Three weeks after planting, I was called out to Mark’s 1,500-acre farm, located south east of Winnipeg, Man., where he grows soybeans, canola and spring and winter wheat. Mark wanted to know why an area of his soybean plant stand was so thin.
The thin plant stand was located in the north 30 acres of an 80-acre field. In the affected area, only one plant was present per 15 feet of row on average. All other plants were missing. The few plants that were still standing were healthy and unaffected by stress or weed competition. Also, affected soybean rows stood beside unaffected rows. In addition, the plants located in the remainder of the field had emerged evenly and were healthy. Neighbouring canola and corn fields were also healthy and showed no signs of disease or pest pressure.
Mark had three possible theories as to what could be causing the problem. Since the 30 acres in question were located in a low-lying area, Mark thought a poor seed bed could be to blame. It was possible the low-lying area was susceptible to excess soil moisture, creating issues such as compaction and limited root growth due to lower oxygen levels and increased phytophthora root rot infection.
He also reasoned above-ground insect feeding could be responsible for the damage in this section of the field.
However, his strongest suspicion was the poor plant stand was caused by planter misses, and it did look a lot like the planter didn’t do its job.
Mark looked dejected. “I guess it’s time for a new planter,” he said.
Mark’s planting records did not shed any light on the issue. As far as I could see, the correct amount of seed was used, and the planter was in good shape with no seed depth or vacuum delivery issues.
In addition, damage from phytophthora root rot was improbable because Mark had used fungicide-treated seed, and the plants left standing in rows did not show symptoms for the disease.
There was also no evidence of above-ground insect feeding, however, I had a theory of my own.
“Let’s wait a few hours and then check for insects,” I said. “I’m pretty sure something is eating your crop before it even gets out of the ground.”
Crop Advisor’s Solution: Cutworm damage can resemble planter misses
After eliminating poor seed bed and equipment failure as possible causes for the missing plants, we searched the affected area for signs of above-ground insect feeding. Although we found no evidence of feeding above ground, by digging in the field in the early morning and late evening we found the insect responsible for the damage — the dingy cutworm.
Mature dingy cutworm larvae can measure up to 3.8 centimetres (1.5 inches) in length. They are grey in colour with light markings down the back, which resemble tire tracks.
Cutworms were feeding on plants below ground and had taken the plants out before they emerged, leaving no visible signs of damage above ground. Mark had used treated seed in neighbouring corn and canola fields, which protected the seed from cutworms. Soybeans in the affected field had been treated with fungicide, but not an insecticide to protect the seed from cutworms.
The cutworms systematically took out each seedling up and down the row, working across the field. The insects were working in the planter rows, which, at first glance, looked like damage caused by planter misses.
We immediately treated the field with species-specific insecticide to control the cutworms. This stopped the crop loss to the rest of the field. Since the infestation was severe, the affected 30 acres yielded next to nothing. However, taking immediate
action contained the damage, and the crop loss.
Insects, such as cutworms, are unpredictable, and it is often difficult to estimate population size as they live in the soil. It’s as important to look for insects below ground as it is to look above ground. Also, don’t make any assumptions about the insect’s life cycle and habits — knowledge of this kind is helpful in the prevention of crop damage and yield loss.
Terry Moyer is a regional sales agronomist for Richardson Pioneer Ltd. at Landmark, Manitoba.