Minnesota Corn: Reducing Bt Trait Acres in 2019? Implications for European Corn Borer

European corn borer larva and damage. Photo: Bruce Potter

Editor’s Note: For accompanying charts and figures see original article at the Source URL below.

The economics of 2018 corn production challenged many farmers to minimize losses per acre. One area some farmers have targeted for reducing costs is hybrid selection. Planting corn hybrids without Bacillus thuringiensis (Bt) proteins for protection against European corn borer (ECB), corn rootworm, or both will greatly 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 only protect the yield potential of a hybrid; yield benefits only occur when targeted insects are above economic 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 2019 fields, planting corn without a Bt trait can work well, if you recognize your insect risk.

Historical, current and future ECB populations

Since the adoption of Bt corn 23 years ago, Bt use rates in Minnesota have grown to approximately 84% of the total acres planted, including the 2018 season. ECB populations throughout most of the Midwest Corn Belt have been effectively suppressed by a similar adoption of Bt.

ECB populations continue to be low in Minnesota where Bt use has remained relatively high since 2007. Low ECB moth flights parallel the low ECB larval populations detected in the fall surveys. Historically low ECB populations have also been documented in Wisconsin, where Bt adoption rates remain high as well.

Survey expanded

In 2017-18, the MN Corn Research and Promotion Council provided funding to increase the number of fields surveyed for ECB damage and overwintering larvae last fall. As part of the project, several farmer cooperators volunteered non-Bt fields for the survey. Importantly, this cooperation allowed us to sample 52 fields that we knew in advance did not have above-ground Bt traits. These fields greatly contributed to our understanding of the current spatial pattern of ECB in the state and revealed where ECB might gain a foothold in non-Bt fields.

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A total of 207 commercial fields were sampled, and thanks to grower and crop consultant cooperators, this included a total of 70 known non-Bt fields. No significant change in the 2018 ECB infestation levels was observed compared to 2017. For example, in the non-Bt fields, we found an average of 0.039/plant (27 larvae total), whereas the randomly sampled fields (137) averaged only 0.008/plant (11 larvae total).

Briefly, the overall population estimate for all commercial fields combined was 0.018/plant in 2018, reflecting historically low levels. These data compare to the state average overwintering larval number per plant was 0.0054/plant in 2017 and this compares to 0.016/plant in 2016 random samples. The average ECB population density in 2017 known non-Bt fields was at 0.029/plant, with the grand total of all 201 fields averaging 0.0114/plant.

While higher than the density in fields at random, the average density in non-Bt fields remains much lower than the traditional economic thresholds or injury levels for ECB. 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. For example, the shaded area in SW MN includes numerous non-Bt fields where we found no ECB tunnels or larvae (Figure 6). In other words, they do not replace scouting for field-specific decisions.

From an area-wide and long-term resistance management view, it is prudent to maintain some ECB in the state as part of the “non-Bt corn refuge for ECB”. Any moths that emerge from non-Bt fields should theoretically have experienced less Bt selection pressure, and ideally will most likely mate with any rare resistant moths that survive from a Bt field.

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 as long as possible. For ECB, this continues to be one of the ongoing success stories with Bt traits.

The risk of ECB developing resistance to Bt is not zero, however, and some continued monitoring of populations in Bt has value. In the case of refuge-in-a-bag (RIB) 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. The effectiveness of the trait and insect biology does make this is less of a concern with ECB relative to some other Lepidoptera such as fall armyworm and corn earworm.

Managing ECB in the absence of Bt

Going into 2019, ECB populations remain generally low. However, scattered reports of damage to non-Bt corn demonstrate ECBs are still present and thus always pose a potential threat in Minnesota.

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 has occurred for several years and non-Bt corn is planted in large contiguous blocks.

Most likely reflecting the higher percentage of non-Bt or conventional corn planted over the past few years, fall ECB populations were most often found in parts of SE, EC, C, WC and NW Minnesota. 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 borer 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 corn growing areas of the state. Multivoltine biotype moths emerge earlier in the growing season.

In southern MN, they are capable of producing two, rarely three, larval generations depending on temperature accumulation and photoperiod cues. Both strains overwinter as 4th or 5th instar larvae, pupate in the spring and moths begin emerging in mid-May or later.

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 the univoltine larvae longer to complete development with an adult flight in-between the multivoltine 1st and 2nd generation moths. Where the univoltine biotype strain of ECB occurs, scouting should focus on fields from pre-tassel to near pollination when the flight is underway, typically mid-July to early August. In areas with biotype mixtures, mixed infestations can occur with overlapping and prolonged scouting windows.

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, several above-ground traits are now less effective against some corn earworm, western bean cutworm and fall armyworm populations. 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. Of course, we’d 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 third or fourth-stage. This limited window means your scouting efforts must be timed well.
  • As corn grows, successive generations appear lower in the corn canopy; insecticide effectiveness declines with greater canopy interception by leaves above the larvae. 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 have tunneled into the stalk, ear shank or ear are not susceptible to insecticide sprays and should not be considered in your spray decision. 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 and 5 gpa is preferred. Performance is enhanced by presence of a heavy dew (favors movement 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 target.

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 its management.
  2. Risk is generally low right now for European corn borer and corn rootworms, but risk is not gone.
  3. You can either choose to ignore that risk (and accept the potential yield loss in your fields) or minimize that risk through active management (scouting + insecticides).

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