A mild winter and spring storm activity are increasing the incidence of migratory insect pests such as the black cutworm and armyworm moths. These are sporadic and nocturnal pests that overwinter in southern states such as Texas and Louisiana are moving north ward as winged adult moths with storm fronts. Adult moths lay eggs in exposed fields on weeds and cover crops where they land and rest. Black cutworm larvae hatch and grow on weeds then move to corn plants to feed on leaves and cut plants reducing crop stands. Armyworm larvae feed on the leaves of grasses such as pasture and ryegrass cover crops. Abundant moth captures have been reported by Purdue University in Indiana and likely occurring in other states in the Corn Belt. Intense trap captures are 9 or more moths caught for two consecutive nights. Early flights do not correlate to economic injury but are a warning to keep watch in fields with a history of cutworm or army worm damage. Black cutworm and armyworm behavior, damage and control is discussed in the following article.
Black cutworms (BCW, Agrotis ipsilon) are a sporadic pest of corn but also attack other crop such as wheat and soybean. Adult moths survive in southern states on weed and crop hosts then hitch a ride northward on the winds and updrafts of storms. Adult moths land in fields where they detect available food sources in fields loaded with broad leaf winter and summer annual weeds before planting, in low lying areas, along woods and streams and in cover crops. No-till and late planted fields are usually infested because weed control is not as complete. Larvae must feed on corn or weeds and cannot go more than 2 weeks without food, so complete weed control 2 weeks before crop emergence is helpful to reduce existing populations. Multiple flights occur through the spring and early summer so vigilance is suggested until corn reaches the V5 growth stage.
BCW larvae should be identified from other cutworms. Dingy cutworm (DCW) can appear at about the same time. DCW overwinter as larvae but are not significant pests. They feed on the leaves but do not cut plants. Other cutworm species such as the clay-backed, sandhill and glassy cutworms are occasionally found. BCW larvae are about 1.5 inches long when fully grown. BCW and DCW cutworms have black tubercles on each segment. BCW have darkened skin granules that make appear greasy. BCW and DCW can be distinguished by the size of the 4 tubercles on each segment. BCW has two small and 2 large tubercles. DCW have four equal sized tubercles.
Scout for BCW up until the V4 to V5 con growth stage. Look in indicator fields that most likely to be infested with heavy weed cover before planting, no-till, along woods streams and low lying areas. Look for cut or wilted plants then dig at the base of plants to find and identify the cutworm and estimate the instar stage or size of the worm. Cutworms feed at night and on cloudy days. Check at least 50 plants in 5 places in the field. If 2-3 percentage of plants are cut or wilted sand larvae are not fully grown at less than ¾ inch long then insecticide application may be necessary.
True armyworms (AW, Pseudaletia unipuncta) are mostly pest of pastures and wheat with some occasional damage to corn. AW prefers grass weeds as suitable hosts. AW arrives in the spring and early summer. AW survives better in the cool and wet springs when natural enemies are reduced. They are most active in the early spring to July. Fall Armyworms (FAW, Spodopera frugiperda) are more prevalent in mid-summer to fall. AW feed at night and hide in crop residue and litter during the day. FAW feed through the day and in the night. FAW can consume much more plant material than other caterpillars. Good grass control before planting reduces the chance of AW outbreak in corn.
AW larvae are dull green to brown with a large dark spot above each proleg. Head capsule is tan to green-brown with dark brown spots. Abdominal body stripes are double with a white bordered orange line. FAW larvae gray to brown-black, an inverted Y mark on the head capsule and longitudinal wide triple stripe pattern.
EVALUATE DAMAGE IN CORN:
Scout for BCW up until the V4 to V5 corn growth stage. Look in indicator fields that most likely to be infested with heavy weed cover before planting, no-till, along woods streams and low lying areas. Look for cut or wilted plants then dig at the base of plants to find and identify the cutworm and estimate the instar stage or size of the worm. Check at least 50 plants in 5 places in the field. If 2-3 percentage of plants are cut or wilted sand larvae are not fully grown at less than ¾ inch long then insecticide application may be necessary.
Scout for AW beginning at field edges where grass weeds are more prevalent. If larvae are found then inspect the field further. Randomly select 20 plants in 5 areas of the field. Control is recommended when 50 percent of plants are infested and larvae less than 1¼ inch are present. Larvae will hide deep in the whorl or in residue near the base of the plant during the day.
- BT corn:
- Hybrids with Herculexâ 1, AgriSure Vipteraâ, and Genuityâ SmartStaxâ are considered effective control of BCW and FAW larvae. AgriSure Viptera also controls AW. However, intense infestations and larger larvae may not be fully controlled.
- Seed treatment:
- High rates of neonicotinoid insecticides could be considered suppression and not full control. Large larvae moving from weeds to corn are difficult to control with seed treatment rates.Neonicotinoid are most effective against hatching and small larvae.
- Rescue insecticide treatments
- Effective rescue of damaged fields is accomplished with the use of pyrethroid insecticides. Granular and liquid soil applied insecticides are useful but will sometimes fail in heavy infestations. A rescue treatment is most effective when timing is correct. See your state extension publications for products and rates.
References and additional information
Note: The information in this issue is based upon field observations and third party information. Since variations in local conditions may affect the information and suggestions contained in this issue, LG Seeds disclaims legal responsibility therefore. Always read and follow label instructions.
- Boyd, Michael and Wayne C. Bailey. 2004. Management of the Armyworm Complex in Missouri Field Crops.4 pp. MU Extension, University of Missouri http://extension.missouri.edu/p/G7115
- FIELD CROPS IPM. Black Cutworm. Purdue University. https://extension.entm.purdue.edu/fieldcropsipm/insects/black-cutworms.php
- Krupke, Christian, John L. Obermeyer and Larry Bledsoe. 2016. CORN INSECT CONTROL RECOMMENDATIONS 2016. E-219W. Purdue University Extension. https://extension.entm.purdue.edu/publications/E-219.pdf
- Krupke, Christian and John Obermeyer. 2017. Healthy Dose of Armyworm Moths. Pest and Crop Newsletter. Issue 3, April 14, 2017. https://extension.entm.purdue.edu/pestcrop/2017/Issue3/
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