Dr. Fei Yang, University of Minnesota, Extension Corn Entomologist, and Dr. Yucheng Wang, University of Minnesota, Research Associate
Corn farmers in Minnesota continue to face significant challenges from the corn rootworm (CRW) complex, primarily the Northern Corn Rootworm (NCR) and Western Corn Rootworm (WCR). Crop rotation has traditionally been a reliable method for managing CRW, as adults predominantly lay eggs in corn fields and the larvae survive only on corn roots. Rotating corn to other crops will disrupt the CRW life cycle and eliminate local populations. While significant Minnesota WCR populations have been limited to continuous corn, a subset of NCR has adapted to crop rotation through an extended diapause trait, which allows eggs to remain dormant in the soil for two or more winters. This adaptation enables larval survival in rotated fields when corn is planted every other year.
In general, nearly all NCR eggs need to rest or diapause through the winters before hatching. However, a proportion of NCR eggs with extended diapause trait can delay hatching, with diapause spanning two or more winters (a small proportion of extended diapause eggs may delay hatching for up to four winters). This extended diapause trait is advantageous to larval survival in every-other-year rotated corn fields.
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| Northern corn rootworm will leave corn to forage on other plants. Photo: Bruce Potter |
In general, nearly all NCR eggs need to rest or diapause through the winters before hatching. However, a proportion of NCR eggs with extended diapause trait can delay hatching, with diapause spanning two or more winters (a small proportion of extended diapause eggs may delay hatching for up to four winters). This extended diapause trait is advantageous to larval survival in every-other-year rotated corn fields.
Research shows that the percentage of extended diapause in an NCR population can vary. Extended diapause in NCR was first reported by Chiang in 1965 in Minnesota, where it was present in only 0.3% of the population. Subsequent studies revealed much higher frequencies: up to 40% in eastern South Dakota by Krysan et al. in 1984, and 14–51% in Illinois by Levine et al. in 1992, with geographical variability. In Minnesota, rotation-resistant NCR issues arose in the early to mid-1980s, early-2000s. During the extended diapause outbreak in the early 2000s, damage was higher in odd vs. even numbered years. These past events have been geographically limited in scope, rising and falling with overall NCR population densities. In addition, French et al. (2014) found that the extended diapause trait in NCR was heritable and potentially influenced by maternal factors.
In 2021, isolated fields of rotated corn with lodging and emerging NCR beetles were observed in areas of SW Minnesota. The situation escalated in 2022 when by late summer, growers and consultants started reporting large numbers of NCR beetles in landscapes across Minnesota. In 2023, more agriculturalists observed root-lodged rotated corn with rootworm-injured roots and NCR beetles. In 2025, the issue appears to have resurfaced, with numerous reports of damage in first-year corn fields (Figures 1 and 2).
In 2021, isolated fields of rotated corn with lodging and emerging NCR beetles were observed in areas of SW Minnesota. The situation escalated in 2022 when by late summer, growers and consultants started reporting large numbers of NCR beetles in landscapes across Minnesota. In 2023, more agriculturalists observed root-lodged rotated corn with rootworm-injured roots and NCR beetles. In 2025, the issue appears to have resurfaced, with numerous reports of damage in first-year corn fields (Figures 1 and 2).
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| Figure 1. Lodged plants (left) and damaged corn roots (right) by northern corn rootworm in first year non-Bt corn field in Minnesota in 2025. (Photo Credit: Fei Yang) |
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| Figure 2. Larvae (left) and adults (right) of northern corn rootworm in first year non-Bt corn field in Minnesota in 2025. (Photo Credit: Fei Yang) |
Notably, the recurrence of extended diapause problems in NCR seems to coincide with odd-numbered years. So far, most damage associated with extended diapause has been observed in hybrids lacking a Bt corn rootworm (Bt-RW) trait. This is unlike WCR, where Bt resistance has been widespread and a persistent concern in Minnesota since 2009. As rootworm management strategies continue to evolve, it is critical to identify and monitor Bt-RW fields experiencing unexpected injury from NCR to guide more effective resistance management approaches.
NCR beetles are very mobile. When fresh silks or pollen are not available, they will readily leave a cornfield to forage. The population of these beetles can be very high when attracted to early or late silking fields in an area. Asteraceae (e.g., ragweed, sunflowers) pollen is especially attractive to NCR beetles. Despite their mobility, current knowledge suggests NCR beetles will return to corn fields to lay eggs.
