European corn borer survey - 2017-2021: Northwest Minnesota

This article was written by Angie Peltier and Bruce Potter (UMN Extension) and Eric Burkness and Bill Hutchison (UMN Dept of Entomology).

Purpose of Study

European corn borer (ECB) larvae tunnel into stalks and ear shanks (Figure 1). Feeding affects the transfer of water and nutrients within the plant and can directly affect yield by reducing kernel weight and number.  ECB feeding can indirectly affect yield when tunnels cause stalk breakage, ear drop, or allow the entry of stalk rot and ear mold fungi.

Figure 1. European corn borer (Ostrinia nubilalis). Clemson University, USDA Cooperative Extension Slide Series, Bugwood.org.

ECB and Bt corn. 

More than 25 years ago scientists found a way to transfer a gene from a soil-borne bacterium called Bacillus thuringiensis (Bt) into the corn genome. Bt corn was approved for commercial use in 1996. Within the corn plant tissues, this gene produces a protein toxic to corn borer larvae. When ingested by larvae, the protein breaks down to a toxin which kills larvae by allowing mid-gut contents to leak into the rest of the body cavity. Additional Bt traits that target different above- and below-ground insect pests have since been incorporated into some hybrids.

The only way to manage ECB before Bt corn was developed, was to closely monitor ECB moth flights and scout for larvae and egg laying. If ECB populations warrant, foliar insecticide applications can provide control if they are carefully timed as the larvae are only susceptible to insecticides for 10 to 14 days. After that time, 3^rd instar larvae begin to tunnel into the stalk, ear or ear shank where they are protected from insecticide applications. This timing can be difficult particularly in areas of the state where both a single generation and multiple generation biotypes of ECB exist. Historically, the single generation (univoltine strain) has predominated in NW Minnesota.

Even the best-timed insecticide application is less effective than growing a hybrid with the Bt trait. Depending on the hybrid and trait package Bt corn can cost up to $20/acre more than conventional seed. In the current economic environment, $20/acre is a big deal and is a major driver of non-Bt corn hybrid seed purchases. During the past 4 years in MN, Bt corn use for above-ground traits for stalk and ear pests has ranged from 85-88% (USDA average).

High adoption of Bt corn has also occurred in NW MN. This has resulted in area-wide suppression of ECB populations, protecting even the non-Bt acres.

Study Objectives. 

Some objectives of the MN Corn Research & Promotion Council-sponsored 2017-2021 fall ECB survey in NW MN are to answer the following questions:

1) Are there changes in ECB population densities over time?

2) To what extent does the area-wide suppression effect occur in the NW?

3) Have any population shifts taken place? ie. is the Bt trait still effective (Bt-resistant corn borer have been found in eastern Canada but fortunately they are a different strain than occurs in MN)  and does ECB in NW MN continue to only produce a single generation of larvae each year? Understanding the number of generations per year is essential for managing ECB in non-Bt corn.

Results

During 2017, 2018, 2019, 2020 and 2021 a total of 13, 30, 55, 28 and 43 commercial fields were surveyed in NW MN, respectively (Figure 2, Table 1). Among the randomly surveyed fields there were also 3 known non-Bt fields in 2017, 21 in 2018, 36 in 2019, 8 in 2020 and 29 in 2021. The data presented in Table 1 summarize the per plant average number of ECB larvae in surveyed fields by year and Bt status.  In 1995, before the 1996 release of ECB Bt hybrids, an average of 1.16 ECB larvae per plant were found in NW MN corn plants.  In 2017 through 2019, randomly surveyed corn fields (likely a mix of Bt and non-Bt fields) had an average of 0 to 0.020 larvae per plant, while the average number of larvae per plant in non-Bt corn fields ranged from 0.0190 to 0.1472 larvae per plant. When compared to randomly surveyed fields, in 2017 there were more than 3.3 times the number of larvae per plant in the non-Bt fields; similarly, when compared to randomly surveyed fields, in 2019 there were more than 14 times the number of larvae per plant in the non-Bt fields. 

Figure 2. Fields in the northwest crop reporting district surveyed for European corn borer in 2017-2021.

 

Table 1. NW MN crop reporting district data for European corn borer larvae and percentage of fields infested in field corn, Minnesota 2017-2021. Baseline data for 1995, prior toBt corn commercialization is also shown (non-Bt fields)*

ECB population densities were very low in all surveyed fields in 2020 and 2021. This may indicate that, even though overall ECB populations are low, they still follow the historical cycle entomologists believe is related to a fungal disease and other parasites causing dramatic declines in high ECB populations every 6-7 years. An additional factor that might have impacted population densities of larvae within plants, is the historic extreme drought conditions that prevailed in NW MN in 2021, as mortality of both eggs and early larval instars has been associated with uninterrupted periods of hot, dry weather. Another key factor is likely the high Bt use rates in NW MN.

Want to learn more?

For additional information about the European corn borer and ECB management, visit this UMN Extension European Corn Borer website. 

This survey was sponsored by the Minnesota Corn Research & Promotion Council.