On Jan. 18, 2023, Bruce Potter, University of Minnesota Extension IPM specialist, and Dr. Anthony Hanson, UMN Extension IPM educator, joined UMN Extension crops educator Ryan Miller for a wide-ranging discussion of European corn borer. This was the second episode of the 2023 Strategic Farming: Let’s talk crops! webinars in this series. 

There are two corn rootworms that affect corn roots in Minnesota, northern corn rootworm (NCR) and the western corn rootworm (WCR). Eggs are laid in cracks in the soil, where they spend the winter, emerging as larvae in the spring to feed on corn roots. “When rootworms feed, they injure corn roots which negatively impacts water and nutrient uptake, lodging and yield,” says Hanson. Although a rare phenomenon, when present in high numbers, adult rootworms can cause severe feeding injury on corn silks which can both negatively impact pollination and increase the risk of ear mold diseases. Continuous corn fields, fields with high beetle populations, many continuous corn fields in the area, fields with early or late silking corn (including volunteer corn) and using the same management tactic repeatedly are at greater risk of building damaging population densities and sustaining significant rootworm injury. 

In sharing the nodal root injury ratings scale when sharing research results, Bruce Potter explained, “Those node injury scales are based on a percentage of nodal roots (usually node 5 through 7) pruned to within an inch and a half of the plant. The scale goes from 0 to 3, and 0 means that none of the roots at those nodes were pruned, a 3 means that 100% of the roots at all three nodes were pruned. Any time you start to see a quarter of a node pruned, you can start to see lost yield potential.”

NCR tends to be more mobile and its eggs more cold-tolerant than WCR, and recently there has been some indications that NCR population densities in northwest Minnesota may be increasing. NCR, which are the predominant species in northern Minn. corn fields, are also important because of a growth habit called “extended diapause” in which some populations have developed a preference for laying their eggs in soybean crops only for their larvae to hatch the following spring into fields that have been planted to corn – essentially crop rotation-resistant rootworms. WCR populations have not yet been documented to undergo extended diapause in Minnesota. 

Western corn rootworms have a long (or should we say infamous) history since the 1950s of evolving to resist many of the management tactics we used to combat them, from resistance to multiple insecticide active ingredients to rotation resistance (in the eastern corn belt). After generations of battling this pest, when Bt hybrids specific to rootworm were first released for widespread sale in 2005 there were high hopes for effective management. When resistance to specific Bt-proteins began to be identified in 2009, a mere 6 years after their release, managing rootworms again became a bit more complicated. 

Resistance to individual rootworm-specific Bt traits has been documented in both NCR and WCR populations. Consequently, knowing a bit about your field’s corn rootworm population (maybe through being a part of the UMN Extension yellow sticky trap corn rootworm monitoring network) can aid in hybrid selection as one seeks to plant hybrids with Bt traits, and/or apply an in-furrow insecticide, or rotate the field out of corn to manage rootworms. 

With new RNAi traited hybrids entering the marketplace, there is some concern that rootworms resistant to both Bt and RNAi will soon follow. To slow the speed of resistance development in rootworms to RNAi, it is not recommended to deploy these hybrids in fields with very high rootworm population densities and Bt effectiveness has declined, as rootworm populations would then just be exposed to a single management tactic, under which resistance is more likely to occur.  

For those that missed this session, it is now available to view on YouTube at https://youtu.be/_R9Hbb7M5lA. For more information and to register to attend other weekly session through the end of March, visit z.umn.edu/strategic-farming.