By Liz Stahl
Although European corn borer (ECB) populations dropped dramatically after widespread adoption of Bt-corn hybrids, the discovery of Bt-resistant populations reminds us to not let our guard down on this pest. Corn rootworm (CRW) is another major corn pest in Minnesota where resistance to Bt traits has led to management challenges. ECB and CRW were the topic of discussion on the March 13th, Strategic Farming: Let’s Talk Crops program with Dr. Fei Yang, Extension Entomologist with University of Minnesota Extension.
European corn borer (ECB)
European corn borer, specifically the larva, caused significant yield losses and economic damage throughout the U.S. Corn Belt prior to the introduction of Bt hybrids. Injury from ECB can lead to stalk breakage, ear droppage, stalk rots, ear rots, mycotoxins, and ultimately yield losses. Control was particularly challenged in MN by the existence of two biotypes, the univoltine biotype, which has only one generation in a year, and the multivoltine biotype, which has at least two generations each year.
Because ECB larvae burrow into the stalks and ears of corn, timing an insecticide application for effective control is challenging. Transgenic Bt corn replaced the use of insecticides for ECB control in many fields, providing excellent control of ECB for almost 27 years in the U.S.
ECB resistance to the Bt technology
Practical resistance in ECB to Bt corn (Cry1F) was first found in four fields in Nova Scotia, Canada in 2018. Since then, practical resistance to Cry1F and other Bt proteins has been documented to widely expand in other locations in Canada, and in 2023 practical resistance in ECB to Bt sweet corn (Cry1A.105 + Cry2Ab2) was found in the U.S. in Connecticut. Although resistance in ECB to Bt corn is still rare in the U.S., insect resistance to Bt proteins poses a significant threat to the long-term sustainability of this technology.
The close proximity of the Canadian populations to MN is concerning as ECB moths can migrate long distances. Monitoring for resistance, use of a high dose/refuge strategy, and pyramiding or stacking traits in hybrids have been effective so far at fending off and slowing down resistance.
University of MN Extension surveyed fields across MN in 2023 for ECB in research supported by the MN Corn Research and Promotion Council. Overall ECB populations were very low. In one non-Bt field in Crookston MN, however, approximately 30 to 35% of the corn plants had been damaged by ECB. ECB larvae were collected, then reared and tested in the lab for resistance. Fortunately, resistance was not detected to any of the Cry Bt proteins currently on the market for control of ECB. Continued monitoring is needed, however, and Dr. Yang requests anyone who sees a problem developing in a field to contact him either using this online report form or email at yang8905@umn.edu.
Corn rootworm (CRW)
As with ECB, the larval stage of CRW causes the most damage to corn, but in this case by feeding on corn roots. Root feeding can hurt corn yield since injured roots take up less water and nutrients, and when severe, can result in root lodging, which can hinder photosynthesis, pollination and harvestability. Feeding wounds can also create entry points for root and crown rot pathogens. Adult CRW beetles can also injure corn by feeding on silks, which can hinder pollination. This feeding can also create an entry point for sap beetles and ear molds.
There are two main species of CRW in Minnesota: The western corn rootworm (WCR) and northern corn rootworm (NCR). WCR tends to be the dominant species in southern MN, while the balance shifts to NCR as you move further north in state. One reason for this shift is the increased cold tolerance of NCR eggs compared to WCR eggs.
Since WCR eggs hatch the following year, problems with WCR are most significant in continuous corn. The WCR variant that lays eggs in soybean fields has not been detected in MN. As a result, rotation is still the number one way to manage WCR. Control of volunteer corn in soybean, however, is important for this strategy to work as WCR beetles will be attracted to volunteer corn in soybean fields, laying their eggs in the volunteer corn.
The first evidence of field-evolved resistance in WCR to Bt-corn (Cry3Bb) was in 2009, six years after commercialization. Resistance to Cry3 and Cry34/35 Bt proteins has since been detected in states throughout the Corn Belt including MN, IA, NE, and ND.
Northern corn rootworm (NCR)
After staying generally low since 2005, populations of NCR are on the rise in Minnesota. Although a portion of NCR eggs will hatch the following year, NCR exhibits extended diapause, where a significant portion of the egg population will hatch two or more years after being laid. This complicates the management of CRW since rotating to soybean every other year can still result in injury from CRW when corn is grown in the rotation. There are many unknowns around extended diapause, and it is currently unclear how many populations show this trait in the state.
Resistance to Bt proteins (Cry3 and Cry 34/35) in North Dakota in NCR was reported in 2019 from a population collected in 2016. Resistance in NCR to these same proteins in Minnesota was reported in 2023 from a population collected in 2019.
The quest to better understand CRW
There are currently many gaps in our understanding of the biology, ecology, genetics and resistance to Bt proteins in NCR and WCR. A better understanding of extended diapause in NCR could also help in the management of this pest.
Dr. Yang has started a greenhouse trial to evaluate the interactions and population dynamics of WCR and NCR. Initial results indicate that WCR tends to out-compete NCR in most conditions, although many factors influence the relationship between these two species. A study conducted two decades ago showed that at 0.5 F, you can see 100% mortality of WCR but only 20 to 50% mortality of NCR. However, given the ongoing changes in climate and the adaptive evolution of insect pests, it's plausible that the biological traits of these species, such as their supercooling and cold hardiness abilities, have significantly evolved. Consequently, current research is focusing on the effects of cold temperatures on WCR and NCR mortality to better predict their population densities and geographical distributions.
Management strategies:
Scout fields to assess the potential for CRW issues the next time corn will be planted in the field. This can be done by using yellow sticky traps to monitor adult corn rootworm beetle populations and/or by digging roots during mid-late July to check for feeding.
Crop rotation is the still the number one tactic to help manage CRW. Check what Bt traits are in the hybrids you plant, rotating effective sources when feasible, and consider pyramid stacks and the use of an insecticide at planting in fields where you anticipate heavy CRW pressure. Rarely would adult CRW beetle control be called for in field corn in Minnesota, and insecticide resistance in CRW has limited the effectiveness of this practice where it has been historically used.
Dr. Yang noted that RNAi offers a different mode of action, or way of controlling CRW, than the proteins in Bt hybrids, but it takes longer to kill CRW than an effective Bt trait. This technology must also be managed with resistance in mind.
Join the webinar series
Join us next week when we welcome Dr. Kiersten Wise, plant pathologist at the University of Kentucky to discuss “Efficacy matters: Fungicides for managing specific corn and soybean diseases”.
University of Minnesota’s Strategic Farming: Let’s talk crops! webinar series, offered Wednesdays through March, features discussions with specialists to provide up-to-date, research-based information to help farmers and ag professionals optimize crop management strategies for 2024.
Source : umn.edu