Anthony Hanson, IPM Extension Educator; Bruce Potter, Extension IPM Specialist; Ken Ostlie & Bill Hutchison, Extension Entomologists; Angie Peltier & Ryan Miller, Extension Educators
If you plan to have non-Bt corn fields in 2023, we are looking for potential European corn borer fall survey sites. To volunteer non-Bt fields for the 2023 fall survey, click here or contact Anthony Hanson at 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.
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 27 years ago, Bt use rates in Minnesota had grown to as high as 86% of the total corn acres planted between 2000 and 2021 according to USDA-NASS data (Fig. 2). During 2022, Bt hybrids comprised 89% of Minnesota corn acres, a slightly higher rate than the 84% national average.
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’s June Agricultural Survey as published in the NASS Acreage Report.
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 2022 (Fig. 3) continue to parallel the low larval population densities detected in the fall surveys.
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 (Figs. 4-7). 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. 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
During 2022, 148 randomly selected commercial fields and 37 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 2022 than in previous years while finds continue to be found in the central and SW region of the state. Overall, 2022 ECB population levels (Figures 5-7) were similar to those in 2017-2021 (Table 1, Fig. 4) and remain at historically low levels averaging 0.0016 larvae/plant.
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 these 2022 maps show tunnels rather than larvae found to better highlight areas where ECB damage was found. Three live larvae were found only at one location in Kanabec county resulting in 0.0016 larvae per plant average across the state (Table 1). However, this field also had the highest number of tunnels in the survey with 14 tunnels found in the 10 plants sampled
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. 7) highlights areas of southwest and central MN that appear to have a higher likelihood of finding ECB. However, problem 2023 fields may still occur in areas of the map where ECB was not detected in 2022.
We also are looking to focus on volunteered non-Bt sample sites in 2023 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 37 fields known to have been planted to non-Bt hybrids were sampled and averaged 0.0216 tunnels/plant compared to the 148 random fields averaging 0.0169 tunnels/plant. While larval or tunneling counts have typically been higher in non-Bt field averages, the previously mentioned Kanabec County site was randomly selected and not a known non-Bt field. Given the high population counts above threshold levels at this field, it would be concerning from a resistance perspective if this was a Bt field, but this area of the state also has a higher rate of non-Bt fields. In cases like these, we will continue to monitor the area for potential issues and contact growers. The prolonged low state-wide ECB population levels suggest Bt traits continue to work well. 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 2022 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. For example, Bt resistance has appeared in ECB in Nova Scotia, Canada, arising from intensive use of hybrids with a non-pyramided single Bt protein (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 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.
Source: umn.edu
Photo Credit: GettyImages-Yasonya
Categories: Minnesota, Crops, Corn