The economics of corn production challenge 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 2022 fields, planting corn without a Bt trait can work well, if you recognize and account 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, 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 the stalks, ears tips, and ear shanks.
This injury reduces yield by interrupting the flow of water and nutrients. Harvestability is reduced when affected stalks lodge or break, or when tunnels in the shank cause ear drop.
Since the adoption of Bt corn 26 years ago, Bt use rates in Minnesota had grown to as high as 84% of the total acres planted in 2017-18. During 2021, Bt hybrids comprised 86% of Minnesota corn acres, 3% higher than 2020, and slightly higher than the 84% national average (Figure 1).
ECB populations in Minnesota, and throughout most of the Midwest, have been effectively suppressed by similar Bt adoption rates. ECB populations continue to be low in Minnesota where Bt use has remained high since 2007.
Low ECB moth flights in 2021 (Figure 2) continue to parallel the low larval population densities detected in the fall surveys (Figures 3-4). Low ECB population densities have also been documented in other states such as Wisconsin, where Bt adoption rates also remain high (Figure 1).
During 2017-21, the MN Corn Research and Promotion Council has provided funding to increase the number of corn fields surveyed for overwintering larvae (Figure 3,5) and ECB injury (Figure 4,5).
During 2021, 176 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 2021 than in previous years.
Overall, 2021 ECB infestation levels observed were similar to 2017-2020 (Table 1, Figure 5) and remain at near historically low levels, averaging 0.0063 larvae/plant. These data compare to the state average overwintering larval number in random samples of 0.0120 in 2020, and 0.0039/plant in 2019.
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The average density in known non-Bt fields remains much lower than the traditional economic threshold levels for ECB (typically greater than 0.5 larvae/plant). Forty fields, known to lack a Bt trait, were sampled and averaged 0.0475 larvae/plant. Be aware that the larvae/plant observations of overwintering ECB can underestimate yield loss from multivoltine populations when late instar larvae are killed by predation or parasitism.
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 individual fields. In other words, they do not replace scouting for field-specific decisions.
Table 1. Statewide data for corn borer larvae in field corn, Minnesota 2017 – 2021.
|Year||Mean ECB larvae/plant (n)|
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.
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 over time, helping conserve the Bt crop technology for long as possible. For ECB, this continues to be one of the ongoing success stories with Bt traits. The 2021 fall survey did not reveal any fields with suspected Bt-resistance, and the prolonged low ECB populations levels suggest Bt traits continue to work well.
The risk of ECB developing resistance to Bt is not zero, however, 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 ears and nearby non-Bt ears via refuge plants can lead to a mosaic 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 2022, 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. 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 reflect local non-Bt dominance in some areas of Minnesota (Figure 4).
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 region 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.
Two ECB biotypes
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 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. 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. 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 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:
- 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.
- 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.
- 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 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, 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 the percentage Bt corn use for the following year.