True Armyworm Larvae Emerge in Southwest Missouri

Published: May 5, 2009

The elevated true armyworm, Mythimna unipuncta, moth numbers observed in Southwest Missouri during past weeks have resulted in the emergence of numerous larvae from eggs this past week. Jay Chism, regional agronomy specialist in Barton County, reported the presence of very small larvae (1st and 2nd instars) in some wheat fields in the Lamar area. Wheat, tall fescue, grass pastures, and occasionally field corn are all hosts of true armyworm larvae. This insect rapidly grows through approximately 7 or more worm stages (instars) as they develop from egg to adult moth. The early instars avoid light and spend much time close to the soil surface and on lower plant foliage. Feeding by early instars is usually minimal, but the amount of damage they cause rapidly increases as the larvae increase in size and move upward on host plants. A total of 2-3 generations may be produced each season, but only the first generation generally causes problems in grass crops and pastures. Later generation larvae tend to move to turf to feed and develop. When numerous moths are produced by numerous larvae, they are readily found around lights and in grass foliage. In past years numbers of moths emerging from the first generation larvae caused problems at several night ball games and other events by flying in extremely high numbers around the lights. Larvae may also cause problems on highways when they move in mass (like their armyworm name implies) and are killed by vehicle traffic. Large slick spots on the road surfaces may form and result in vehicle accidents. True armyworm larvae do not feed on legumes, only grasses.

Scouting: Larvae of true armyworm are often active at night or on cloudy days as they avoid light. To determine the presence of small larvae scout plant debris on the ground and for feeding damage on lower plant foliage. As larvae increase in size, they will feed during both night and day periods and move upward on host plants as they consume foliage. Larvae are relatively easy to identify as this insect has four pairs of abdominal prolegs located on the abdomen near the tail end of the worm and an addition pair of anal prolegs located at the very tail of the insect. The indentifying characteristic to look for is a dark brown to black triangle located on each of the 8 feet of the four pairs of abdominal prolegs. Another identifying characteristic is the presence of a broad orange or salmon colored line running down each side of a larva from head to tail.

Economic Threshold: (corrected threshold for field corn)

Tall Fescue and Grass Pastures: Occasional severe pest of grass seed and forage fields. Treat when an average of 4 or more half-grown or larger worms (½ inch to 1 ½ inch larvae) per square foot are present during late spring and before more than 2% to 3% of seed heads are cut from stems.

Wheat: Treat when an average of 4 or more half-grown or larger worms per square foot are present during late spring and before more than 2% to 3% of seed heads are cut from stems.

Field Corn: Treat seedling corn when 25% or more of plants are being damaged. Control is justified after pollen shed if leaves above ear zone are being consumed by larvae.

Potential Black Cutworm Problems Elevated for Central, Northeast, and Southeast Counties.

Captures of black cutworm moths during past weeks may result in cutting of corn plants by this pest beginning next week. The highest captures have been in Northeast Missouri where numbers of moths collected at the Greenley Center have been extremely high. Audrain and Callaway counties in Central Missouri and several traps in Southeast Missouri also report elevated numbers of moths. Although no reports of larval damage from black cutworm have been reported at this time, the potential for damage in areas receiving high numbers of moths does exist. To see the trap captures and predicted dates of first cutting by 4th instar black cutworm go to www.IPM. missouri.edu. Leaf feeding is often visible in black cutworm infested fields about 7 days prior to first cutting. The economic threshold for black cutworm suggests that 2-3% or more of seedling corn plants cut justifies the application of a rescue insecticide. Late planted corn is most at risk as large worms can cause severe injury to small corn seedlings. Seed treatments (250 rate) will provide up to about 50% control of this pest in most fields.

Greenbug and Oat Bird Cherry Aphids in Wheat Fields

Most wheat fields in Missouri are supporting populations of these two aphids, although beneficial insects such as ladybird beetles are helping to reduce pest numbers. Both aphid species can transmit Barley Yellow Dwarf disease, although this transmission causes more severe problems in wheat when infections are transmitted to wheat plants during fall and less so during spring seasons. Both aphids also cause direct damage to wheat plants by using their piercing-sucking mouthparts to suck and feed on plant juices.

The greenbug aphid is a traditional pest of wheat in Missouri after being transported into the state by winds blowing in from more southern locations. This aphid does not overwinter in Missouri, but populations may build to economic levels anytime during the growing season. The greenbug is a small, pear-shaped, pale yellow to pale green, 1/16-inch aphid with a distinct dark green line running the length of the body. The legs, cornicle (tailpipes), eyes are black in color. This aphid is often found in small colonies on the leaf surface of wheat. They tend to be problems in fall of year when conditions are mild and dry. They often initially produce red speckling at the feeding site, but may cause plant stunting to plant death.

The bird cherry-oat aphid is olive green in color with reddish-orange coloring surrounding the base of cornicles. Both species of aphids are born alive and can produce a generation about every 7-10 days when conditions are favorable. The antennae, eyes, tips of legs, and cornicles are black in color. This aphid can be found throughout the year in Missouri as it is capable of overwintering in most years.

The economic thresholds for the greenbug aphid is 25-50 aphids or more from emergence to jointing and 7-15 bird cherry-oat aphids or more during the fall and 12-25 aphids or more during spring. Scouting for greenbug aphids is easier as they tend to be in colonies on leaf surfaces. In contrast, the bird cherry-oat aphids tend to be found low in the plant canopy and found as individuals scattered over leaf surfaces. During winter this aphid often overwinters in plant debris near plant crowns.