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Reducing Bt-trait acres in 2021 Minnesota corn production? Implications for European corn borer

Bruce Potter, Extension IPM Specialist; Ken Ostlie, Bill Hutchison, Extension Entomologists; Angie Peltier & Anthony Hanson, Extension Educators

The recent economics of corn production have challenged many 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), corn rootworm, or both will reduce seed costs. However, if not careful, farmers could inadvertently reduce crop revenues, if they select hybrids without considering yield potential or insect populations in their fields.

Yield potential is the first thing to consider when selecting a corn hybrid. Bt traits protect the yield potential of a hybrid, 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 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 insect losses. In many 2021 fields, planting corn without a Bt trait can work well, if you recognize and account for your insect risk.

Historical, current, and future ECB populations

Since the adoption of Bt corn 25 years ago, Bt use rates in Minnesota have grown to as high as 85% of the total acres planted in 2017. During 2020, Bt hybrids comprised 83% of Minnesota corn acres, identical to the national average (Figure 1). 

Figure 1. Adoption of Bt corn hybrids. The Bt varieties include those that contain more than one gene that can resist different types of insects (e.g., European corn borer; corn rootworms) since 2000. Source: USDA, Economic Research Service using data from USDA, National Agricultural Statistics Service, June Agricultural Survey as published in the NASS report Acreage.

ECB populations in Minnesota and throughout most of the Midwest have been effectively suppressed by similar adoption rates of Bt. ECB populations continue to be low in Minnesota where Bt use has remained high since 2007. Low ECB moth flights (Figure 2) parallel the low ECB larval population densities detected in the fall surveys (Figures 3-5). Low ECB population densities have also been documented in Wisconsin, where Bt adoption rates remain high as well (Figure 1). 

Figure 2. MN black-light trap ECB captures for 2020. The June peak corresponds 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, for all locations. Source: MN Extension IPM Program: MN ECB black light trap captures.

ECB larvae sampling

During 2017-20, the MN Corn Research and Promotion Council provided funding to increase the number of fields surveyed for overwintering larvae (Figure 3) and ECB damage (Figure 4). During 2020, 166 randomly selected commercial fields were evaluated for the presence of overwintering ECB larvae. Fall overwintering larval population densities in WC and NW Minnesota were somewhat lower in 2020 than in previous years. 
Figure 3. Historical overwintering fall ECB populations (1995-1997) comparing a pre-Bt era infestation peak (1995), with the early years of commercialization (1996-1997), and with recent years (2018-2020). Source: UMN Extension IPM Program (E.C. Burkness, W.D. Hutchison, A. Peltier & B.D. Potter).

Figure 4. Changes in ECB damage (tunneling) from 2018-2020. Source: UMN Extension IPM Program (E.C. Burkness, W.D. Hutchison, A. Peltier & B.D. Potter).

Overall, 2020 ECB infestation levels observed were similar to 2016-2019 (Table 1, Figure 5) and remain at near historically low levels, averaging 0.012 larvae/plant. These data compare to the state average overwintering larval number in random samples of 0.0039/plant in 2019, 0.0080/plant in 2018, 0.0054/plant in 2017, and 0.016/plant in 2016. The average density in non-Bt fields remains much lower than the traditional economic threshold levels for ECB (typically greater than 0.5 larvae/plant). It is important to remember that these numbers are state averages, and the maps represent interpolated spatial data and do not reflect the densities within an individual field. In other words, they do not replace scouting for field-specific decisions.  

Table 1. Statewide data for European corn borer larvae in field corn, 2017 - 2020.
Year Mean ECB larvae/plant
2017 0.0054 (147)
2018 0.0080 (137)
2019 0.0039 (152)
2020 0.0120 (166)

Figure 5. Relative location of fields sampled in 2019 (left) and 2020 (right) for ECB. Legend: Gray - no ECB detected, Yellow - tunnels only (no larvae), Red - tunnels with larvae. (E.C. Burkness, Travis Volmer, W.D. Hutchison, & B.D. Potter).

From an area-wide and long-term resistance management view, it is prudent to maintain susceptible ECB in the state. Non-Bt, or “refuge,” corn should produce moths that reflect the resistance level found in the local ECB population. When Bt-resistance levels are low, non-Bt corn fields will produce primarily susceptible moths. Any moths that emerge from non-Bt fields should theoretically have experienced less Bt selection pressure and ideally will most likely mate with the rarer resistant moths that survive from Bt fields. Such matings are therefore designed to assist in keeping the frequency of resistance genes low, and functionally recessive. The subsequent ideal outcome is that susceptible genes dominate over time and help conserve the Bt technology long as possible. For ECB, this continues to be one of the ongoing success stories with Bt traits. The 2020 fall survey did not reveal any fields with suspected Bt-resistance, and the prolonged low populations levels suggest the Bt-ECB traits continue to work well. 

The risk of ECB developing resistance to Bt is not zero, however, and continued monitoring of ECB populations in Bt has value. For example, Bt resistance has appeared in ECB in Nova Scotia, Canada, arising from intensive use of one Bt trait. (Note: this ECB biotype does not occur in Minnesota). 

One situation bears scrutiny with ear-feeding caterpillars, such as ECB. In the case of “refuge-in-a-bag” fields, pollen shed between the Bt and refuge plants can lead to a mosaic of Bt expression in pollen and kernels, 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 and corn earworm; 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 2021, ECB populations should remain generally low statewide. However, scattered reports of damage to non-Bt corn demonstrate ECB is still present and thus always reflects a yield threat in Minnesota. Crop consultants and crop advisors often note 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 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 most often found in parts of SE, EC, C, WC and NW Minnesota (Figure 4). As growers choose to plant less Bt corn, these populations should be expected to increase. 

Another variable to consider is that two biotypes of ECB continue to be present in Minnesota. A 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 produce 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. While ECB populations remain susceptible, this means Bt-corn can have a bigger impact on ECB multivoltine populations. Conversely, the multivoltine biotype might be expected to develop resistance more quickly when it appears. 

Risk of yield loss from ECB can be reduced if you scout fields and apply a labeled insecticide where needed. 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 ears; a more challenging situation 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. 

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 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 ECB attack beyond the 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 timed well. 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. End result: insecticide effectiveness declines with greater canopy interception by leaves 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 insecticide applications. Re-evaluate the field closer to 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 (favors movement of insecticide into whorl or leaf axils) and diminished when using lower water volume, when leaves are dry (no movement to leaf axils) and when hot temperatures increase evaporation of smaller spray droplets before they hit the target. Avoid 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. Moving away from Bt traits means 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 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. 1), especially given the past 8 years where Bt corn has averaged ~80% use in MN; also note that historically low ECB infestations have also occurred in WI for the past 8 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 Bt hybrids (and cost), typically plan for gradual reductions, e.g., from 80% Bt the previous year, to 70-75% Bt acres. This approach allows flexibility in reducing costs while not exposing the entire farm to 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, evaluate the control obtained from scouting and insecticide use on non-Bt (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 the percentage Bt corn use for the following year.

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