Figure 1. CRW larva.Northern corn rootworm (NCRW) is a significant pest in the Corn Belt and has adapted to crop rotation in areas by a mechanism known as extended diapause.
The NCRW populations exhibiting extended diapause can survive two or more years as eggs in the soil until corn returns to the crop field.
An integrated management approach should be leveraged for this adapted corn pest.
Corn Rootworm has been a perennial problem for most corn growing regions since the 1940s. The pest has thrived in Midwestern areas where corn acres are dense. Repeatedly planting fields to corn over consecutive years is advantageous to Corn Rootworm survival since eggs laid by adult beetles in the summer will not hatch until being exposed to a period of cold temperatures throughout the winter. This period of over wintering referred to as “diapause” normally only occurs the first year before hatching begins the following spring. Presence of a host crop like corn to feed on the following spring is therefore critical to larva (Figure 1) survival the following season. The unique biology of this pest has historically allowed crop rotation to be a highly effective management practice up until more recent years.
Corn Rootworms Evolve to Overcome Rotation
Figure 2. NCRW beetle adult.There are four species of corn rootworms present in North America, but Western Corn Rootworm (WCRW) and Northern Corn Rootworm (NCRW) are the most economically important. Adult NCRW are uniquely identified by the solid green color of the elytra wing cover (Figure 2) and are recently getting more attention due to how their lifecycle has evolved. NCRW has genetically adapted to corn-soybean crop rotations by extending its diapause period, where the eggs remain dormant in the soil for multiple years before hatching. Delaying egg hatch for multiple seasons gives NCRW more opportunities to reestablish itself in a year following soybeans. This phenomenon has yet to be observed in the WCRW species and adds complexity to decision making processes when crop rotation has been serving as the primary management practice. Extended diapause NCRW populations were first observed in the mid-1980’s and have fluctuated in presence over the years. In 2023, Golden Harvest agronomists started seeing larger numbers of NCRW beetles and root damage from larva in rotated corn fields, indicating extended diapause was likely present. Distribution of 2023 observations largely aligned with historically reported geographies where extended diapause was known to exist, although it was also found in a few counties outside the normal range (Figure 3). Observations from 2023 are a good indicator that NCRW may reappear in the same fields in the 2025 season, even if soybeans are planted in 2024.
Understanding Extended Diapause
Understanding NCRW extended diapause is crucial for effectively managing the pest. All corn rootworm eggs need to diapause over the winter before being able to hatch in the spring. Repeated use of crop rotation has imposed a selection pressure for NCRW individuals with a longer diapause duration that gives the best chance for survival. This adaptation of lengthening the overwintering dormancy period to span two (up to four) winters allows the rootworm population to survive crop rotation or harsh environmental conditions. Research shows that the extended diapause trait is not ubiquitous across all NCRW populations, meaning that not all eggs will delay hatching for multiple seasons.1 It can be common for 50-60% of the eggs to hatch the first year, while the remainder hatch in following years.3,4
Figure 3. Geography of historically observed extended diapause in NCRW and the counties where NCRW extended diapause was observed by Golden Harvest Agronomists in 2023.Scouting to Determine Risk
Regular monitoring or scouting of corn fields for root injury and adult beetles can help evaluate the severity of infestation and future risk. Scouting can involve techniques such as pre-season DNA soil sampling analysis, in-season adult sticky traps, and late-season root evaluations. Corn Rootworm populations can be highly sporadic from field to field and within fields. Scout enough areas to fully represent whole fields.
Adult beetle traps can be a good indicator of future NCRW populations but can have challenges when estimating two or more years in advance. Traditionally yellow sticky traps thresholds of two or more corn rootworm beetles/trap/day have suggested that alternative management may be needed the following year if planting corn, regardless of species.2 Since NCRW beetles can migrate short distances from neighboring fields, it cannot be assumed that beetles observed in rotated corn fields are always a result of extended diapause. There is not a lot of data available to correlate sticky trap captures with NCRW extended diapause risk or injury but researchers in Minnesota have been using four or more NCRW per plant as a threshold for determining risk of extended diapause in corn-soybean rotations. The higher threshold levels account for an additional year of egg mortality and consider that only a portion of the population will have delayed hatch.1 Digging roots and noting larvae feeding in rotated corn fields is a better indicator of extended diapause when assessing risk of future problems. It may still be possible to have root damage in rotated fields that was not due to extended diapause if volunteer corn was present and attracted beetles into soybean fields the prior year.
Management Strategies
It is important to note that the management of NCRW extended diapause requires a comprehensive and adaptive approach. Individual management practices such as short-term crop rotations may no longer provide adequate protection if NCRW extended diapause is present in an individual field. Regular monitoring and understanding of local population dynamics is critical to developing long term economical solutions.
Multiple management practices exist for protecting against NCRW species with extended diapause
Longer Crop Rotation Durations: Short term rotations involving corn every other year may no longer be effective, but diversifying rotations with a third-year non-host crop or multiple years of alfalfa can still be effective where possible.
Corn Rootworm Traits: Dual mode of action corn rootworm traits like Duracade® and Agrisure® Total trait stacks can be highly effective. Be mindful that repeated use of the same trait could select for resistant rootworm populations. Rotating management options and modes of action can help minimize this.
Soil-Applied Insecticide: Multiple options now exist for applying soil-applied insecticides such as Force® through planters.
Foliar insecticides: Well-timed foliar insecticide applications can effectively reduce the number of gravid females prior to laying eggs. Multiple applications may be needed to effectively control beetles that have emerged at different timings.
References
1Yang, F. and B. Potter. 2023. Northern corn rootworm and extended diapause problems increase in areas of Minnesota. Minnesota Crop News, University of Minnesota Extension. https://blog-crop-news.extension.umn.edu/2023/07/northern-corn-rootworm-and-extended.html
2Dunbar, M. and A. Gassmann. 2013. Abundance and distribution of western and northern corn rootworm (Diabrotica spp.) and prevalence of rotation resistance in eastern Iowa. J. Econ. Entomol. 106: 168-180.
3Krysan, J.L., J.J. Jackson, and A.C. Lew. 1984. Field termination of egg diapause in Diabrotica with new evidence of extended diapause in D. barberi (Coleoptera: Chrysomelidae). Environmental Entomology, 13: 1237-1240.
4Levine, E., H. Oloumi-Sadeghi, and J.R. Fisher. 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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