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Northern corn rootworm and extended diapause problems increase in areas of Minnesota

Dr. Fei Yang, Extension corn entomologist, and Bruce Potter, Extension IPM specialist

A new old problem

Northern corn rootworm beetle.
In southern Minnesota, western corn rootworm (WCR) tends to be the dominant species in continuous corn. Likely due to greater cold tolerance of overwintering eggs, the northern corn rootworm (NCR) increasingly becomes the dominant species as one moves north within the state. 

While significant Minnesota WCR populations have been limited to continuous corn, a portion of NCR has adapted to short corn rotations with an extended diapause genetic trait. Most recently, rotation-resistant NCR issues have arisen in the early to mid-1980s, early-2000s. These past events have been geographically limited in scope, rising and falling with overall NCR population densities.

Minnesota’s NCR populations have been generally low since 2005. In recent years, most corn rootworm management has focused on WCR in continuous corn in the southern part of Minnesota. Meanwhile, over the past few years, the NCR populations have been quietly on the rise in the background. During 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.

The extent of rootworm injury from both species is still unclear for 2023 as areas of drought have likely intensified the impact of root injury on yield while minimizing lodging. However, this year, more agriculturalists have observed root-lodged rotated corn with rootworm-injured roots and NCR beetles. Portions of West Central Minnesota are now seeing NCR extended diapause problems for the first time.

So far, extended diapause issues have primarily been observed in hybrids without a Bt corn rootworm trait. This differs from WCR where Bt resistance has been a concern in Minnesota since 2009. However, NCR populations collected in 2016 have been documented to be resistant to Bt traits in North Dakota (Calles-Torres, et. al., 2019). As rootworm management strategies are developed, identifying Bt-RW fields with unexpected injury due to NCR is important.

This year, a mosaic of highly variable crop stages caused by drought-delayed corn emergence, delayed planting, or replanting of drowned areas is influencing beetle emergence and leading to the aggregation of beetles in particular fields or areas within fields. This aggregation increased concerns over silk feeding affecting pollination. It will also influence which fields, or areas of fields, will see the most egg rootworm laying.

What is extended diapause?

A portion of Minnesota NCR populations display the extended diapause trait. Simply, nearly all NCR eggs need to rest or diapause during the winter before hatching in the following spring. The extended diapause trait delays hatching, with diapause spanning two winters (a small proportion of extended diapause eggs may delay hatching for up to four winters). This 2-year extended diapause trait is advantageous to larval survival in every-other-year rotated corn fields. Further, other than exposing eggs to an additional year of weather and predation, there is no penalty for the trait in continuous corn.

Research from multiple states shows that the percentage of extended diapause in an NCR population can vary. Krysan et al. (1984) reported 40% from eastern South Dakota, while Levine et al. (1992) reported 14 to 51% in IL, with variation based on geographic locations. French et al. (2014) found that extended diapause was heritable and influenced by the NCR female.

NCR beetles are very mobile. When fresh silks or pollen are not available, they will readily leave a cornfield to forage. Populations 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. Unlike the WCR variant that occurs in the eastern corn belt, our current knowledge is that foraging NCR beetles return to corn fields to lay eggs.

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.

So now what?

The increased extended diapause populations have complicated corn rootworm management. Until NCR populations decline once again, more rotated corn fields will require rootworm management while the mobility of NCR and extended diapause makes it important to pay attention to details.

1) Determine your field's risk

Keep in mind that rootworm populations are very field-specific. Use whole plant counts or yellow sticky traps to determine rootworm populations. Use scouting to help evaluate your fields’ risk from NCR and extended diapause. However, placing too few traps or scouting only a small part of the field is likely to provide information that is misleading and perhaps costly.

Because NCR beetles are so mobile, they may require more traps per field to adequately assess their populations. Different trap types, and perhaps even color differences among dye lots, can also affect capture rates. For example, NCR captures are higher on Sentry Multigard than Pherocon AM traps (Calles-Torrez, et al., 2020; Leaf and Ostlie unpublished).

Whole plant counts can vary by the field’s developmental stage, day, and even time of day especially early and 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 MN, 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. (MN data suggests it can be as high as 4 or more (Leaf and Ostlie, unpublished). We do not have a lot of 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.

2) Develop a strategy for NCR high-risk fields.

Hybrids with vigorous root systems may better tolerate rootworm feeding to some extent. Controlling broadleaf weeds such as ragweeds and pigweeds will help prevent aggregating NCR beetles in or near the field.

Consider an effective at-plant insecticide if your planter allows and previous years' NCR pressure indicates risk for continuous or rotated corn.

Currently, Bt-RW traited hybrids still appear effective on many NCR populations. Consider a Bt-RW hybrid for rotated fields at high risk for NCR. Be wary of Bt-resistant WCR in continuous corn fields with mixed NCR and WCR populations. Expect overuse of Bt to accelerate the development of resistant NCR populations.

Foliar insecticide applications for adult control are an option but their timing is different from insecticide applications for silk clipping or from fungicide applications. As long as Bt hybrids remain effective, “beetle bombing” will have limited utility for NCR.

For fields with extremely high beetle populations, an extra year between corn crops (for example corn, soybeans, wheat, and corn) can reduce populations of extended diapause NCR. Small grain and broadleaf crops in general, and sorghums are other non-hosts. We do not know of NCR populations where more than 50% of the individuals have extended diapause. Therefore, planting a second year of corn is unlikely to trip up extended diapause NCR and can make things worse.

The areas with extended diapause this year are likely to see problems in 2024 rotated corn. Additionally, areas with high NCR beetle populations in the fall of 2022 are also likely to see issues in 2024.

We would appreciate hearing of field locations with extended diapause and fields with NCR problems in fields planted to Bt-RW traited corn hybrids.

Selected references

Calles-Torrez, V., Knodel, J.J., Boetel, M.A., French, B.W., Fuller, B.W. and Ransom, J.K., 2019. Field-evolved resistance of northern and western corn rootworm (Coleoptera: Chrysomelidae) populations to corn hybrids expressing single and pyramided Cry3Bb1 and Cry34/35Ab1 Bt proteins in North Dakota. Journal of economic entomology, 112: 1875-1886.

Calles-Torrez, V., M.A. Boetel and J.J. Knodel. 2020. Corn rootworm survey in North Dakota and a comparison of two sticky traps. J. Appl. Entomol. 144: 897–910.

French, B.W., Coates, B.S. and Sappington, T.W., 2014. Inheritance of an extended diapause trait in the N orthern corn rootworm, Diabrotica barberi (Coleoptera: Chrysomelidae). Journal of Applied Entomology, 138: 213-221.

Krysan, J.L., Jackson, J.J. and Lew, A.C., 1984. Field termination of egg diapause in Diabrotica with new evidence of extended diapause in D. barberi (Coleoptera: Chrysomelidae). Environmental Entomology, 13: 1237-1240.

Levine, E., Oloumi-Sadeghi, H. and Fisher, J.R., 1992. Discovery of multiyear diapause in Illinois and South Dakota northern corn rootworm (Coleoptera: Chrysomelidae) eggs and incidence of the prolonged diapause trait in Illinois. Journal of Economic Entomology, 85: 262-267.

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