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Scouting, spray timing critical for western bean cutworm control

Conditions are right for a western bean cutworm year in Ontario corn and it’s time to scout — but likely not yet to spray.

Western bean cutworm (WBC) is now the most economically damaging pest in Ontario corn. It feeds on the tassel and ears of corn and doesn’t particularly affect yield, but the feeding allows in disease that affects the quality of the crop.

“You need to time scouting for at least tasselling,” according to Tracey Baute, the Ontario agriculture ministry’s field crop entomologist in Ridgetown. If WBC moths lay eggs before tasselling, the larvae will die. Target scouting for pretassel and full tassel; the larvae will feed on the tassel, then migrate to the ear.

This year, with the corn crop mostly planted later than usual, “we will have ideal growth stage at the same time as peak moth flight and egg laying,” Baute said. Peak moth flight is usually in the last week of July, which will line up nicely with the tasselling of a good proportion of the Ontario corn crop.

This year’s long planting season also translates to corn on the same farm at drastically different maturity levels — which could mean more time spent scouting in the province.

Corn fields need to be scouted over about three weeks, starting with early tasselling. The threshold is five per cent of plants with eggs in order to spray. But that doesn’t mean you spray right away.

“I’m worried everyone’s going to spray too early,” Baute said. “All larvae can’t survive on tassel all season, so some have to push way down to the ear.”

Delaying spraying until the ears have some silk has the added bonus of being able to combine a fungicide application with an insecticide application in one pass, she said.

The sprays have no residual for WBC — another reason that the spray needs to be timed correctly, Baute noted. Some of the sprays have translaminar properties, or systemic action through the leaf, but as the WBC are not foliar feeders, there’s no control on the insect that way.

There’s an extensive network of WBC traps across the province, the data from which feeds into the CornPest.ca website. Maps show moth spread across the province.

Moth traps, however, only show the potential for a problem. Fields have to be scouted and eggs identified at the right timing to justify a spray pass.

Not every field will be at risk from WBC, a relatively recent arrival in the province. Hotspots, on sandy soils where the pest can overwinter, include the Tillsonburg and Bothwell areas, but moths are now being found in a much wider area.

The WBC trapping network’s map for the week of July 10 to 17, 2017. Dots represent the number of moths captured during the week: red, 251-500; orange, 101 to 250; yellow, 51 to 100; green, one to 50; blue, zero.
photo: Screengrab from CornPest.ca

Biotech traits provide little control, as the WBC has resistance to all Bt traits other than the Viptera trait.

Growers of corn with the Viptera trait are encouraged to check to see if corn ears have had feeding in the fall, and to let Baute know if they do.

Among sprays, Coragen is the most popular, but Baute encouraged rotation with other products such as Matador or Voliam Express in order to help stave off resistance to Coragen.

“Just don’t panic and pull the trigger too soon.”

About the author

Field editor

John Greig is a field editor for Glacier FarmMedia.

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