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Reducing Bt trait acres in 2024 Minnesota Corn Production? Implications for European corn borer

Anthony Hanson, IPM Extension Educator; Fei Yang & Bill Hutchsion, Extension Entomologists, Bruce Potter, Extension IPM Specialist; Angie Peltier, Ryan Miller, & Liz Stahl, Crops Extension Educators

If you plan to have non-Bt corn fields in 2024, we are looking for potential European corn borer fall survey sites. To volunteer non-Bt fields for the 2024 fall survey, click here or contact Fei Yang (yang8905@umn.edu) or Anthony Hanson (hans4022@umn.edu).

The economics of corn production challenge farmers to minimize production costs. Hybrid selection is one way to reduce costs. Planting corn hybrids without Bacillus thuringiensis (Bt) proteins for protection against European corn borer (ECB) (Fig. 1), corn rootworm, or both will reduce seed costs. However, farmers could inadvertently reduce crop revenues by selecting hybrids without carefully considering hybrid yield potential or insect populations in their fields.
Figure 1. European corn borer tunneling in corn stalk.

Yield potential is the first thing to consider when selecting a corn hybrid. Bt traits protect a hybrid’s yield potential, but yield benefits only occur when targeted insect population densities are above economic injury levels. When insect pressure is low or absent, economic benefit with trait-protected hybrids only occurs if higher seed costs are offset by greater yields. Switching to less-expensive non-Bt seed can be a good strategy when yields are comparable or when seed cost savings exceed any reduced yield potential plus prospective losses due to insect injury. In many 2023 fields, planting corn without a Bt trait can work well, if one recognizes and accounts for potential risk from ECB and other insect pests. 

Historical, current, and future ECB populations

Between its introduction to the state in the 1940s and the advent of corn hybrids with Bt traits in the mid-1990’s, ECB was responsible for significant yield losses throughout MN. After initially feeding on leaf tissue or pollen, 3rd stage larvae injure corn as they tunnel into stalks, ears tips, and ear shanks. This injury reduces yield by interrupting the flow of water and nutrients. Harvestability may also be reduced when affected stalks lodge or break, or when tunnels in the shank cause ear-drop. 

Multivoltine ECB populations, or biotypes, exist in the state. The univoltine biotype that produces a single generation each year was the first type introduced into the U.S. and historically predominated in the northern and central corn-growing areas of the state. After the adoption of Bt, higher ECB densities have typically occurred in areas where univoltine populations predominate and Bt adoption rates are somewhat lower. The multivoltine biotype, which historically predominated in southern MN, typically produces two, or rarely three, larval generations depending on temperature accumulation and photoperiod cues. Multiple generations mean that the multivoltine biotype can be exposed to two or more rounds of selection to overcome Bt resistance each year. There is an overlap with both multi- and univoltine populations that can complicate ECB management in non-Bt hybrids.

Since the adoption of Bt corn 28 years ago, Bt use rates in Minnesota had grown to as high as 86% of the total corn acres planted between 2000 and 2023 according to USDA-NASS data. During 2023, Bt hybrids comprised 85% of Minnesota corn acres, which was the same as the national average.

ECB populations in Minnesota, and throughout most of the Midwest, have been effectively suppressed by these Bt adoption rates. ECB populations continue to be low in Minnesota where Bt use has remained high since 2007. Low ECB moth flights in 2023 (Fig. 2) continue to parallel the low larval population densities detected in the fall surveys, although some flights of ECB were seen near Owatonna peaking at 14 moths in a single night. While Owatonna had the largest moth catches last year, those values are in line with typical variation in low-level populations we've documented during the 2020s. 
Figure 2. MN black-light trap captures of ECB in 2023. The June captures correspond to 1st generation and the mid-August to September flight corresponds to the 2nd generation. The 2nd generation ECB flight can overlap the univoltine moth flights that occur in July to August. Moths continued to be trapped at historically low numbers at all locations. Source: MN Extension IPM Program: MN ECB blacklight trap captures.

