Tough winter for bean leaf beetle06/01/2018 | Crop Production Research, Soybean News, Economics, Weather
By Matthew Wilde, ISA senior writer
Mother Nature hampered northern Iowa farmers’ efforts to get soybeans planted, but she may help protect plants as they grow.
Bean leaf beetle winter mortality rates were highest in the northern part of the state, according to estimates recently released by Iowa State University (ISU) Extension and Outreach. An overwintering survival model developed by ISU projects only 3 percent of the yield-robbing pests in north central Iowa endured the winter and 11 percent lived in northwest and northeast Iowa.
The mortality rate significantly decreased farther south, ranging from 77 percent in east central Iowa to 64 percent in the southeast corner of the state.
Bean leaf beetle adults are susceptible to cold weather and most will die when air temperatures fall below 14 degrees. Fluctuating temperatures can negatively influence spring populations, research shows.
“I’ve been out in the fields and haven’t seen many at all,” said Scott Nelson, Iowa Soybean Association (ISA) On-Farm Network® director. “But that doesn’t mean beetles won’t be a problem.”
When bean leaf beetle populations reach damaging levels in soybean fields, Nelson said yield reductions of 5 to 15 percent or more are common. The insects can transmit bean pod mottle virus and feed on cotyledons, leaves and pods.
Wayne Fredericks, a former ISA president, finished planting on Monday — a couple weeks later than he would have liked.
Even though late winter storms, roller-coaster temperatures and spring rains delayed seeds getting into the ground in northern Iowa, Fredericks is glad the weather helped keep potentially damaging bean leaf beetles at bay.
“It is good numbers will be down,” he said. “Usually it’s cold enough up here that overwintering beetles are not much of an issue.”
Beetles on the move?
Beetles protect themselves during the winter by crawling under plant debris and loose soil. In the spring, the insects emerge and migrate to available hosts like alfalfa and various clovers. Eventually, beetles move to preferred hosts like soybeans as plants emerge.
Overwintering and first-generation beetles rarely cause economic damage to soybeans due to lower numbers and rapid plant growth, experts say. Terry Basol, an ISU regional field agronomist based near Nashua, said he’s only recommended a farmer spray once to prevent damage in the early soybean growth stages.
But the presence of overwintering beetles or lack-there-of is an indicator of building first and second generations (which overwinter) later in the season. That’s when damage and yield loss is typically a concern.
“A smaller population of overwintering beetles translates to lower second-generation numbers,” Basol said. “That’s good news for northern Iowa farmers who dealt with a harsh winter and spring.”
The statewide-predicted bean leaf beetle mortality rate was the highest during the 2013-14 winter since tracking began in 1989, ISU data shows. The bean leaf beetle mortality rate in central Iowa was 75 percent this year, 4 percentage points higher than the 29-year average.
Scout and treat — if needed
Basol suggests farmers scout soybean fields early for beetles (and other pests) to determine if larger numbers are likely later in the season that may warrant spraying — especially as pods form and fill.
At $10-per-bushel-soybeans and a control cost of $15 per acre, ISU research indicates treatment thresholds are:
- More than six first-generation beetles found in a 3-foot row or more than 25 in 20 sweeps (using sweep net). Usually done during the late vegetative or early reproductive stage.
- More than 23 second-generation beetles found in a 3-foot row or more than 95 in 20 sweeps. Usually done in late August or early September.
“I’m more worried about the second-generation, which can clip pods, defoliate plants and spread bean pod mottle virus,” Basol said.
Pyrethroid sprays can be used to control the beetles. Nelson said seed treated with insecticides, like Gaucho or Cruiser, is effective controlling beetles so they don’t become a problem.
“You will see the dead carcasses on the ground,” he said, referring to fields planted with treated seed.
Even though overwintering beetles moving to crops are expected to be low this year, ISU experts urge farmers, especially in southern Iowa, to scout if:
- Soybeans are planted near alfalfa or if the field has the first emerging soybeans in the area.
- Fields have a history of bean pod mottle virus.
- Food-grade or seed fields where reductions in seed quality from bean pod mottle virus can be significant.
To learn more about bean leaf beetles, click here to go to the Soybean Research & Information Initiative website. Or, click here for information from ISU.
Contact Matthew Wilde at email@example.com.
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