the changes in the effectiveness of products and allow farmers to make appropriate planting decisions.

Technical Abstract: The western corn rootworm, Diabrotica virgifera virgifera LeConte, is commonly managed with corn (Zea mays L.) expressing insecticidal proteins from the bacteria Bacillus thuringiensis (Bt). Under laboratory conditions, populations of western corn rootworm have been selected for resistance to each commercially available protein targeting this pest. In addition, field populations of western corn rootworm have resistance to at least two of the four Bt proteins targeting this pest. In the current study, progeny of a western corn rootworm population collected from a Minnesota corn field that was planted to SmartStax® corn were evaluated for resistance on corn hybrids expressing the individual constituent proteins Cry3Bb1 or Cry34/35Ab1 and in diet assay with overlays of either protein. Results from seedling mat assays, greenhouse assays, and diet toxicity assays suggest that this population has resistance to Cry3Bb1 and is beginning to develop resistance to Cry34/35Ab1. In diet toxicity assays, larvae of the MN population had resistance ratios of 4.71 and >13.22 for Cry34/35Ab1 and Cry3Bb1, respectively, compared with the control populations. In the seedling mat assay, survival of the MN population on the Cry34/35Ab1 hybrid compared to its near isoline were not significantly different, unlike the control population in these assays. For all Cry3Bb1 assays, the MN population survived at greater frequency than the control population. In addition, larval head capsule width and dry weight were not significantly different on Cry3Bb1-expressing corn when compared with larvae recovered from the near isoline corn, suggesting resistance to Cry3Bb1 was nearly complete. In all on-plant assays, larvae of the MN population had smaller head capsule widths and lower dry weights when reared on corn expressing Cry34/35Ab1 when compared with larvae reared on the near-isoline, indicating any level of resistance present was incomplete. Overall, these results document resistance to Cry3Bb1 and an incomplete resistance to Cry34/35Ab1 in a SmartStax performance problem field.