ECB fall survey

During 2017-23, the MN Corn Research and Promotion Council has provided funding to increase the number of corn fields surveyed for ECB tunneling and overwintering larvae. These maps were produced by obtaining tunnel and larval counts per 10 plants in two fields per county in the state excluding north-central and northeastern counties with limited corn acres. Scouts examined plants for evidence of stalk damage and determined whether the damage was due to ECB feeding or other causes, such as disease. Historically, the Minnesota Department of Agriculture organized similar surveys, which allows us to make very generalized comparisons of ECB populations at their peak just prior to the beginning of Bt use to current-day infestation levels (Fig. 3).

Figure 3.  Historical overwintering fall ECB larval populations (1995-1997) from previous fall surveys comparing a pre-Bt era infestation peak (1995), with the early years of Bt hybrid commercialization (1996-1997), and with recent years (2019-2021). Source: UMN Extension IPM Program.

During 2023, 143 randomly selected commercial fields and 40 volunteered non-Bt fields were evaluated for the presence of overwintering ECB larvae. Fall overwintering larval population densities in WC and NW Minnesota were somewhat lower in 2023 than in previous years while finds continue to be found in the central and SW region of the state. Overall, 2023 ECB population levels were similar to those in 2017-2023 (Table 1, Fig. 4) and remain at historically low levels averaging 0.0011 larvae/plant. 
Figure 4. Number of tunnels found at all field corn sites (non-Bt and random) with interpolation (color shading) averaging values at up to the six nearest sites.


Be aware that the number of larvae per plant observed in fall surveys can underestimate damage that year from multivoltine populations when late instar larvae are killed by predation or parasitism, so this 2023 map shows tunnels rather than larvae found to better highlight areas where ECB damage was found. Two live larvae were found only at one location in Polk County resulting in 0.0011 larvae per plant average across the state (Table 1). The only other location where tunnels were found in Benton County did not have any larvae in the sampled plants (Fig. 4).

Table 1. Statewide data for European corn borer larvae in field corn, Minnesota 2017 - 2023.

Year

Mean ECB larvae/ plant (fields sampled)

2017

0.0054 (147)

2018

0.0080 (137)

2019

0.0039 (152)

2020

0.0120 (166)

2021

0.0063 (188)

2022

0.0016 (185)

2023

0.0011 (183)

It is important to remember that the number or larvae per plant in the table are state averages, and the maps represent interpolated spatial data and do not reflect the densities within individual fields. In other words, they do not replace scouting for field-specific decisions. The composite map of randomly sampled and non-Bt sites (Fig. 4) highlights areas of northwest and central MN that appear to have a higher likelihood of finding ECB. However, problem 2024 fields may still occur in areas of the map where ECB was not detected in 2023.

We also are looking to focus on volunteered non-Bt sample sites in 2024 to get a better estimate of background populations in the state and because ECB is difficult to find in randomly selected sites. The average density in known non-Bt fields still remains much lower than the traditional economic threshold levels for ECB that are typically greater than 0.5 larvae/plant (or 50 larvae/100 plants on the maps). The 40 fields sampled known to have been planted to non-Bt hybrids averaged 0.0125 tunnels/plant compared to the 143 random fields averaging 0.0007 tunnels/plant. The prolonged low state-wide ECB population levels suggest Bt traits continue to work well for the time being.

Resistance management

From an areawide and long-term resistance management view, it is prudent to maintain Bt-susceptible ECB in the state. Non-Bt, or “refuge,” corn is necessary to produce moths that have had less exposure to Bt traits as larvae, and thus should continue to be susceptible to Bt (see Ostlie et al. 1997). Theoretically, susceptible moths emerging from non-Bt fields should be more abundant than rare resistant survivors from Bt fields and available to mate with ECB moths emerging from nearby Bt fields. Such matings between moths from non-Bt and Bt fields should continue to keep the frequency of Bt-resistance genes low in the local ECB population. The subsequent ideal outcome is that susceptible genes dominate the larger ECB population over time, helping conserve Bt technology for as long as possible. ECB Bt traits continue to be an ongoing success story. The 2023 fall survey did not reveal any fields with suspected Bt-resistance, though it did highlight areas to continue monitoring in the state.

