The mere mention of armyworms can cause angst in those who have experienced outbreaks, and the news of armyworms in the area can trigger unnecessary insecticide applications.
Fortunately, other than taking some time, scouting for armyworms is fairly straightforward and the larvae are easily controlled with insecticides (Figure 1).
The true armyworms (Mythimna unipuncta, formerly known as Pseudaletia unipuncta, Haworth) are relatives of cutworms and are in the Noctuidae moth family.
Don’t confuse armyworms with the tent caterpillars that feed on broadleaf trees and shrubs. Some mistakenly call these armyworms.
Armyworms are native to eastern North America, but they cannot overwinter in Minnesota.
Each spring, armyworm moths 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. Field edges near lodged grassy areas, lodged small grains and corn that had areas of heavy grass weed problems earlier should be checked. Armyworm infestations have also been associated with corn planted into rye cover crops.
Larvae feed in the area where they hatched until they pupate in the soil or run out of food.
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. Larvae are approximately 1½ inches long when mature.
When these larvae begin to move underground to pupate, the year’s risk is over.
The larvae can range from tan and olive to nearly black in color. Regardless of color, they can be distinguished by a series of lengthwise stripes on the body. Two pale orange to pink stripes with white borders separated by a dark stripe on the side of the body are diagnostic (Figure 2).
The net-like pattern on the head and a dark band at the base of the abdominal prolegs are also identifying characteristics.
The true armyworm prefers to feed on grasses. When their food source is depleted, they will 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.
In previous infestations, armyworms have cleaned out the weedy grasses in a sunflower or soybean field and ignored the broadleaf crop.
Occasionally, they’ve been reported as a pest on some broadleaf crops, but the larvae may have simply been migrating after their favored food was depleted.
Armyworm larvae have their share of natural enemies. They are 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.
The presence of a large migration flight of true armyworm into Minnesota can be detected with black light traps. The capture of moths can predict when a problem is likely and when it will occur but, because immigrant moths can re-migrate, not 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 that an insect was present (Figure 3). When live larvae are present, there’s potential for future damage.
Where to look for them
Armyworm larvae, like some cutworms, tend to feed at night and hide throughout the day. They may also be active on cloudy days.
During the heat and bright sunlight, larvae often hide under leaf litter on the ground. Therefore, 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”.
How to scout
Preliminary scouting for armyworms in small grains, field edges and even grassy areas within row crops can be done with a sweep net. Once armyworms are found, switch to a crop specific scouting method.
Because grassy weeds are attractive to egg-laying moths, pay close attention to field borders and areas within the field that have or had high grass weed pressure. If not killed before moths arrive, grass cover crops, particularly winter rye, may also be attractive egg laying sites.
Examine plants for feeding damage and larvae. Larvae can often be found in the whorl, and the nighttime feeding often occurs in the whorl.
Treat whorl stage corn when 25 percent of plants have 2 larvae per plant or 75 percent of plants have one larva or more. On tassel stage corn, minimize defoliation at or above the ear leaf.
Managing armyworm in corn
The Handy Bt Trait Table, developed by researchers at Michigan State University, shows which Bt proteins control various insect species. While several Bt traits control fall armyworm (FAW) only, the Viptera trait is promoted for control of true armyworm.
All Bt traits can have difficulty controlling large populations of large armyworm larvae. Typically, we deal with larger, less susceptible larvae moving from weeds and field borders into corn. Secondly, insects must eat the Bt to be affected. As a result, damage could occur with very high armyworm populations on the move, even with an effective Bt protein.
Do not base treatment decisions solely on field-edge populations. The presence of live armyworm larvae should be confirmed before an insecticide is applied. Treating populations that are starting to pupate or are heavily parasitized with insecticide is not recommended.
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 are not 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 is important to check the pre-harvest interval of any small grain pesticide. Take precautions to protect pollinators, particularly as corn nears tassel stage.
True armyworm lookalikes in corn & spring cereal crops
Be aware that there can be an armyworm imposter lurking on field edges
Several species of cutworms may be found in corn and small grain crops. These will have five proleg pairs like armyworm. Anticipate increased populations of both migratory and overwintering species to be higher in weedy fields or cover crop fields.
Grass sawfly larvae range from tan to green (Figures 4 and 5). They are in the order Hymenoptera (bees and wasps) rather than Lepidoptera (butterflies and moths).
The fleshy prolegs are a giveaway, because they number more than five. In the Lepidoptera, the prolegs number 5 or less. Lepidoptera caterpillar prolegs have minute hooks (crochets) while those of sawflies do not.
Sawflies can clip small grain heads, but infestations heavy enough to require treating are rare.
Wheat head armyworm are common insects but rarely a pest of Minnesota cereal crops. There are a number of related species that are not easy to differentiate. The larva tends to feed on the heads of cereals.
Reviewed in 2018