With the economics of 2018 corn production challenging many farmers, University of Minnesota Extension specialists say that planting hybrids without Bacillus thuringiensis proteins for protection against European corn borer and corn rootworm would greatly reduce seed costs.

Based on a survey conducted last fall on ECB damage, Bruce Potter, Extension Integrated Pest Management specialist, and Ken Ostlie and Bill Hutchison, Extension entomologists, wrote in a recent Crop News blog that planting corn without a Bt trait can work if farmers recognize their insect risk.

However, if not careful, farmers could inadvertently reduce crop revenues if they select hybrids without considering yield potential or insect populations in their fields.

Cost savings per acre could range from $8 to $25 without the CRW trait, and up to $40 or more when a straight conventional corn hybrid is planted, Potter estimates.

“It really depends on the corn hybrids, but I am probably pretty close,” he adds.

The scientists came to their conclusion after reviewing data obtained from field surveys conducted last fall for ECB damage and overwintering larvae. The Minnesota Corn Research and Promotion Council provided funding to increase the number of fields surveyed.

As part of the project, several farmer-cooperators volunteered non-Bt fields for the survey. This helped the U-M scientists by allowing them to sample 52 fields they knew in advance did not have aboveground 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,” the scientists noted in their blog.

A total of 201 fields across all major Minnesota corn-producing counties were sampled last fall for ECB.

Of 149 randomly selected and sampled fields, the state average overwintering larval number per plant was 0.0054 per plant in 2017, they reported. This compares to the random sample of 0.016 per plant in 2016.

In contrast, the average ECB population density in known non-Bt fields was at 0.029 per plant, with the grand total of all 201 fields averaging 0.0114 per plant.

ECB populations down, but pest still exists
Going into 2018, ECB populations remain generally low, they reported. However, scattered reports of damage to non-Bt corn demonstrate ECBs are still present and pose a prospective 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,” they wrote. “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 southeast, east-central, central, west-central and northwest Minnesota. As growers choose to plant less Bt corn, these populations should be expected to increase.”

The scientists explained that two biotypes of ECB borer have been introduced into Minnesota. A univoltine biotype that produces a single generation of eggs and larvae each year was the first type introduced into the U.S, and it historically predominates in the northern corn-growing areas of the state. Multivoltine biotype moths emerge earlier in the growing year. In southern Minnesota, they are capable of producing two — rarely three — larval generations, depending on temperature accumulation and photoperiod cues. Both strains overwinter as fourth or fifth instar larvae and pupate in the spring, and moths begin emerging in mid-May or later.

The scientists say that risk of yield loss from ECB can be reduced if farmers scout fields and apply a labeled insecticide where needed.

“Early- and late-planted fields will be most attractive to egg-laying first- and second-generation moths of the multivoltine biotype, respectively,” they wrote. “These fields should be scouted for ECB if planted to a hybrid without an aboveground Bt trait. In contrast, it takes the univoltine larvae longer to complete development with an adult flight in-between the multivoltine first- and second-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 late-season scouting attention to detect potential ECB resistance and attack by other ear-feeding caterpillars, they add. If farmers see an unusually high proportion of injured plants, they should notify their seed dealer.

ECB, scouting and insecticide applications
The scientists note that a limited window exists for scouting, as larvae are susceptible to insecticides for 10 to 14 days during each generation — from hatching to tunneling of third- or fourth-stage. So, scouting efforts must be timed well. Plus, as corn grows, successive generations appear lower in the corn canopy, and insecticide effectiveness declines with greater canopy interception by leaves above the larvae. The percentage of control for well-timed applications declines from 85% with the first generation to 50% with the second.

Larvae tunneled into the stalk, ear shank or ear are not susceptible to insecticide sprays and should not be considered in your spray decision, they add. Re-evaluate the field closer to application if there is a scheduling or weather-related delay in getting the field sprayed.

And keep in mind that water volume is critical with aerial applications. The more, the better. Five gallons per acre is preferred.

Bt-free?
Moving from Bt traits to reduce costs? The scientists urge farmers to 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 its management.

• Risk is generally low right now for European corn borer and corn rootworms, but risk is not gone.

• You can either choose to ignore that risk and accept the potential yield loss in your fields, or minimize that risk through scouting and insecticides.

Since the adoption of Bt corn 22 years ago, Bt use rates in Minnesota have grown to approximately 80% of the total acres planted, including the 2017 season.

For more information, read the Extension blog.

Source: University of Minnesota Extension Crop News blog