The risk of ECB developing resistance to Bt is not zero, however, so continued monitoring of ECB populations in Bt corn fields has value. Resistant populations have been documented across the Canadian border from Minnesota, so the northwest portion of the state will likely need continued monitoring. More information on resistant populations and screening in Minnesota is available in an earlier article.

One situation bears scrutiny with ear-feeding caterpillars, such as ECB. In the case of “refuge-in-a-bag” fields, pollen-shed among Bt ears and nearby non-Bt ears via refuge plants can lead to a mosaic of levels of Bt expression in pollen and kernels of non-Bt ears – potentially reducing refuge efficacy and increasing selection pressure for resistance. This mosaic in kernel Bt expression is a concern for ECB and other caterpillar pests such as fall armyworm (FAW) and corn earworm (CEW); there are several cases of Bt resistance with FAW and CEW globally on multiple crops, including corn.

Managing ECB in the absence of Bt

Going into the summer of 2024, ECB populations should remain generally low statewide. However, scattered reports of damage to non-Bt corn demonstrate ECB is still present and thus continues to reflect a yield threat in Minnesota. Growers, crop consultants and advisors, and those of us with Extension often see building corn borer populations after several years of planting non-Bt corn. That said, a temporary increase in acres planted to non-Bt corn should not dramatically increase the risk of economic damage from ECB in the near-term, particularly if the non-Bt fields are surrounded by several Bt fields. However, this risk likely increases as the proportion of local fields planted to non-Bt hybrids increases, particularly where the local shift away from Bt dominates the locale for several years and where non-Bt corn is planted in large contiguous blocks. Most likely, the higher fall ECB populations observed in some fields reflects local non-Bt dominance in some areas of Minnesota (Fig. 6). Because ECB colonizes over 100 host plants, such as sweet corn, peppers, green beans, and wild and cultivated hemp, there are ample hosts available in the Midwest for ECB to maintain low-level populations; the species will not go extinct any time soon. Therefore, if growers choose to plant less Bt corn longer-term, ECB populations would be expected to increase.

Scout fields

Risk of yield loss from ECB can be reduced if you scout fields and apply a labeled insecticide where needed. Another variable to consider is that two biotypes of ECB continue to be present in Minnesota. Early and late-planted fields will be most attractive to egg-laying 1st and 2nd generation moths of the multivoltine biotype, respectively. These fields should be scouted for ECB if planted to a hybrid without an above-ground Bt trait. In contrast, it takes univoltine larvae longer to complete development, so moths of this biotype produce an adult flight in-between the multivoltine 1st and 2nd generation moths. Where the univoltine biotype strain occurs, scouting should target fields from pre-tassel to near pollination when the flight is underway, typically mid-July to early August. Scouting should focus on small larvae in leaf axils and on ears which can be more challenging since larvae are no longer congregated in the whorl. In areas with biotype mixtures, mixed infestations can occur with overlapping and prolonged scouting windows throughout the summer. Degree-day models can help time scouting efforts but are not a substitute for being in the field.

Bt corn should also receive some scouting attention late season to detect potential ECB resistance and attack by other ear-feeding caterpillars. While ECB resistance to Bt has not been detected in the Midwest, several above-ground traits are now less effective against other pest species, such as corn earworm, western bean cutworm, and fall armyworm. Occasionally, refuge plants may be attacked but look for evidence of ECB injury and presence beyond the field’s proportion of refuge plants. In particular, examine leaf feeding from first-generation corn borers in earlier planted fields, stalk and ear tunneling in late-silking fields from univoltine and second-generation corn borers, ear feeding from corn earworm and western bean cutworm, and late-whorl and ear feeding from fall armyworm. If you do detect an unusually high proportion of injured plants, confirm you planted a hybrid or hybrids with above-ground Bt traits and notify your seed dealer. Independent confirmation is important so ask a trusted ag advisor to investigate or confirm your suspicions and test plants for Bt expression. Of course, we would appreciate a “heads-up.”

