Elson Shields, Entomology Department, Cornell University
The excessive wet soil conditions during the 2017 corn rootworm (CRW) hatching period during late May – early June caused a major reduction in corn rootworm adult populations during the 2017 growing season. Adult surveys in most fields during early August showed a scarcity of adult beetles during the egg-laying period. As a result, most fields in NY will have a reduced risk for CRW damage during the 2018 growing season. In these lower risk fields, CRW management costs can be reduced by growing non-Bt-CRW corn and using either a reduced rate of soil insecticide or the 1250 rate of seed treatment. First year corn is never at risk from CRW and therefore Bt-CRW corn, a soil insecticide or the 1250 rate of seed treatment is an unnecessary expense. This includes any application of Capture in the pop up fertilizer. Well drained fields which did not experience the typical periods of water logged soils during late May – early June 2017 will be at higher risk from CRW injury in 2018 and should be managed accordingly. These higher risk fields may benefit from planting Bt-CRW corn varieties. In “normal” years, the risk of economic damage from CRW is 0% – 1st year corn, 25%-35% – 2nd year corn, 50%-70% – 3rd year corn and 80%-100% for 4th year and longer continuous corn.
Status of Bt-CRW resistance in the US:
CRW Bt resistance continues to build across the corn growing regions of the US with multiple localized resistant populations identified for each of the Bt-CRW traits. Cross resistance has been identified within the Cry3 family (Cry3Bb1-Yieldgard Rootworm, eCry3.1Ab-Duracade, mCry3A-Agrisure RW) and if one of the Cry3 traits are failing in your field, the planting of another toxin within the Cry3 family may lead to disappointing CRW management results. Resistance has also been reported in several states to Cry 34Ab1/Cry35Ab1. There has been no reported cross resistance between the Cry3 family of toxins and Cry34Ab1/Cry35Ab1 toxin combination.
The rootworm Bt-toxin pyramids consist of two different Bt-RW toxins in the same plant. Some seed companies have included two different toxins from the Cry3 family where cross resistance has been reported where other seed companies utilize the pyramid mix of a toxin from the Cry3 family and Cry34Ab1/Cry35Ab1 where no cross resistance has been reported. If control failures have been reported in your fields/region to any one of the Cry3 family of toxins, planting a pyramid composed of two different Cry3 toxins is not recommended. Instead, it is a better CRW resistance choice to plant a pyramid consisting of a Cry3 toxin with the Cry34Ab1/Cry35Ab1 toxin.
A very handy resource to identify the Bt traits in your corn varieties is the annually updated Bt trait table. The 2018 Handy Bt Trait Table for US Corn Production is made available by Dr. Chris Difonzo, MSU, Dr. Pat Porter, Texas A&M and Dr. Kelley Tilmon, OSU can be found at the following URL:
https://lubbock.tamu.edu/files/2018/01/BtTraitTableJan2018.pdf
As Bt –CRW traits are failing to resistance by corn rootworm, the promise of the next effective trait is ever appealing. The development of the RNAi technology against CRW has been touted as the next effective plant incorporated toxin with a very slim chance of resistance development by CRW. However, it only took about 20 million individuals from a single Illinois continuous corn field and a few generations to generate an RNAi resistant laboratory population. In addition, field results with RNAi containing corn varieties suffer a noticeable amount of root feeding damage before the slow-killing toxin kills the insect larvae. As a result, the new RNAi technology will not be the “silver bullet” everybody has hoped for. Stewardship of the Bt technology has become increasingly important in areas where Bt resistance has not been reported because the next technology needs effective Bt toxins to help it out.
Bt Trait Stewardship Suggestions:
A few simple management adjustments can go a long way in preserving the efficacy of the Bt-CRW traits in NY.
- Long-term corn fields need to be rotated to a non-corn crop on a regular basis. Continuous corn matched with a long-term use of same Bt-CRW trait promotes the development of a resistant population.
- Rotate toxins between the Cry3 family and Cry34Ab1/Cry35Ab1 toxins. There is no recorded cross resistance between these two groups of toxins.
- Use the Bt-CRW technology only in fields of 3rd and longer continuous corn fields. Rotate the toxin groups and rotate the long-term corn to at least 1 year away from corn to break the CRW cycle.
- Plant some fields to non-Bt-CRW varieties and use either a granular soil insecticide or the 1250 rate of seed treatment. Liquid insecticides in the popup fertilizer are not effective and not recommended.
