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Serving: IA

Scout for Western Bean Cutworms Now

Entomologists encourage monitoring for WBC egg masses and young larvae.

Entomologists confirm early captures of western bean cutworm moths in Iowa, Indiana, Illinois, Nebraska, Wisconsin, Minnesota and Missouri. Corn producers should begin scouting now for egg masses and young larvae. Timing an insecticide application can be critical, and applications should be made before larvae enter the silks.

Marlin Rice, professor of entomology at Iowa State University, reports that as of July 10, 76 of Iowa's 99 counties have reported WBC moth captures. Captures at this point are fairly widespread with high trap captures in Clay, Cherokee, Guthrie, Hardin, Humboldt, Monona and Woodbury counties.

According to John Obermeyer, IPM specialist at Purdue University, northwestern Indiana pheromone trap cooperators are reporting higher early trap counts in comparison to last year.

"It is too early to speculate on this year's flight intensity, but numbers so far are higher than last year," says Obermeyer. "Now is the time to scout for egg masses and young larvae."

The life of a WBC

Adult WBC moths emerge in late June through July, mate and begin laying eggs immediately. WBC moths lay eggs in masses from five to 200. Eggs will turn purple by the fifth day of development and hatch as larvae one or two days later.

Young WBC larvae feed on tassels and silks, but eventually tunnel through the silk channel to reach the developing kernels. Direct yield loss occurs as larvae consume all or parts of developing kernels. Partially consumed kernels may be attacked further by ear molds or secondary insect feeders that enter the ear through the WBC feeding channel.

Because of the labor intensive nature of scouting, the critical timing needed for insecticide applications and the possibility that multiple treatments may be necessary, insecticides may not be an economical or effective solution to the WBC problem.

Fields planted with in-plant control of WBC with the Herculex® I and Herculex® XTRA insect protection traits have shown to be effective in protecting corn against WBC. Comparing fields with Herculex I or Herculex XTRA to fields without this technology - a conventional hybrid or a hybrid with another Bt trait - is the best way to evaluate the trait's effectiveness against WBC.

The Herculex I gene protects the corn plant against European and southwestern corn borer, western bean cutworm, black cutworm, fall armyworm, corn earworm, while the Herculex® RW trait protects against western and northern corn rootworms. The Herculex XTRA insect protection trait contains both Herculex I and Herculex RW traits.

Scouting recommendations

Obermeyer suggests that egg scouting begin once moths become active.

"In five different areas of a field, inspect 20 consecutive plants for egg masses which are laid on the upper surface of the top leaves of corn," says Obermeyer. "Also look for larvae that may have hatched and crawled to the whorl and begun to feed. For hybrids lacking resistance to WBC, a treatment threshold of 8 percent of the plants with an egg mass and/or larvae in the tassel is suggested."

Obermeyer adds that timing is critical. As soon as pollination begins, larvae will make their way into the ear via the silks and become impervious to insecticides.

The 8 percent nominal threshold was developed at the University of Nebraska, notes Rice.

"A nominal threshold is one that is based upon a person's understanding of the pest's biology tied together with field experience, and it is rarely based on rigorous research," says Rice. "The University of Nebraska threshold was developed during the years when field corn was worth about $2 per bushel, but with corn in the $3.50-per-bushel range now, it would make sense to cut the threshold in half, to 4 percent, of the plants infested with an egg mass."

The University of Nebraska recommends the following nominal economic threshold:

  • Eight percent of plants with egg masses or small larvae
  • If eggs have hatched, spray at 95 percent tassel emergence
  • If tassels already are emerged, spray when most of eggs are expected to hatch

"It sometimes doesn't hurt to get a second opinion," says Rice. "Last winter, I spoke to Dr. Earle Raun, a private crop consultant from Nebraska. Dr. Raun's threshold is about half of the University of Nebraska threshold, or right in the ballpark, if we halved the Nebraska threshold based on a current higher cash value for the crop. The perspective I appreciate with this threshold is that it incorporates a 'field experience' component and increases as the crop matures. This suggests that it would require more insects to cause economic damage in the later plant stages."

Nominal threshold recommended by Dr. Earle Raun:

  • Five percent of plants with egg masses or small larvae on silking/blister/early milk-stage corn (R1 to early R3)
  • Twenty percent of plants with egg masses on mid-milk-stage corn (R3)
    No threshold on late-milk/dough/early-dent-stage corn (late R3 to early R5) unless seed production field
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