Beginner

Scientific Name
Mythimna unipuncta

Identification

• Larvae:
  • Dull-green to brown
  • White-bordered stripes running laterally along the body
  • Dark, diagonal band at the top of each proleg
  • As large as 5 cm (2 in.) in length at maturity
• Adults:
  • Nocturnal and rarely seen
  • 4 cm (1 ½ in.) wingspan
  • Pale grey-brown forewings with a white spot near the centre
• Armyworms attack young corn plants, often eating everything but the stalk and the leaf mid-vein  
• Plants will outgrow moderate leaf-feeding as long as the growing point is not damaged

Often Confused With
Variegated cutworm
Fall armyworm

Period of Activity
Larvae are usually active for 3- 4 weeks from late May to early July, depending on the year.  Armyworm are not present every year, however on occasion the populations may build to excessive levels and cause significant damage to emerging corn crops.

Scouting Notes
Examine 10 sets of 10 plants per field.  Early infestations are often found at the field edges.  Armyworms feed almost exclusively at night.  If possible, scout in the early morning or early evening.  During the day, larvae often hide in the leaf whorl, in crop debris or under soil clods.  Look for feeding damage, frass and larvae on the young seedlings throughout June and early July.

Thresholds

On seedling corn, an insecticide may be warranted if more than 10% of the plants show feeding damage.  Once the plants reach the mid-whorl stage, they are more tolerant of feeding injury and the threshold increases to 50% of the plants with damage.  Larvae that have reached 4 cm (1 ½ in.) in size are ready to pupate and have finished feeding on the crop.

Advanced

Scientific Name
Mythimna unipuncta

Identification
Larvae are dull-green to brown with white-bordered stripes running laterally along the body.  There is a dark, diagonal band at the top of each proleg.  They are as large as 5 cm (2 in.) in length at maturity.  The head is yellow-brown with a network of dark brown lines creating a pattern.

The adults are nocturnal and rarely seen.  They have a 4 cm (1 ½ in.) wingspan with pale grey-brown forewings with a white spot near the centre.

Armyworms attack young corn plants, often eating everything but the stalk and the leaf mid-vein.  Plants will outgrow moderate leaf-feeding as long as the growing point is not damaged.

Often Confused With
Variegated cutworm
Fall armyworm

Biology
In early spring, the moths prefer to lay their eggs in grassy vegetation. Larvae hatch from the eggs and feed at night or on overcast days for approximately one month.  There are two generations per year, but it is the first generation that tends to do the most damage in corn in Ontario.

Period of Activity
Larvae are usually active for 3- 4 weeks from late May to early July, depending on the year.  Armyworm are not present every year, however on occasion the populations may build to excessive levels and cause significant damage to emerging corn crops.

Scouting Notes
Examine 10 sets of 10 plants per field.  Early infestations are often found at the field edges.  Armyworms feed almost exclusively at night.  If possible, scout in the early morning or early evening.  During the day, larvae often hide in the leaf whorl, in crop debris or under soil clods.  Look for feeding damage, frass and larvae on the young seedlings throughout June and early July.

Thresholds
On seedling corn, an insecticide may be warranted if more than 10% of the plants show feeding damage.  Once the plants reach the mid-whorl stage, they are more tolerant of feeding injury and the threshold increases to 50% of the plants with damage.  Larvae that have reached 1.5” (4cm) in size are ready to pupate and have finished feeding on the crop.

Management Notes

• Naturally occurring beneficial organisms and parasites often keep armyworm populations from building. 
• When scouting, look for the eggs of parasites on the backs of the armyworm larvae.  These small, oval, yellowish eggs are laid by parasitic wasps, infesting the worms upon hatching.  Where parasites are present, sprays may not be required.
• Maintain good grassy weed control both in and around the field.
• Sprays to control large larvae are usually unnecessary.