Notes on European corn borer, scouting and insecticide applications

  • Larvae are susceptible to insecticides for 10-14 days during each generation, from hatching to tunneling of the third or fourth stage. This limited window means your scouting efforts must be well-timed. Timing can be difficult in areas with mixed univoltine and multivoltine biotypes.
  • As corn grows and the plant loses its whorl where larvae like to congregate, successive generations occupy leaf axils and ears lower in the corn canopy. Thus, insecticide effectiveness declines with greater canopy interception by leaves positioned above the larvae. Consequently, percentage control for well-timed applications declines from 85% (1st generation) to 70% (univoltine) to 50% (2nd generation). Expect control with insecticides, even if timed well, to be noticeably less effective than Bt traits (>99.5%).
  • Larvae that tunnel into the stalk, ear shank, or ear are not susceptible to insecticide sprays and should not be considered in your spray decision. Make sure there are still enough exposed, susceptible larvae to justify an insecticide application. Re-evaluate the field closer to the time of application if there is a scheduling or weather-related delay in getting the field sprayed.
  • With aerial applications, water volume is critical… the more the better, with 5 gpa preferred. Performance is enhanced by heavy dew, as it favors movement of insecticide into the whorl or leaf axils, and diminished when using lower water volume, when leaves are dry as there is no insecticide movement to leaf axils and when hot temperatures increase evaporation of smaller spray droplets before they reach the plant. Avoid the use of spreader / sticker types of adjuvants.

When moving away from Bt traits to reduce costs, keep in mind three important considerations:

  1. Bt traits are a form of insurance. The question for you as a grower on a field-by-field basis is if that insurance and the associated cost is warranted or not. Moving away from Bt traits means both that you are assuming the risk of insect attack and timely scouting will be critical for optimal management. 
  2. Statewide, risk is generally low for yield loss from ECB, but risk is not zero. Looking ahead to this summer, the current low risk of ECB infestations in MN is based on two assumptions: the low-density larval trends observed over the past 10 years will continue, and the pest will continue to be susceptible to all or most of the Bt proteins present in Bt corn hybrids. Each assumption is reasonable based on Bt adoption rates in MN (Fig. 2), especially given the past 9 years where Bt corn has averaged ~80% of the corn grown in MN; historically low ECB infestations have also occurred in WI over the past 9 years, with 65-75% Bt corn use.
  3. If you choose to plant fewer acres of Bt corn this year, and accept the risk of potential yield loss, you will need to minimize that risk through more active management (scouting + insecticides). Recently, growers who have reduced their reliance on (and cost of) Bt hybrids typically plan for gradual reductions, e.g., reducing Bt acreage from 80% the previous year, to 70-75% the following year. This approach allows flexibility in reducing costs while not exposing the entire farm to the risk of ECB damage. Of course, when reducing the use of Bt hybrids, growers also need access to non-Bt hybrids that will still provide the necessary agronomic traits, desired maturities for their location, and competitive yields. Annual corn hybrid yield trials are published each year, and available through UMN Extension, MN Crop News, and Extension services in nearby states. After reducing Bt acres, growers should annually evaluate yield performance of Bt and non-Bt fields, and evaluate the control obtained from scouting and insecticide use on non-Bt corn (noted above). Knowledge of each of these factors, along with knowledge sharing of any ECB “hot spots” on neighboring farms, is all valuable information in assessing what percentage of Bt corn to grow the following year.
This work was supported, in part, by the farm families of Minnesota and their corn checkoff investment. To volunteer non-Bt fields for the 2024 fall survey, click here or contact Anthony Hanson at hans4022@umn.edu.

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