Agronomists Warn Corn Rootworm Pressure is Rising
Producers have barely even begun to harvest this year’s corn crop but already need to start making decisions for next year. One of those is assessing corn rootworm threat, which agronomists say is likely to be high in 2021.
Ron Beyer, a Golden Harvest agronomist who works with farmers in northwestern Iowa says, “Particularly in corn-on-corn acres, we’re seeing some of these populations just explode.” He also warns about the looming risk of northern corn rootworm, the numbers for which were high in some northwestern Iowa fields in 2019 due to extended diapause, says Beyer. Extended diapause occurs when eggs laid by predominant northern corn rootworm beetles remain dormant in soil through the next year when soybeans are planted.
Other agronomists across the Corn Belt cite similar findings. Regardless of the reasons – from near-perfect growing conditions, which translated to good insect-breeding conditions in some areas, to delayed planting or overly dry conditions this spring in other areas – the bottom line seems to be that CRW pressure is higher than normal this year. Andy Heggenstaller, head of agronomy for Syngenta Seeds, said in late August that the company’s CRW monitoring program was showing the heaviest pressure in northern Illinois, Iowa, Wisconsin, southern Minnesota, Nebraska, and Colorado. Heggenstaller said at the time they were some of the highest CRW populations they have recorded in the past five years.
CRW has managed to thwart every control measure that agriculture has thrown at it since scientists first discovered the northern corn rootworm in 1820 followed by western corn rootworm in 1867, the two most destructive species. Across the U.S. Corn Belt, it is considered the number one threat to corn yields, causing an estimated $1 billion in losses every year.
Crop rotation with soybeans began to fail on a mass scale in the mid-90s, which new Bt rootworm-resistant seed traits hoped to resolve. However, a number of factors led to the first case of western corn rootworm resistance to Bt corn in Iowa in 2009. Since then, resistance to the four types of Bt proteins used in traited hybrids has been on the rise across the Corn Belt. Below are more details about corn rootworms and some of the tools recommended to help mitigate their damage:
- Lifecycle: Western and northern corn rootworm have only one generation per year. Eggs of both species are deposited in the soil by female beetles from mid-summer until autumn. The eggs overwinter and begin hatching from late May to early June in most areas of the Midwest. The newly hatched larvae seek out and feed on corn roots. The larvae pass through three stages, or instars, before pupating in the soil. Primary larval damage is caused by the later instars, typically from mid-June through mid-July.Since beetles lay their eggs almost exclusively in cornfields, populations of rootworm larvae used to be successfully avoided by following a good corn rotation plan. However, in the Midwest, northern rootworm biotypes have been found that have adapted to an every-year rotation. These biotypes remain in the egg stage over the following season and hatch the next season when corn is again in the field.
- Crop Damage:Larval feeding on corn roots may reduce yields. Injured root tips feature brown lesions. In some fields, entire nodes of roots may be pruned severely. Pruned roots are less capable of supplying water and nutrients to the growing ears and moderate to severe root pruning may result in lodging and significant losses at harvest. Larval injury also may make roots more susceptible to root and stalk rot fungi. High adult densities may clip silks resulting in poor pollination and reduced kernel set.
- Scouting:Scouting for corn rootworm adults is important not just to protect corn pollination (obviously too late this year) but also to determine the damage potential for next year’s crop from high adult populations laying eggs during the current growing season. Farmers can evaluate the impact of rootworms by digging and washing corn roots in late season. Learn more about evaluating roots HERE .
A 2013 Iowa State University study determined a 1.5-rootworm-beetle-per-sticky-trap-per-day level in soybeans (or two beetles per trap per day in corn) indicates rootworm damage may occur the following year, says Joe Spencer, University of Illinois (U of I) entomologist. Measuring rootworm damage in a field is also critical to assess whether or not a resident rootworm population might be resistant to a particular Bt trait package. The Iowa State Nodal Injury Scale (NIS), developed by Iowa State Entomologists, is the industry standard for quantifying root damage and focuses on root pruning caused by late instar larvae. For reference, a rating of 0.5 (one half of one node pruned) is considered unexpecteddamage to a pyramided Bt corn plant, and could be evidence of resistance.
- Mitigation:In cases where resistance is evident, it’s recommended preventive measures like rootworm traits and/or a soil-applied insecticide be used, says Spencer. Many experts recommend growers that find unexpected damage in traited hybrids should rotate to soybeans in 2021. Dr. Nick Seiter, Field Crop Entomologist from U of I says the worst thing a producer could do from a resistance-management standpoint would be to plant continuous corn with the same trait package after observing unexpected damage in a field the previous year. “As we have seen with herbicide resistance over the last several years, over-reliance on the same tools in the same fields will yield a predictable outcome,” says Seiter. (Sources: AgInsider, Bayer Crop Science, University of Illinois, NCSTate Extension, AgDaily)
