David Wangila, University of Minnesota Dept. of Entomology, Postdoctoral Researcher
Fei Yang, University of Minnesota Extension corn entomologist
Beetles of the two corn rootworm species, northern corn rootworm and western corn rootworm, are now emerging and flying in cornfields across Minnesota. Corn rootworm is the most damaging insect pest of corn in the state. The larvae feed on corn roots, leading to root injury (Image 1), decreased plant growth, plant lodging (Image 2), and reduced yield. Additionally, adult beetles feed on the tassel and silks, affecting pollination. Peak beetle emergence typically coincides with the onset of silk. The best period to scout for rootworms is usually the first three weeks of August.
With the availability of Bt corn for controlling rootworm larvae, growers could simplify management by focusing on corn hybrids and insecticides rather than field-by-field corn rootworm scouting.
However, western corn rootworm has developed widespread resistance to all Bt proteins (including the Cry3 and Cry34/35 groups), leaving RNA interference (RNAi) as the only effective technology for rootworm management. Compared to Bt proteins, RNAi acts more slowly in killing rootworm larvae. All these situations could create strong selection pressure on rootworms, potentially accelerating the evolution of resistance to the RNAi technology. More recently, northern corn rootworm has developed resistance to both Cry3 and Cry34/35 protein in some fields in North Dokata (2019) and in Minnesota (2023). The ongoing Bt resistance issues in rootworm pose significant challenges and risks for corn production in the state. Unfortunately, many growers are unaware of the corn rootworm status in their fields, such as, is the rootworm population high in the field? is it necessary to use Bt traits? Is there any Bt resistance problems in the field? Should I switch traits? Should I add soil insecticide? Should I control adult beetles? Or should I rotate crops?
Scouting for rootworms could answer these questions by providing critical information on population status, root injury and beetle abundance. Rootworm damage primarily occurs below ground, making it less obvious until it reaches a critical level that affects plant stability, leading to lodging. By evaluating the extent of root injury, you can determine the effectiveness of your corn rootworm management practices and identify any early warning signs of resistance to Bt traits on your farm. You can also use the whole plant counts and yellow sticky traps for scouting beetle abundance, predicting the population density and risks for the following season.
Timely scouting for root injury and beetle activity is crucial for Bt resistance management. Early detection provides valuable insights for devising appropriate management strategies for resistant insects. To determine if rootworms are developing resistance in your area, we recommend scouting and collecting beetles for Bt toxin bioassays.
Fei Yang, University of Minnesota Extension corn entomologist
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| Image 1. Corn roots damage by corn rootworm. |
With the availability of Bt corn for controlling rootworm larvae, growers could simplify management by focusing on corn hybrids and insecticides rather than field-by-field corn rootworm scouting.
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| Image 2. Lodged plants due to corn rootworm feeding on roots. |
Scouting for rootworms could answer these questions by providing critical information on population status, root injury and beetle abundance. Rootworm damage primarily occurs below ground, making it less obvious until it reaches a critical level that affects plant stability, leading to lodging. By evaluating the extent of root injury, you can determine the effectiveness of your corn rootworm management practices and identify any early warning signs of resistance to Bt traits on your farm. You can also use the whole plant counts and yellow sticky traps for scouting beetle abundance, predicting the population density and risks for the following season.
Timely scouting for root injury and beetle activity is crucial for Bt resistance management. Early detection provides valuable insights for devising appropriate management strategies for resistant insects. To determine if rootworms are developing resistance in your area, we recommend scouting and collecting beetles for Bt toxin bioassays.
Seeking volunteers
If you observe heavy rootworm infestation in your cornfields, especially with Bt traits in 2024, we are seeking volunteers to collect these insects for testing. We will assay the insects and provide you with reports on their Bt resistance levels. To volunteer for the 2024 collection, please contact Fei Yang (yang8905@umn.edu) or David Wangila (dwangila@umn.edu).
Kahler, A. L., A. E. Olness, G. R. Sutter, C. D. Dybing, and O. J Devine. 1985. Root damage by western corn rootworm and nutrient content in maize. Agron. J. 77: 769-774.
Ludwick, D. C., L. N. Meihls, K. R. Ostlie, B. D. Potter, L. French, and B. E. Hibbard. 2017. Minnesota field population of western corn rootworm (Coleoptera: Chrysomelidae) shows incomplete resistance to Cry34Ab1/Cry35Ab1 and Cry3Bb1. J. Appl. Entomol. 141: 28-40.
Oleson, J.D., Y. L. Park, T. M. Nowatzki, and J. J. Tollefson. 2005. Node injury scale to evaluate root injury by corn rootworms (Coleoptera: Chrysomelidae). J. Econ. Entomol. 98: 1-8.
Tinsley, N. A., R. E. Estes, and M. E. Gray. 2013. Validation of a nested error component model to estimate damage caused by corn rootworm larvae. J. Appl. Entomol. 137: 161-169.
Zukoff, S. N., K.R. Ostlie, B. Potter, L. N. Meihls, A. L. Zukoff, L. French, M. R. Ellersieck, B. W. French, and B. E. Hibbard. 2016. Multiple assays indicate varying levels of cross resistance of MON88017-selected field populations of the western corn rootworm to mCry3A, eCry3.1Ab, and Cry34/35Ab1. J. Econ. Entomol. 109: 1387-1398.
Select references
Gray, M. E., and K. L. Steffey. 1998. Corn rootworm (Coleoptera: Chrysomelidae) larval injury and root compensation of 12 hybrids: an assessment of the economic injury index . J. Econ. Entomol. 91: 723-740Kahler, A. L., A. E. Olness, G. R. Sutter, C. D. Dybing, and O. J Devine. 1985. Root damage by western corn rootworm and nutrient content in maize. Agron. J. 77: 769-774.
Ludwick, D. C., L. N. Meihls, K. R. Ostlie, B. D. Potter, L. French, and B. E. Hibbard. 2017. Minnesota field population of western corn rootworm (Coleoptera: Chrysomelidae) shows incomplete resistance to Cry34Ab1/Cry35Ab1 and Cry3Bb1. J. Appl. Entomol. 141: 28-40.
Oleson, J.D., Y. L. Park, T. M. Nowatzki, and J. J. Tollefson. 2005. Node injury scale to evaluate root injury by corn rootworms (Coleoptera: Chrysomelidae). J. Econ. Entomol. 98: 1-8.
Tinsley, N. A., R. E. Estes, and M. E. Gray. 2013. Validation of a nested error component model to estimate damage caused by corn rootworm larvae. J. Appl. Entomol. 137: 161-169.
Zukoff, S. N., K.R. Ostlie, B. Potter, L. N. Meihls, A. L. Zukoff, L. French, M. R. Ellersieck, B. W. French, and B. E. Hibbard. 2016. Multiple assays indicate varying levels of cross resistance of MON88017-selected field populations of the western corn rootworm to mCry3A, eCry3.1Ab, and Cry34/35Ab1. J. Econ. Entomol. 109: 1387-1398.
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