Importantly, the presence of NCR beetles alone in rotated fields does not confirm extended diapause. The NCR beetles you observe in corn may have emerged there, or they may have moved in from another field in the area, perhaps continuous corn. Don’t assume that NCR beetles you observe in rotated corn are extended diapause without evidence of rootworm-injured corn roots. However, the presence of high beetle populations may indicate a risk for corn in that field next year or in the case of extended diapause two years later.
The increased extended diapause populations have complicated rootworm management. Until NCR populations decline once again, more rotated corn fields will require rootworm management while the mobility of NCR and extended diapause make it important to pay attention to details.
Because NCR beetles are highly mobile, more traps per field may be needed to accurately estimate their populations. Trap performance can vary by type and even by dye lot. For example, Sentry Multigard traps generally capture more NCR beetles than Pherocon AM traps.
The whole plant counts are influenced by factors such as crop development stage, weather conditions, and time of day, especially early or late in the season. Will the field be rotated? The current threshold for continuous corn is ¾ - 1 WCR or NCR beetle/plant. Data on NCR extended diapause thresholds for rotated corn are sparse. In Minnesota, we have been using 4 or more NCR/plant to account for the additional year of egg mortality and only a portion of the population with the trait.
Sticky trap thresholds as low as 2 beetles/trap/day have been suggested. Minnesota data suggests it can be as high as 4 or more. We have very limited data correlating sticky trap captures with NCR subsequent extended diapause injury. However, a sticky trap network funded by the Minnesota Corn Research and Promotion Council might hold some clues as previously trapped rotated fields return to corn.
In summary, scouting is critical to assessing risk from NCR and extended diapause, but using too few traps or incomplete scouting methods may lead to misleading and potentially costly decisions. We would appreciate hearing of problem field locations experiencing extended diapause and NCR issues, and fields planted with rootworm-traited corn hybrids. Please contact me at yang8905@umn.edu
NCR beetles are very mobile. When fresh silks or pollen are not available, they will readily leave a cornfield to forage. The population of these beetles can be very high when attracted to early or late silking fields in an area. Asteraceae (e.g., ragweed, sunflowers) pollen is especially attractive to NCR beetles. Despite their mobility, current knowledge suggests NCR beetles will return to corn fields to lay eggs.
Importantly, the presence of NCR beetles alone in rotated fields does not confirm extended diapause. The NCR beetles you observe in corn may have emerged there, or they may have moved in from another field in the area, perhaps continuous corn. Don’t assume that NCR beetles you observe in rotated corn are extended diapause without evidence of rootworm-injured corn roots. However, the presence of high beetle populations may indicate a risk for corn in that field next year or in the case of extended diapause two years later.
The increased extended diapause populations have complicated rootworm management. Until NCR populations decline once again, more rotated corn fields will require rootworm management while the mobility of NCR and extended diapause make it important to pay attention to details.
Determine your field’s risk
Corn rootworm populations are highly field-specific, so accurate scouting is essential. Use whole plant counts or yellow sticky traps to assess beetle populations. Scouting only a small portion of the field or using just one or two traps per field may lead to unreliable conclusions and poor management decisions.Because NCR beetles are highly mobile, more traps per field may be needed to accurately estimate their populations. Trap performance can vary by type and even by dye lot. For example, Sentry Multigard traps generally capture more NCR beetles than Pherocon AM traps.
The whole plant counts are influenced by factors such as crop development stage, weather conditions, and time of day, especially early or late in the season. Will the field be rotated? The current threshold for continuous corn is ¾ - 1 WCR or NCR beetle/plant. Data on NCR extended diapause thresholds for rotated corn are sparse. In Minnesota, we have been using 4 or more NCR/plant to account for the additional year of egg mortality and only a portion of the population with the trait.
Sticky trap thresholds as low as 2 beetles/trap/day have been suggested. Minnesota data suggests it can be as high as 4 or more. We have very limited data correlating sticky trap captures with NCR subsequent extended diapause injury. However, a sticky trap network funded by the Minnesota Corn Research and Promotion Council might hold some clues as previously trapped rotated fields return to corn.
In summary, scouting is critical to assessing risk from NCR and extended diapause, but using too few traps or incomplete scouting methods may lead to misleading and potentially costly decisions. We would appreciate hearing of problem field locations experiencing extended diapause and NCR issues, and fields planted with rootworm-traited corn hybrids. Please contact me at yang8905@umn.edu
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