Extensive injury by corn rootworm (CRW) to pyramid Bt rootworm (Bt-RW) hybrids was observed in several Ontario counties in 2020, suggesting that CRW populations have developed resistance to multiple Bt-RW proteins.
The Canadian Corn Pest Coalition (CCPC) and OMAFRA have developed mitigation recommendations for continuous corn producers, in hopes of knocking back the resistant corn rootworm population present in Ontario. Producers with high risk fields of continuous corn production (3+ years) with a history of repeated Bt-RW hybrid use need to take measures over the next few years in hope of restoring the durability of current Bt-RW hybrids: Mitigation Measures to Manage Bt Resistant Corn Rootworm in Ontario
Bt resistance will now always remain within the CRW populations in Ontario. At best, through implementing mitigation measures above, we can knock CRW populations back to where resistant individuals are not as prevalent within these populations. Once these populations are reduced, a long term responsible approach will be needed for rootworm management. This includes only planting corn for 3 years in a row in any given field, with only one year requiring the use of Bt-RW hybrids. This results in less selection pressure towards Bt resistance in the rootworm population. This strategy will only work if fields are rotated out of corn in Year 4: Long-Term Responsible Use of Bt Hybrids for Rootworm Management. This long term approach to responsible use should be considered by all continuous corn producers including those who are not currently experiencing unexpected rootworm injury on Bt-RW hybrids.
This two page Decision Tree Diagram by OMAFRA helps to summarize the measures outlined in the two documents above.
Taking these actions now is the best chance to manage resistant CRW populations before they become widespread in Ontario.