Armyworms build populations in southeast Idaho

By John O’Connell

Capital Press

Armyworms have surfaced in high populations for the first time in southeast Idaho.

ABERDEEN, Idaho — Growers in southeast Idaho’s Pleasant Valley area should keep an eye on winter wheat fields for a destructive species of worm that hadn’t previously posed a threat in the region, University of Idaho Extension experts warn.

UI Extension cereals pathologist Juliet Marshall said armyworms wiped out an entire field of volunteer wheat that the grower had already harvested but irrigated to spur germination and clean the bed for planting spuds next season.

After confirming the presence of armyworms for the grower — who initially worried the volunteer wheat was dying due to a soil toxicity problem — Marshall also found the pests in volunteer grain in two other nearby fields.

Marshall said it’s unlikely many of the worms will survive through the winter. However, she believes they could still pose a nuisance this fall in emerging winter wheat, planted following wheat.

UI Extension entomologist Arash Rashed believes growers in the south Pleasant Valley area should also monitor for the pest next spring, concerned since they’ve surfaced in high numbers this season, they could develop into a more common problem.

Marshall said the armyworms have been in the area in the past but may have increased their numbers this season due to a hot summer.

“I’ve seen armyworms and cutworms before, but never at a level that they took out the entire field,” Marshall said.

She said the worms target cereals but will consume most other crops if they have limited options.

Rashed said natural enemies of the pest, including predatory beetles, parasitoids and pathogens, have helped keep its populations in check. He said the presence of five worms per square foot in a small grain field at planting and early growth would require treatment with a foliar insecticide, applied in patches and a surrounding buffer where infestations occur. Furthermore, he said tillage effectively controls them.

“This pest is usually associated with no-till or low-till fields,” Rashed said.

At this time of year, Rashed said the pest can be found in its larval and pupal stages. He said they cause patchy damage, defoliating plants and eating flag leaves, flowers and kernels, and it’s easy to spot larvae on the ground in fields. The worms often have orange and black stripes along their bodies and a pale-colored, inverted Y on their faces. Pupae are about an inch long and are a shiny reddish-brown color.

Adult moths, which have brownish wings with a white spot at the center, typically fly into Idaho from southern states.

Rashed said the first wave of moths should arrive here in April or May, and they can produce multiple generations. He advises growers to scout fields for them every two weeks next spring.

“Starting in April and May, I would start looking in field margins because they tend to be there, and they may consume natural vegetation and start moving into grains,” Rashed said.


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