Armyworm Larvae Have Been Spotted

Isolated reports of true armyworm larvae in Essex County have come in. This does not mean that we will experience the same kind of infestations as last year but it does tell us that we all need to get out and scout wheat and corn fields thoroughly. Unfortunately for growers in Essex County, they tend to give the neighbouring counties a heads up on armyworm activity. All counties in the southwest (Essex, Chatham-Kent, Lambton, Elgin) should have the fields scouted within the next 5-7 days and every 5 days after that. Counties a little further east and north should be on the alert and be scouting within the next 10 days and weekly thereafter.

Figure 1. Full grown true armyworm larvae.

Scouting is best done in the evening after dusk when the larvae are most active (bring a flashlight), but if scouting during the day, look for crop damage including notches in the leaves or leaves being stripped to the midrib and signs of brown frass along the ground. Those fields showing these signs should be scouted again in the evening to get a sense of the number and size of larvae present. Scout both the margins of the wheat and corn fields as they could be marching from one field to the next but also scout throughout the interior of the field. Unfortunately with the delay in weed control this year, eggs could have been laid throughout the corn fields, not just the field margins or neighbouring wheat fields.

Remember size does matter..at least when it comes to insects. Any larvae larger than 1 inch are difficult if not impossible to control.

Thresholds for Corn

With early season feeding, insecticide may be warranted in seedling corn if there are two or more unparasitized larvae per seedling and feeding damage exceeds 10%. For corn past the whorl stage, if 50% of the plants are showing damage and are infested with larvae smaller than 2.5 cm (1 in.), then insecticide treatment may be warranted.

Thresholds in Wheat

Four unparasitized larvae, smaller than 2.5 cm (1 in.), per square foot, if head clipping is occurring.

Thresholds in Forages

Armyworm can choose to feed on the grasses within the forage crop too. Control is warranted when five or more larvae (smaller than 2.5 cm) per square foot are found. In seedling crops, two to three larvae (smaller than 2.5 cm) per square foot may warrant control.