Location: Plant Genetics Research2012 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.
3. Progress Report:
Until the recent development of resistance in the field, insecticidal proteins expressed in transgenic corn were highly effective in rootworm management. Development of insect resistance in the field was not surprising given that laboratory selection, regardless of the insecticidal proteins used, always gave rise to resistant insects. Given rootworms developed resistance to transgenic crops in the field, it is critical to stay ahead of the curve. We continued to select colonies of the western corn rootworm aimed at evaluating “refuges” (plantings of non-Bt corn) for delaying resistance to specific transgenic corn expressing the insecticidal protein Cry34/35Ab1. Rootworm colonies were selected using both “seed mix” and “block refuge” scenarios along with unselected colonies and colonies selected after survival on 100% rootworm insecticidal corn. These colonies provide understanding to what extent the mandatory planting of non-Bt corn assists in the delay in the development of resistance to insecticidal transgenic corn targeted toward corn rootworms. For the other objective of the project, we continued to select for sources of maize naturally resistant to western corn rootworm larval feeding, using maize germplasm sources including the Germplasm Enhancement of Maize (GEM) project. We evaluated ~100 new maize lines not previously evaluated from the GEM program. We conducted one generation of selection on a corn population (CRW17) in order to improve resistance to western corn rootworm larval feeding. The overall research project is aimed at understanding the nature of resistance, any cross resistance that may occur, and whether or not refuges work in delaying resistance for low-dose rootworm products. This research serves as an alert to the U.S. corn industry of additional potential problems. Given that resistance has developed in the field to transgenic products targeting corn rootworms, our cross resistance work will enable industry to understand its transgenic options, and our native resistance breeding program will fill the management vacuum in the long run. Overall, this project separately facilitates 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.
1. Development of a western corn rootworm (WCR) colony with resistance to Bt corn. The study and development of colonies of western corn rootworms (WCR) resistant to insecticidal transgenic corn can help us understand how such resistance occurs in nature and slow its progress. An ARS scientist in Columbia, Missouri, developed a laboratory derived colony of resistant WCR from survivors of WCR fed on the newest commercial type of transgenic corn targeting rootworms. Testing in the greenhouse demonstrated the utility of the development of resistant WCR colonies as a tool for evaluating strategies designed to slow the development of the occurance of natural resistance in WCR in the U.S. corn growing regions. Availability of resistant colonies will facilitate understanding of the resistance and allow for cross resistance studies with other and future insect control strategies that use a plant biotechnological approach.
Hibbard, B.E., Frank, D.L., Kurtz, R., Boudreau, E., Ellersieck, M.R., Odhiambo, J. 2011. Mortality impact of transgenic maize roots expressing eCry3.1Ab, mCry3A, and eCry3.1Ab + mCry3A on western corn rootworm larvae in the field. Journal of Economic Entomology. 104(5):1584-1591.