Black Cutworm Moth Numbers Low Except for SE Missouri

Published: April 17, 2012

Numbers of black cutworm moths remain low in most areas of the state, although moderate numbers of moths have been captured consistently in Southeast Missouri during the past few weeks. Black cutworm moths typically begin arriving in Missouri from more southern states about April 1 with peak flights occurring about April 10 annually. In 2012 first captures of black cutworm moths were reported from the Nevada area during the second week of March. Even though moth numbers are moderately elevated in southeast counties and moths arrived several weeks early in Nevada in southwest Missouri, no "intensive moth captures" have been reported. The predictive model is designed to merge moth captures from pheromone traps with historic and current weather data to predict the date of first cutting by 4th instar stage black cutworm larvae. At this time the risk of economic infestations in field corn is reduced except for possibly Southeast Missouri counties. However, it is still possible that moth flights may continue for several weeks and cause problems in both corn and soybean fields. 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.

In 2011 black cutworm problems were somewhat reduced in corn as compared to previous year. However, many soybean fields did experience black cutworm infestations that caused damage ranging from minor stand loss to total loss of plant stand. Black cutworm larvae occasionally cause damage to seedling soybean when their feeding activities result in high plant mortality. In most years, low populations of black cutworm larval in soybean go unnoticed as surviving plants often compensate for lost plants. However, moderate to heavy feeding by black cutworm larvae on soybeans seedlings may substantially reduce crop yields and result in replant situations. 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% or more cutting is a more reasonable economic threshold for black cutworm infestations in seedling soybean. Both corn and soybean producers are encouraged to monitor crop stands beginning at plant emergence and continuing at least twice weekly for the 6 to 8 weeks. Seed applied insecticide treatments on soybean should help reduce black cutworm larval numbers, but may not prevent economic damage in situations where larval feeding is severe. In corn, these seed treatments often control about 50% of black cutworm larvae under heavy infestations. They should work better in soybean where the treated seeds are grouped closer together in the row. If rescue insecticide applications are necessary in corn or soybean, be sure to follow label directions, precautions, and restrictions for the crop being rescued.

True Armyworm Larvae Reported in Tall Fescue and Wheat

Wheat, tall fescue, grass pastures, and occasionally field corn are host plants of true armyworm (Mythimna unipuncta formerly Pseudaletia unipuncta). Infestations of small armyworm in grass pastures and wheat fields were reported during the past few days from numerous tall fescue and wheat fields in the southern half of the state. True armyworm larvae are very small in size and numbers relatively low at this time. However the potential for this pest to cause severe damage in most host crops is elevated by the presence of small larvae this early in the year. The major damage to tall fescue and other grass pastures is destruction of plant foliage along with some cutting of seed heads. Heavy true armyworm infestations may defoliate and consume 100% of the grass foliage and move to feed in adjoining grass pastures before 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.

True armyworm populations have been light in wheat this spring. Few fields have required an insecticide application and no head cutting has been reported. Economic Threshold: 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. 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.

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. 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. True armyworm can be a severe pest on field corn and generally cause excessive defoliation and plant mortality.