Understanding the Impact and Measures Needed to Address Bt Resistant Rootworm

Why be concerned about the Bt resistant rootworm populations showing up in Ontario? Dr. Jocelyn Smith, University of Guelph Ridgetown Campus stands in a field that has cross resistant rootworm.  The grower experienced more than 50% yield loss in this 2nd year silage corn field planted with a pyramid hybrid in 2020.  Jocelyn discusses the impacts of these resistant rootworm populations and the mitigation measures needed to knock back these populations. Following these mitigation measures in Ontario is our best chance of maintaining the durability of Bt rootworm hybrids in the future.

Crop rotation away from corn is the best option to mitigate these rootworm populations. Rootworm larvae need corn roots to survive. If corn is not planted into these fields, the resistant rootworm population will crash. Non-corn options for livestock producers can be found here in this previous FCN article on non-corn options.

If you must plant corn, plant a non-rootworm Bt hybrid and use additional root protection tools like soil insecticide or seed treatment. That way, we are not continuing to expose rootworm to the Bt proteins they are becoming tolerant to.

Switching to a different pyramid Bt rootworm hybrid (contains two or more proteins targeting rootworm) will not mitigate this issue.  Cross resistance means that these rootworm are tolerant to multiple Bt proteins..even proteins they haven’t been exposed to yet because most of the rootworm Bt proteins currently used are closely related. And the resistant traits will remain within these populations from now on. Using a pyramid hybrid in these fields results in the rootworm either both Bt proteins are compromised and rootworm causes significant injury or only one protein still effective, essentially making the hybrid a single trait hybrid. Research indicates that resistance can quickly develop within 3 years of use of single trait hybrids.

More detailed information on mitigation measures can be found in the previous FCN article: Mitigation Measures for Bt Resistant Corn Rootworm