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Strategic Farming: Let's talk crops! session talks corn insect pests

western corn rootworm, and WCR feeding injury
By: Angie Peltier, UMN Extension crops educator

On January 18, 2023, Bruce Potter, University of Minnesota Extension IPM specialist, and Dr. Anthony Hanson, UMN Extension IPM educator, joined UMN Extension crops educator Ryan Miller for a wide-ranging discussion of insect pests of corn in Minnesota. This was the second episode of the 2023 Strategic Farming: Let’s talk crops! webinars in this series.

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European corn borer (ECB)

In recent years, UMN Extension entomologists, specialists and educators have conducted a European corn borer survey in Minnesota, continuing a tradition that began in 1963 by the Minnesota Department of Agriculture. This survey serves multiple purposes, including improving our understanding of: 
  1. Whether ECB Bt traits are still effective
  2. How Bt hybrids affect ECB population dynamics in fields planted to hybrids with and without ECB Bt traits
  3. The footprint of different ECB biotypes (univoltine=single generation per year; bivoltine=two generations per year; or a combination)
Briefly, 10 plants in each of two corn fields sampled randomly and two fields that are known to be growing non-Bt corn per county are surveyed for ECB feeding injury; those plants with injury are dissected to count the number of larvae and measure tunnel length, which is related to yield loss.

Since the surveys began, ECB population densities hit a peak of nearly 400 borers per 100 plants in 1995, the year before genetically modified hybrids expressing a Bt protein specific to ECB were available to plant. On average fewer than 2 borers per 100 plants have been found in the survey since that time.

While there is no substitution for scouting one’s own corn fields for ECB injury symptoms, by and large the recent results of these surveys point to several conclusions: 
  1. Overall the Bt trait is resulting in area-wide suppression of ECB populations even in fields not planted to Bt corn
  2. The ECB Bt trait is still effective against ECB larvae. 
Scouting is recommended for those that plant non-Bt corn, as there is a narrow window between when larvae emerge from their eggs and when they begin boring into the stalk, where insecticides can no longer reach the larvae. Scouting of Bt fields is also recommended to quickly identify any Bt trait failures .

In addition to ECB feeding injury and larvae, UMN Extension personnel are also examining corn plants for both signs of the tar spot fungus, a pathogen capable of causing significant yield losses in Minnesota corn, and stalk rot.

Cover Crops and Insect Pests

Questions submitted in advance by people registering for the 2023 Strategic Farming: Let’s Talk Crops! sessions indicated that several were interested in learning more about how adding a cover crop to the larger cropping system can impact insect pests in the cash crop.

True armyworms (TAW) were an issue for some people’s cash crops in 2022 that had seeded a rye cover crop in fall 2021. Like other migratory pests that are unable to survive tough northern winters, armyworm moths travel north with weather systems each spring from where they spend their winters in the southern US. While it is difficult to predict where TAW moths will drop out of the storms that bring them north, what is certain is that they will be attracted to lush, green grasses (such as a rye cover crop) as a spot to lay their eggs. Once the rye is terminated with an herbicide, TA larvae will be looking for another plant to feed on, making cash crop seedlings ‘appetizing’. Although soybeans are as likely to suffer significant armyworm feeding injury, corn crops grown in fields with a rye cover crop are at particular risk. Corn fields with heavy grass weed pressure is also vulnerable.

While rare in MN, fall armyworms arrive later in the growing season than TAW and can lay their eggs in a newly planted rye cover crop, injuring seedlings and both armyworm species can move into corn from lodged grasses in field borders.

Timely termination of your rye cover crop and scouting your cash crop for armyworm feeding injury and larvae are two important practices for managing armyworms.

Corn Rootworms

There are two corn rootworms that affect corn roots in Minnesota, northern corn rootworm (NCR) and the western corn rootworm (WCR). Eggs are laid in cracks in the soil, where they spend the winter, emerging as larvae in the spring to feed on corn roots, negatively affecting both water and nutrient uptake. Although a rare phenomenon, when present in high numbers, adult rootworms can cause severe feeding injury on corn silks which can both negatively impact pollination and increase the risk of ear mold diseases. Continuous corn fields, fields with high beetle populations, many continuous corn fields in the area, fields with early or late silking corn (including volunteer corn) and using the same management tactic repeatedly are at greater risk of building damaging population densities and sustaining significant rootworm injury.

NCR tends to be more mobile and its eggs more cold-tolerant than WCR, and recently there has been some indications that NCR population densities in northwest Minnesota may be increasing. NCR, which predominate in more northern MN corn fields, are also important because of a growth habit called “extended diapause” in which some populations have developed a preference for laying their eggs in soybean crops only for their larvae to hatch the following spring into fields that have been planted to corn – essentially crop rotation-resistant rootworms. WCR populations have not yet been documented to undergo extended diapause in Minnesota.

Western corn rootworms have a long (or should we say infamous) history since the 1950’s of evolving to resist many of the management tactics we used to combat them, from resistance to multiple insecticide active ingredients to rotation resistance (in Illinois and Wisconsin). After generations of battling this pest, when Bt hybrids specific to rootworm were first released for wide-spread sale in 2005, there were high hopes for effective management. When resistance to specific Bt-proteins began to be identified in 2009, a mere 6 years after their release, managing rootworms again became a bit more complicated.

Resistance to individual rootworm-specific Bt traits has been documented in both NCR and WCR populations. Consequently, knowing a bit about your field’s corn rootworm population (maybe through being a part of the UMN Extension yellow sticky trap corn rootworm monitoring network) can aid in hybrid selection as one seeks to plant hybrids with Bt traits, and/or apply an in-furrow insecticide, or rotate the field out of corn to manage rootworms.

With new RNAi traited hybrids entering the marketplace, there is some concern that rootworms resistant to both Bt and RNAi will soon follow. To slow the speed of resistance development in rootworms to RNAi, it is not recommended to deploy these hybrids in fields in which Bt is no longer effective, as rootworm populations would then just be exposed to a single management tactic, under which resistance is more likely to occur.

Fielding audience questions

Potter and Hanson answered many audience questions including about managing insect pests in organic corn production systems, how the 2022 dry fall will affect pests in 2023, how the 2022/2023 winter conditions will affect winter survival, whether there is an interaction between N and P deficiency and rootworm injury, how the RNAi technology is most likely to be deployed.

Thanks to the Minnesota Soybean Research & Promotion Council and the Minnesota Corn Research & Promotion Council for their generous support of this program!

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