Location: Plant Genetics Research2011 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
Related project 3622-21220-006-00D was a bridging project which expired with the approval of the current five year project, which began February 10, 2011. Insecticidal proteins expressed in transgenic corn (Bt corn) are highly effective in rootworm management. However, in nine of nine attempts with all available proteins, laboratory selection on transgenic insecticidal proteins targeted toward western corn rootworm has resulted in resistance to levels of insecticidal protein seen in commercially available transgenic corn in the selected insect population. Given recent examples of insect pests becoming resistant to transgenic crops in the field, it is critical to stay ahead of the curve. In the few months since the current project began, we have 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 (Bt). 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. Currently, we are beginning the fourth generation of selection. These colonies should help us understand to what extent the mandatory planting non-Bt corn assists in the delay in the development of resistance to insecticidal transgenic corn targeted toward corn rootworms. In the other primary objective of the project, we continued selecting for sources of natural resistance in maize to western corn rootworm larval feeding from sources including the Germplasm Enhancement of Maize (GEM) project. We evaluated ~100 new germplasm 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 long-term goals of our overall research program are 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 will serve as an alert to the U.S. corn industry of potential problems. Should resistance develop in the field to transgenic products targeting corn rootworms, our cross resistance work will help the industry to understand its transgenic options, and our native resistance breeding program could also help fill the management vacuum. 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. Western corn rootworm susceptibility to genetically modified corn expressing an insecticidal protein. Genetically modified corn with resistance to corn rootworm larval feeding offers a viable alternative to insecticides for managing the most economically important insect pests of corn. Maintaining insect susceptibility to genetically modified crops (resistance management) is in the interest of growers, the U.S. Environmental Protection Agency (EPA), and the seed industry, but requires an understanding of corn rootworm biology that does not currently exist. The mortality and biological responses of western corn rootworm larvae to exposure (feeding) to three separate lines of genetically modified corn expressing insecticidal proteins (Bt corn) relative to survivorship on non-modified control corn (control plots) were evaluated by ARS scientists in Columbia, MO. Many more beetles (890) emerged from control plots while the average beetle emergence from the three different genetically modified corn lines was 19, 2, and 1 when averaged across many plots in five different environments. There was also an 8 d delay in beetle emergence from one of the genetically modified corn lines and beetles recovered from the genetically modified corn lines were slightly more likely to be females than those emerging from non-modified controls. These quantitative data are important to seed companies and resistance management professionals in their attempts to optimize resistance management plans for transgenic corn and are required by the EPA for effective regulation. The information from these studies are critical in the struggle to maintain agricultural productivity as insect pests continue to threaten crop yields and the need to reduce chemical insecticide use becomes essential for protecting our environment.
Meihls, L., Higdon, M.L., Ellersieck, M., Hibbard, B.E. 2011. Selection for resistance to mCry3A-expressing transgenic corn in western corn rootworm. Journal of Economic Entomology. 104(3):1045-1054.