True armyworm is a migratory pest from the southern U.S. Each spring, volunteers help us monitor for true armyworm moths during April and May and weekly updates are posted on the ICM Blog. Although a trapping threshold does not exist to indicate whether a certain area might be at high risk for true armyworm feeding, we can use information from the trapping network to guide scouting efforts. Farmers and agronomists in the central portion of Iowa have noticed increased true armyworm activity this year despite relatively low trap captures, particularly in fields with rye cover crops. Fields with reduced tillage, no tillage, or that had a cover crop should be scouted for true armyworms in June.
Adult true armyworm moths are attracted to fields that contain living ground cover like grassy weeds or cover crops such as rye. Upon arrival, the female moths lay eggs on green plants. After hatching, the young larvae feed on these plants until they are completely consumed or killed (with herbicides or tillage). When the initial hosts are no longer available, the larvae typically move to corn, which is preferred over soybeans. This spring, agronomists have reported seeing true armyworms primarily in soybean fields with recently terminated rye cover crops. Feeding by true armyworm larvae on soybean leaves has been noted (Photo 1), and larvae are active under thick rye residue.
True armyworm larvae are identified by dull orange stripes found on each side of their body (Photo 2). Overall body color ranges from green to orange to gray or black. In addition to the six legs found on the thorax, true armyworm larvae also have four pairs of prolegs on the abdomen with dark bands on the outer side of each proleg (Photo 3). True armyworms have a network of black lines present on their orange head capsule.
Much like the fall armyworm that visited Iowa last fall, true armyworm gets its name from the larvae’s behavior of moving to new food sources in large groups. Large infestations can defoliate fields overnight, leaving only the stalks and major leaf veins. Two to three generations can occur in Iowa throughout the summer, and true armyworm larvae can feed on corn plants throughout the entire season. Injury is usually most severe early in the season when plants are small
Corn: True armyworm feeding typically begins on the lower leaves of corn plants (Photo 4). As this leaf tissue is removed, the larvae will move to the upper leaves and continue feeding. True armyworms do not tunnel into the stalk and will not feed on the growing point of larger plants. Small plants typically recover from true armyworm feeding and outgrow the defoliation.
Soybean: Broadleaf crops, like soybean, are typically only fed upon after grasses in the area are consumed or terminated. Although soybean is not a preferred host, the growing point is exposed early in the season and soybeans may be more susceptible to stand losses. True armyworm larvae begin feeding from the leaf edges, leaving ragged holes.
High risk fields should be prioritized when scouting. Fields with early-season grassy weeds, a grass cover crop, or grassy field borders are attractive to egg-laying females. With the cool, wet spring we experienced, fields where cover crops were terminated late could have been egg-laying sites for female moths and should be prioritized.
Make sure to scout the entire field to look for larvae and evidence of feeding. Take note of how many plants have defoliation, the number of larvae present, and the size of the larvae. Larvae generally feed on leaves during the cooler parts of the day (early morning, evening, or night) or on cloudy days. When it is warm, larvae will hide in the soil, crop residue, or the whorl of corn plants. Large larvae consume more tissue but will generally be done feeding in a few days. Insecticides should target young, small larvae that will be feeding for a long time (Photo 5). It is not uncommon to see a range of larval sizes in a single field (Photo 5).
Thresholds and Treatment
In most years, natural enemies (parasites, predators, and pathogens) keep populations in check. For corn seedlings (VE – V2), it is recommended that treatment occur if 10 percent or more of the seedlings are injured and larvae less than ¾ inch in length are still present. For corn that is V7 – V8, treatment should be considered when larvae are less than ¾ of an inch long, there are more than eight larvae per plant, and 25 percent of the leaf area has been removed. In reproductive stages, focus on minimizing defoliation at or above the ear. Larvae that are less than ¾ inch in length will feed for another week or so and may cause additional injury. In soybeans, treatment may be warranted if defoliation reaches 30% during the vegetative stages or 20% during reproductive stages.
If treatment is necessary, many insecticides are labeled for true armyworm. Follow all instructions on the label to ensure good control. Remember that nearly all insecticides work by contacting the insect’s body; if true armyworm larvae are feeding or hiding under dense residue (Photo 6), insecticides are unlikely to make contact and are ineffective. Target applications when larvae are actively feeding on corn foliage to ensure good contact.
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