True Armyworm Larvae Reported in Tall Fescue and Wheat

Published: May 30, 2013

Wheat, tall fescue, grass pastures, and occasionally field corn are host plants of the true armyworm (Mythimna unipuncta formerly Pseudaletia unipuncta). Infestations of true armyworm larvae have been reported from both grass pastures and wheat fields in southwest, central, and east central regions of the state. True armyworm larvae range in size from very small ¼ to ¾-inch in some tall fescue fields and grass pastures and ½ to 1-inch in size in some wheat fields, especially in far southeast Missouri counties. Some tall fescue fields, grass pastures, and wheat fields have required insecticide applications the past two weeks to reduce true armyworm numbers to below economic threshold levels. Many other fields have low numbers of larvae present which probably will not reach economic levels during spring of 2013. However, the potential for problems with true armyworm infestations remain. Scouting of fields for the presence of true armyworm larvae should continue for at least the next 2-3 weeks. Fields of tall fescue, grass pastures, and wheat should be scouted at least 2-3 times weekly to determine if larval numbers and damage are increasing to intolerable levels.

Whether an economic infestation of this pest develops in your fields is determined by several factors such as 1) high numbers of true armyworm moths migrating into your area during early spring, 2) the presence of cool, wet weather during spring, 3) presence of lush growth of grasses (especially tall fescue) and wheat during spring, and 4) lack of beneficial insects. In 2012 numbers of true armyworm moths and larvae were greatly reduced in Missouri and most southern states by drought conditions. At the same time, beneficial pathogens and biological control agents also were reduced under drought conditions, so fewer beneficial are present during spring and summer of the following year. So far in 2013, moth flights have been light to moderate with few moths captured in traps, but instead have resulted in sustained low numbers of moth captures in southwestern Missouri counties over the past few weeks and to a lesser extent into Central Missouri areas from around Mexico to Montgomery City and the Herman area. The types of damage to tall fescue and grass pastures are destruction of plant foliage along with cutting of seed heads. Heavy true armyworm infestations may defoliate and consume 100% of the grass foliage and seed heads and then move to adjoining grass pastures before continue feeding and eventually reaching maturity.

True armyworm larvae hatch from spring laid eggs and rapidly grow 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, become more active during daylight hours, and move upward on host plants to feed. 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. 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: True armyworm moths have grayish-brown to tan colored forewings, with a white spot located in the center of each forewing, and grayish-white to pale hindwings. Larvae are almost hairless with smooth bodies. Although very small larvae are often pale green in color, they quickly change to yellowish-brown or tan bodies with tan to brown heads mottled with darker brown patterns. Three distinct broad, longitudinal dark stripes run the length of the body with one occurring on the back and one each running down each side. An additional one or more orange lines can be found running the length of each side of the body from head to tail. Larval identifying characteristics include the presence of four pairs of abdominal prologs located in the center of the larva and a single pair of anal prologs present at the tail end of the larva. Each abdominal proleg will have a dark brown to black triangle located on the foot of the proleg. These dark triangles are good identification characters as few other larvae possess this characteristic. 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. Small larvae are best scouted during late afternoon, evening , and early morning hours. As larvae increase in size, they will feed during both night and day periods and move upward on host plants as they consume foliage. Larger larvae tend to remain on the upper regions of host plants.

Economic Thresholds: 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 in tall fescue seed fields.

Economic Thresholds: 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. Statewide true armyworm populations have been light in wheat this spring, although many fields in southwest Missouri have required an insecticide application for control of this pest. To this point in the season only vegetative feeding of wheat has been reported, with no cutting of wheat heads observed. Although foliage feeding always occurs with true armyworm larvae, cutting of wheat heads from plants occurs only in some years. The trigger to begin cutting of wheat heads by this pest is unknown. Wheat should be monitored several times per week after heading as true armyworm larvae can cut most heads from plants in a 2-3 day period once they begin cutting heads. Heads are typically cut by the larvae chewing perpendicular (straight across the stem) just below the seed head. Sometimes voles and field mice will cut heads, but they generally cut the entire stem by biting diagonally across the stem. Once cut by rodents, the cut stems and heads are often piled in small bundles on the ground, which does not occur if true armyworm larvae are responsible for the cut heads.

