Weeds, Insects & Diseases

Western bean cutworm [Loxagrotis albicosta], Lepidoptera


Insect Biology

Western bean cutworm (WBC) is a common pest of corn. It goes through four stages of development – egg, larvae, pupa, and adult (moth). The insect overwinters as larvae or pre-pupae in the soil. In June and early July the larvae pupate and emerge from...


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Description

Young larvae are dark brown and are easily identified by a faint diamond marking on their backs. Mature larvae turn to a gray to a pinkish brown with three short white stripes directly behind the head capsule. Older larvae (3rd-5th instar) have three...


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Management

Scouting for WBC is important: scout 20 plants in five areas of the field from early July to the end of August, particularly when the crop is in the pre-tassel to full tassel stages. Focus on the top three-to-four upper leaves of the plant. Look for ...


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Insect Biology

Western bean cutworm (WBC) is a common pest of corn. It goes through four stages of development – egg, larvae, pupa, and adult (moth). The insect overwinters as larvae or pre-pupae in the soil. In June and early July the larvae pupate and emerge from the soil as moths. The moths mate, and the females deposit eggs in clusters of up to 200 eggs on the upper side of pre-tasseled corn leaves. Once the corn crop is in tassel or beyond, they prefer to lay their eggs on the underside of dry bean leaves or later-planted corn fields still in pre-tassel stages in July and August. In ideal conditions with warm weather, this can occur earlier in the season. Eggs hatch in about five to seven days.

The new larvae are the most damaging – they seek out the whorl of the corn plant to begin feeding on the newly emerged tassel and pollen. As the ear shoots emerge, the larvae migrate there and enter through the tip or side husk to feed on silks and developing kernels. Numerous WBC can be found in a single ear. And because the larvae are very mobile, they can disperse from the original egg site to other plants in the vicinity both up and across corn rows.

Besides yield loss, WBC can also be associated with quality concerns (mould or mycotoxin development) as ear feeding allows an entry point for secondary infections. Mature larvae drop to the ground when fully developed and burrow into the soil to overwinter as pupa in earthen chambers.

Larvae Description

Young larvae are dark brown and are easily identified by a faint diamond marking on their backs. Mature larvae turn to a gray to a pinkish brown with three short white stripes directly behind the head capsule. Older larvae (3rd-5th instar) have three characteristic dark brown stripes immediately behind the head. These larvae can grow to a length of about 40 mm.

  • Western bean cutworm larva
  • Damage to corn cob
  • Western bean cutworm

Pupae Description

Pupae are dark brown and about 19 mm long.

Adult Description

Adult moths are easy to identify from other corn pests. Each wing of the moth has a white band running along the edge or margin of the wing and has a spot or “moon” and comma-like mark approximately two-thirds of the way down the wing.

WBC eggs are the size of a pinhead, pearly white when first laid, and are shaped like tiny cantaloupes. As the eggs mature, they turn to tan, then purple in colour before hatching.

Management

Scouting for WBC is important: scout 20 plants in five areas of the field from early July to the end of August, particularly when the crop is in the pre-tassel to full tassel stages. Focus on the top three-to-four upper leaves of the plant. Look for egg masses and young larvae. If egg masses are found but are still white (newly laid), flag the plant and return in a few days to determine if the eggs are turning purple indicating that they are ready to hatch. Eggs hatch one or two days after turning purple.

Insecticides can be effective in infested crops. It is important to observe egg hatch to know when young larvae will be active since this is the stage when insecticide application is most effective. Once larvae enter the ear they are protected by the enclosed husk and difficult to control.

Additionally, preventative measures can be taken by planting BT corn hybrids (containing Vip3A protein) in fields with a history of WBC infestation.

Source
http://www.omafra.gov.on.ca/english/crops/pub811/pub811.pdf

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