Armyworm in small grains
Armyworm is an occasional pest of Minnesota small grains (Figure 1). Scouting for armyworm is fairly straightforward and the larvae are easily controlled with insecticides.
The true armyworms, referred to as Mythimna unipuncta and formerly known as Pseudaletia unipuncta (Haworth), are relatives of cutworms and are in the Noctuidae moth family.
Armyworms are native to eastern North America, but they can’t overwinter in Minnesota.
Each spring, armyworms migrate north like their black cutworm cousins. Adult moths are blown in on southerly storm fronts in late spring and early summer. There can be several flights of armyworm moths in a single summer.
Because migrant moths drop out of low-level jet streams with thunderstorms, armyworm infestations are sometimes found in areas that have received hail.
Armyworms tend to be more abundant in cool, wet years. Heat and dry weather are hard on armyworm eggs and small larvae.
Armyworm moths target specific environments to lay their eggs.
Areas of dense grasses are favored egg-laying sites. They prefer field edges near lodged grassy areas, lodged small grains and corn with previous heavy grass weed problems. Armyworm infestations have also been associated with corn planted into rye cover crops.
Armyworms have multiple, but distinct, generations in Minnesota. There are six larval instars (stages), and most of the vegetation is consumed during the last week of larval life. When mature, larvae are approximately 1.5 inches long.
When these larvae begin to move underground to pupate, the year’s risk is over.
Armyworm larvae can range from tan and olive to nearly black. Regardless of color, you can distinguish them by the series of lengthwise stripes on their body. Identifying characteristics include:
Two pale orange to pink stripes with white borders separated by a dark stripe on the side of the body (Figure 2).
Net-like pattern on the head.
A dark band at the base of the abdominal prolegs.
Larvae feed in the area where they hatched until they pupate in the soil or run out of food. Armyworms, like some cutworms, tend to feed at night and hide throughout the day.
The true armyworm prefers to feed on grasses. When their food source depletes, they’ll migrate in groups to find a new food source.
These migrating swarms or “armies” eat and destroy crops as they move. They can easily cross a road and feed well into a field on the other side in a single night. Outbreaks tend to occur when moist, lush vegetation is available.
Armyworm larvae have their share of problems. They’re often heavily parasitized by flies and wasps, and they can be infected by fungal and virus diseases. Eggs of fly parasites can sometimes be seen behind the heads of larvae, and the cocoons of parasites cover some infested larvae.
Scouting and management
Black light traps can detect the presence of a large migration flight of true armyworm into Minnesota. Capturing moths can predict when a problem is likely and when it’ll occur. However, because immigrant moths can re-migrate, it can’t predict where the problem will occur.
Pheromone traps for true armyworm are available. However, what the captures mean in relation to crop damage is unclear.
Chewing damage on crop leaves and the presence of frass (insect fecal pellets) on plants and on the ground indicate an insect was present. The presence of live larvae indicates there’s potential for future damage.
Armyworm larvae, like some cutworms, tend to feed at night and hide throughout the day. They’re most active at night and cloudy days. During the heat and bright sunlight, larvae often hide under leaf litter on the ground.
Scouting and insecticide applications are often more effective near dawn and dusk and on cloudy days.
When disturbed, armyworms drop to the ground and curl into a C-shape to “play possum.”
You can do preliminary scouting for armyworms with a sweep net. Pay close attention to areas that are lodged, near lodged grass borders or areas of grassy weeds.
You may have an armyworm problem—and should estimate the population—if you can:
Easily find larvae with a sweep net.
Find feeding damage in the foliage, but no other responsible insect pests.
To estimate how many armyworms you have, do the following in at least five locations within the field:
Vigorously shake plants and examine the ground beneath, making sure to check under debris and soil clods.
Count the number of larvae on the ground in a square foot area.
If you have more than four to five armyworms per square foot, treat an with insecticide.
The larvae occasionally clip heads and, when this damage is significant, it can require treating at lower populations. Head clipping is a behavioral change and usually occurs after leaves have been defoliated or senesce. If you scout at dusk, you’ll often find the larvae at the top of the plant.
Armyworms prefer broadleaves, but they can make a mistake just like anyone else. We’d be a little nervous that an under-seeded alfalfa crop may also be damaged. A barley or wheat crop may have more armyworm pressure than oats, but all are hosts.
The treatment threshold for armyworm larvae in small grain is four to five per square foot.
Before you treat, consider:
Don’t base treatment decisions solely on field-edge populations, as populations are often highest on field edges.
Confirm the presence of live armyworm larvae before applying an insecticide.
Applying insecticide to populations that are starting to pupate or are heavily parasitized isn’t recommended. By the time armyworms are more than 1.5 inches long, they’ve stopped feeding and are getting ready to pupate (Figure 3). At this point, the damage has already been done and control applications probably won’t provide an economic return.
Partial field or border insecticide treatments for armyworm are often sufficient when infestations are well-identified by scouting early or when armyworms populations are migrating. Treat several boom widths ahead of the infestation.
Long insecticide residuals aren’t needed because of the short time a larval generation is damaging. Many insecticide products are labeled and effective. Refer to the insecticide label for rates. It’s important to check the pre-harvest interval of any small grain pesticide.
True armyworm lookalikes
Be aware there can be armyworm imposters lurking on field edges in Minnesota spring cereal crops.
Grass sawfly larvae range from tan to green (Figures 4 and 5). They’re in the order Hymenoptera (bees and wasps) rather than Lepidoptera (butterflies and moths).
A giveaway is the fleshy prolegs. Grass sawfly have more than five of them, while the Lepidoptera order has five or fewer. Lepidoptera caterpillar prolegs have minute hooks (crochets), while sawfly prolegs don’t.
Sawflies can clip small grain heads, but infestations heavy enough to require treating in Minnesota are extremely rare.
Wheat head armyworm are common insects but are rarely a pest of Minnesota cereal crops. There are a number of related species that aren’t easy to differentiate. The larva tends to feed on the heads of cereals.
You may find several species of cutworms in corn and small grain crops. These will have five proleg pairs, like armyworm. You can expect the populations of both migratory and overwintering species to be higher in weedy fields or cover crop fields.
Reviewed in 2018