Armyworms

Cutworms and armyworms can move in large masses. Here, winter cutworm larvae have invaded a home lawn.

The term ‘armyworm’ refers to various species of noctuid moths that have gregarious larvae. This means that eggs are laid in groups, and when they hatch, the caterpillars move across the landscape en masse. Crop damage potential is high due to the abundance and rapid movement of the defoliating larvae.

Common species we have in this region include:

Bertha Armyworm (Mamestra configurata – BAW) occurs at low densities, so any detection of a moth in a pheromone trap is reason to intensify field scouting.

True Armyworm (Mythimna unipuncta (Pseudaletia unipuncta) – TAW) is mostly a pest of grasses, and can cause considerable damage.

Western Yellowstriped Armyworm (Spodoptera praefica – WAW) is a native that has increased in apparency over the past few years. It has a wide host range.

*** More information is available at our other blog “Cutworm Central” ***

Monitoring

Moth flights are monitored with pheromone traps, which can help detect emergence or arrival in a region. If adults are noted, field scouting is recommended. Armyworm damage can be extensive, but also difficult to detect*. Walk across rows and look for ‘strikes’ = small clusters of 1-2 plants with leaf damage. Larvae may be resting near or under the soil surface during the day, so digging up to 2″ is necessary. Other species have ‘climbing’ larvae, so they may be present on foliage, but tend to drop quickly when disturbed.

*See this June 2018 blog post re: scouting for armyworms

First instar bertha armyworm larvae

Management

Biological Control: Parasites (flies, wasps) and predators (ground beetles, assassin bugs, spiders, birds, rodents) are important regulators of armyworm populations, but control is not predictable nor reliable. Groups of birds in grass fields or turf can serve as an indicator of possible armyworm infestation. Entomopathogenic viruses also help to keep populations in check.

Cultural Control: Most armyworms rely on weed hosts to lay eggs. Common lambsquarters is particularly attractive. Manage weeds in and around fields. Fall tillage, where applicable, can help expose overwintering stages to decay and predation.

Chemical Control: Chemical controls are targeted at larvae, and work best when larvae are small. Therefore, field scouting is crucial to determine armyworm pressure as soon as possible. Applying sprays at night may improve control. Consult the PNW Insect Management Handbook for chemical recommendations.