High corn rootworm (CRW) pressure in 2020 is challenging current Bt rootworm hybrids and several Ontario growers have reported unexpected injury by CRW. This indicates that possible cross resistance has occurred, where even pyramid hybrids with multiple rootworm traits are impacted. Currently, we know that several fields with injury in Huron, Perth and Durham Counties are being investigated. There are likely more fields impacted outside of these known areas where use of Bt rootworm traits in continuous corn is prevalent.
Scouting in the next few weeks for corn rootworm injury is important to detect and report potential cases. We highly recommend scouting before early September while rootworm adults are still active. This will allow corn company reps and provincial specialists to assess the field to determine if resistance is occurring. This requires adults to be present to collect and run bioassays on and for green tissue be present on the plants to verify that they are indeed expressing Bt proteins.
Scout these high-risk fields first:
- Continuous corn production and repeated use of Bt corn rootworm hybrids
- High adult CRW populations this year or the previous year
- Lodging or goose-necking where other factors like wind, compaction and herbicide carry over have been ruled out.
Scouting includes documenting high population of adults and looking for lodging or goose-necking that is not likely due to wind, compaction and herbicide injury. Most importantly, dig up plants to determine if root injury is higher than expected. Bt rootworm hybrids contain only moderate doses at best, which means that there will be some level of survival of the population and some expected root clipping. On pyramid (multi-trait) hybrids, a root injury score of more than 0.5 on the Iowa Node Injury Scale (0-3.00) should be reported. Single trait rootworm hybrids with a rating of >1.0 indicates the need to report. More information on scouting guidelines to determine rootworm resistance is on the Field Crop News website at: https://fieldcropnews.com/2020/08/rootworm-scouting-guidelines/
Reporting potential resistance cases is important because rootworm adults are mobile. Resistant populations won’t remain in the immediate field but will spread to nearby corn fields. Knowing where fields are will help enable communication with other continuous corn growers in the area who may not be aware of a problem. Reporting also ensures that sound rootworm management measures can be implement by continuous corn growers for 2021 to try to preserve the effectiveness of Bt rootworm hybrids for Canadian corn growers.
Corn rootworm resistance to Bt traits are widespread in the United States. Several mitigation measures have been used with varying degrees of success. Once resistance occurs, rootworm Bt hybrids will no longer be an effective tool for protection against rootworm injury. No new Bt rootworm traits are coming to market in the immediate future. Our best option is to try to reduce the prevalence of resistance individuals in the population through effective management efforts.
The most successful option to reduce rootworm populations is to rotate out of corn for a minimum of one year. Rootworm larvae require corn roots to survive. However, rotating out of corn is challenging for livestock producers. For silage producers, a viable alternative is to double crop cereal rye or winter triticale with sorghum-sudangrass. This alternative can provide similar yield and fibre quality without sustaining a rootworm population. Information on this option is also available on Field Crop News at: https://fieldcropnews.com/2020/08/forage-options-to-replace-silage-corn/
Other crop alternatives and effective rootworm management options for 2021 will be distributed in the next few weeks through OMAFRA, the CCPC, seed providers and others.
In cases where CRW injury to a Bt rootworm hybrid is suspected, growers should report the issue to their seed supplier and inform the executive or provincial members of the Canadian Corn Pest Coalition (CCPC; https://www.cornpest.ca/contact-us/executive-members/).
|Weekly Weather Summary August 17-23|
|This table developed by OMAFRA using data from Agriculture and Agri-Food Canada and Environment Canada. Max and Min Temps show the extremes that occurred for the 7-day period.|