2013 Annual Report
1a.Objectives (from AD-416):
Objective 1: Develop lines of the western corn rootworm (WCR) resistant to transgenic corn and investigate the biology, pest/host interactions, and fitness costs of resistant and control colonies as they relate to resistance management and rootworm biology.
Objective 2: Develop and release corn germplasm with native resistance to major corn insect pests such as western corn rootworm, and determine genetic and chemical mechanisms of resistance.
1b.Approach (from AD-416):
For Objective 1, we will develop colonies with resistance to Cry34/35Ab1 and test the effectiveness of different refuge types to delay resistance. We will then evaluate our mCry3A-resistant colony for the heredity of this trait, whether the trait is maintained when selection is removed, and whether there are fitness costs associated with the trait. Finally, we will evaluate cross resistance in rootworm colonies selected for resistance to one rootworm product on other single and stacked trait products.
For Objective 2, we will systematically screen exotic and GEM (Germplasm Enhancement of Maize) corn germplasm, identify potential sources of resistance, verify resistance, and move into adapted germplasm. In addition, we will conduct recurrent selection with the CRW17 synthetic population for resistance to western corn rootworm larval feeding damage. Finally, we will evaluate the CML333 family of the NAM (Nested Association Mapping) population for resistance to western corn rootworm larval feeding and identify Quantitative Trait Loci associated with reduced damage.
The study and development of colonies of corn rootworms resistant to transgenic corn that carry an insecticide can help us understand how such resistance occurs and help us to slow resistance in current and future control measures. The selected laboratory colony of the western corn rootworm was established by rearing larvae on transgenic eCry3.1Ab-expressing corn. The eCry3.1Ab protein is a partially engineered insecticidal protein originally derived from a bacteria and engineered to be expressed in corn. Two colonies were developed including a “selected” colony (from adults isolated and grown on transgenic corn) and a “control” colony (from adults isolated and grown on control corn). In laboratory and greenhouse experiments, there was no significant difference in the number of larvae recovered from transgenic and isoline maize for the selected colony, whereas this difference was significant for the control colony. Laboratory-selected resistance has developed in western corn rootworm populations to all insecticidal proteins currently registered for corn rootworm management, which emphasizes the importance of adhering to resistance management plans for maintaining product efficacy.
For the other objective of the project, we conducted another generation of selection on a corn population (CRW17) in order to improve resistance to western corn rootworm larval feeding. We teamed with scientists from the University of Illinois and industry to conduct genomic selection of CRW17 and compare to selection already performed. We also evaluated ~100 new maize lines not previously evaluated from the Germplasm Enhancement of Maize (GEM) project.
Overall, these projects separately facilitate the development of both native and transgenic sources of resistance to the corn rootworm by developing sources of native resistance and providing biological data for resistance management strategies for Bt corn and benefits U.S. corn growers and their seed providers.
Larval movement in seed mix refuges of newest Bt pyramid targeting rootworms affects beetle emergence. Transgenic (genetically modified) corn with resistance to corn rootworm larval feeding has been widely adopted because of the value it offers to growers for managing the most economically important insect pest of corn. Rootworms have developed resistance to other control options, so maintaining susceptibility to transgenic crops (resistance management) is critical. The Environmental Protection Agency requires non-Bt corn to be planted in addition to Bt corn to slow the development of rootworm resistance to Bt by producing susceptible beetles to mate with resistant beetles. ARS scientists evaluated a new method of planting non-Bt in which it is mixed with Bt seed and planted together. With this technique, movement of insect pests from non-Bt to Bt, and perhaps in the reverse direction, can speed the development of resistance in some situations. ARS scientists documented that movement of rootworm larvae between Bt and non-Bt will occur, but likely will not speed the development of resistance because of the low dose of the Bt product. This information is important for the optimization of resistance management plans for transgenic corn.
Clark, T.L., Frank, D.L., French, B.W., Meinke, L.J., Moellenbeck, D., Hibbard, B.E. 2012. Mortality impact of MON863 transgenic maize roots on western corn rootworm larvae in the field. Journal of Applied Entomology. 136:721-729.
Meihls, L.N., Higdon, M.L., Elliersieck, M.R., Tabashnik, B.E., Hibbard, B.E. 2012. Greenhouse-selected resistance to Cry3Bb1-producing corn in three western corn rootworm populations. PLoS One. 7(12):e51055. Available http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0051055.
Devos, Y., Meihls, L.N., Kiss, J., Hibbard, B.E. 2013. Resistance evolution to the first generation of genetically modified Diabrotica-active Bt-maize events by western corn rootworm: management and monitoring considerations. Transgenic Research. 22:269-299.
Robert, C.A., Erb, M., Hiltpold, I., Hibbard, B.E., Gaillard, M.D., Bilat, J., Degenhardt, J., Cambet-Petit-Jean, X., Turlings, T.C., Zwahlen, C. 2013. Genetically engineered maize plants reveal distinct costs and benefits of constitutive volatile emissions in the field. Plant Biotechnology. 11:628-639.
Robert, C.A., Frank, D.L., Leach, K.A., Turlings, T.C., Hibbard, B.E., Erb, M. 2013. Direct and indirect plant defenses are not suppressed by endosymbionts of a specialist root herbivore. Journal of Chemical Ecology. 39:507-515.