Economic Threshold: 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. Larvae of this pest can severely damage corn when high larval populations defoliate plants to the point of killing them. Producers are encouraged to scout corn plants weekly for the presence of true armyworm larvae. Although seedling plants are most at risk during this time of the year, corn plants can be defoliated throughout the growing season. True armyworm can be a severe pest on field corn and generally cause excessive defoliation and plant mortality. Similar to black cutworm larvae, late planted corn is at the highest risk of seedling damage by true armyworm larvae.

Due to the high commodity prices for grass hay, wheat, and field corn, the economic thresholds listed in the previous text may need to be adjusted downward to better reflect when it is economically feasible to treat an emerging pest population.

True Armyworm

Grass Sawfly

Confusion over Larvae of the True Armyworm and Grass Sawfly. Some reports of true armyworm larvae in wheat have resulted when the larvae found in fields are actually larvae of the grass sawfly. Whereas the true armyworm larvae is a lepidoterous pest related to moths and butterflies, the grass sawfly is a hymenopteran pest which is related to bees and wasps. Grass sawfly larvae may possess 5-8 pairs of abdominal prologs (7 or more common) as compared to four pairs of abdominal prologs of true armyworm larvae. Grass sawfly larvae are generally found near the flag leaf or on upper foliage of wheat and are somewhat transparent and pink to light yellowish green in color. Grass sawfly is a minor pest of wheat in Missouri with most larvae being parasitized. Only rarely do they reach populations levels where levels of plant defoliation justify control of larval infestations. They have been reported to cut wheat heads in more northern states, but this is not common observed in Missouri. In contrast, true armyworm larvae can cause significant damage to a wheat crop by both heavy vegetative feeding and severe clipping of wheat heads.

Black Cutworm Moth Numbers Low in Most Missouri Corn Fields - Although reports of black cutworm larvae and damage remain low at this point in the season, the potential for damage from this pest remains until corn plants exceed the four leaf state of growth. The most severe scenario for black cutworm damage to field corn is for large larvae to feed on emerging and recently emerged corn seedlings. Spring rains have slowed corn planting throughout most of Missouri this year. Similarly, black cutworm moth flights have remained low for most areas of the state. Spring rains favor the insect, but low moth flights favor the corn producers. Odds are that corn producers in Missouri will not encounter severe damage from black cutworm larvae this season. However, if heavy or sustained moth flights were undetected in an area, then large larvae may be present to feed on late planted corn seedlings. Thus, weekly or twice weekly scouting of corn seedlings for the presence of black cutworm larvae and other pests is essential to prevent severe damage to seedling corn plants. Scout corn plants regularly through the four leaf stage of plant growth. The economic threshold for black cutworm in field corn is to treat when ¾-inch or smaller larvae cause 2 to 3 percent or more of seedling plants to wilt or be cut above or below ground up to the 5-leaf stage of corn seedling growth. Several management options are available for black cutworm in field corn. Many corn hybrids now contain insecticidal traits which provide good protection from black cutworm larvae. Seed treatments will provide some control, but may be overwhelmed by heavy black cutworm larval populations. Soybean may also be attacked by black cutworm larvae, although this is far less common than attacks on field corn. Historically a good economic threshold for black cutworm feeding on soybean seedlings is 20% or more cutting of soybean seedlings. With higher commodity prices for soybean, this economic threshold is probably too conservative. As commodity prices go higher, producers can afford to treat pest infestations at lower thresholds. With this in mind, an economic threshold of 10 - 15% or more cutting may be a more reasonable economic threshold for black cutworm infestations in seedling soybean.