When Armyworms Come to Town By Drs. Heather Darby and Sid Bosworth, University of Vermont Extension Agronomists You hear that armyworms have been spotted in Vermont or surrounding states. What steps should you take? First, don’t panic! The best first step is to go out and scout your corn and grass fields for armyworms.
When full grown, the caterpillars can be almost 1.5 inches long. They are usually greenish or brownish, but can be almost black. The sides and back of the caterpillar have light colored stripes running along the body. Armyworm caterpillars normally feed at night and much damage can occur before they mature. Their preferred foods are grasses including corn, small grains, and forage species. They first strip the leaves and then consume midribs, seed heads and finally stems. They will feed on other plants if grasses are unavailable. Feeding will start on the lower leaves and move upwards. A large population can strip an entire field in just a few days. When the field is eaten, they “march’ to adjacent fields.
Corn Fields In corn, armyworms usually confine their feeding to the leaf margins except when populations are very large, and then they consume all the leaves except for the tougher midrib. Feeding starts on the lower leaves and as these leaves are eaten, the armyworms move to the upper leaves. Look for ragged holes and pellet-like droppings in the whorls and scattered on the ground. Larvae do not tunnel into the stalk and they do not feed on the growing point, at least on larger plants. Cornfields that are most susceptible include fields that are either minimum or no-tilled into grass sod or fields infested with grass weeds. Cornfields that have grassy weeds sprayed with a post-emergence herbicide should be closely scouted as the weeds begin to die. Armyworms, if present, will move immediately to the corn. In some fields, armyworms have eaten the young corn nearly to the ground. Many farmers ask if their corn plants can recover from the defoliation. It is recognized that a corn plant can recover from severe degrees of defoliation; the extent of recovery will depend upon the stage of plant growth at the time of damage and the extent of the damage. Generally, crop loss is negligible when defoliation occurs very early in the growth of the plant. The corn crop can recover easily since its growing point remains below ground and there is still potential to produce more leaves. However, as the plants mature the level of recovery will be greatly reduced. Armyworm damage to cornfield.
Insecticide applications to control armyworm should be judiciously applied, see Table 1 for Vermont product recommendations. Decisions to control armyworm with an insecticide should be based on crop size or stage, armyworm size, crop damage and anticipated movement of larvae from one field to another. Besides the expense of application, insecticides could also kill natural insect predators that normally control armyworm. According to Cornell recommendations, an insecticide should only be applied to corn in the whorl stage if most plants are showing damage and about three larvae per plant are found. Penn State recommendations are to treat only when 25% of plants are damaged or killed. Larvae size is also important. If armyworms are less than 3/4 inch in length they still have another week or so to feed. If larvae are mostly 1 1/2 inches in length, then they are nearly done feeding and very little additional leaf injury will occur so the field should not be sprayed; it is too late for the insecticide to be of any economic benefit. A border 20 to 40 feet wide treated with insecticide will prevent armyworms from invading from or into an adjacent field or non-infested area within a field.
Grass Hay and Perennial Pastures Generally, armyworms first feed on the blades of the grass leaves disregarding the sheaths and stems until last. In legume/grass mixtures, the legume can remain standing with all the grass stripped out clean. However, under high larvae populations, even the legume can be consumed. The question concerning perennial grasses is whether permanent damage has occurred to the stand. From our experiences in 2001 (the last time we experienced a serious outbreak of armyworm), the level of damage varied tremendously. For heavily infested fields (“thousands of worms”), the larvae devoured the grass plants all the way down to the crown. Recovery was very slow to almost non-existent. Many of these fields were reseeded. However, most of fields had only damage to that growth and, once harvested, regrowth was normal. Other fields with more severe damage showed slower regrowth, but eventually fully recovered. Pastures can also be affected by armyworm. In the 2001 outbreak, one farmer noted that although all the grasses were eaten, about 50% of his pasture was still made up of clovers, dandelion, plantain and other broadleaf forbs. He had his animals graze the pasture immediately and it did fully recover later in the season. Before making a quick decision to till the field and reseed, it is important to wait and observe regrowth. Once regrowth commences, a small amount of manure or fertilizer could also help with the recovery. Overseeding with clovers the following spring (frost seeding) could also help Armyworms in New York increase desirable species. field. There is little to no information on threshold levels for grass hay fields. Based on small grain recommendations, we suggest using 4 to 5 larvae (less than 3/4 inch in length) per square foot. Be sure to check under debris as well as on the plants for larvae. If the field is cut as a control strategy, be sure to continue monitoring surrounding fields and regrowth. In mixed legume/grass fields, the armyworm will only start eating legumes when all other resources are consumed. If there are nearby corn, small grain, or grass fields, it may be prudent to spray a 20 to 40 foot border to kill invading larvae. Learn more about true armyworm at: go.uvm.edu/truearmyworm. For more information on scouting and control options, please contact Dr. Heather Darby at (802) 524-6501 or Dr. Sid Bosworth at 802) 656-0478.
Table 1. Product Recommendations from the Vermont Agency of Agriculture Product Name
EPA Reg. No.
RUP
Active Ingredient(s)
Misc
Cavalry Insecticide
100-1112-534
y
Lambda-Cyhalothrin
Lambda-T Lamcap II Warrior II With Zeon Technology Insecticide Grizzly Too Insecticide Calvalry II
100-1112-5905 100-1295
y y
Lambda-Cyhalothrin Lambda-Cyhalothrin
100-1295 100-1295-1381 100-1295-534
y y y
Lambda-Cyhalothrin Lambda-Cyhalothrin Lambda-Cyhalothrin
Lambda T-2
100-1295-5905
y
Lambda-Cyhalothrin
Besiege Insecticide
100-1402
y
Lambda-Cyhalothrin/Chlorantraniliprole
PyGanic Crop Protection EC 1.4 II
1021-1771
n
Pyrethrins
OMRI
PyGanic Crop Protection EC 5.0 II Lambda-Cyhalothrin 1 EC Insecticide
1021-1772 228-708
n y
Pyrethrins Lambda-Cyhalothrin
OMRI
Baythroid XL
264-840
y
Beta-Cyfluthrin
Mustang Insecticide
279-3126
y
Cypermethrin-S
Mustang Maxx Insecticide
279-3426
y
Cypermethrin-S
Tombstone Insecticide
34704-912
y
Cyfluthrin
Tombstone Helios Insecticide
34704-978
y
Cyfluthrin
Entrust
62719-282
n
Spinosad
OMRI
Entrust SC Silencer
62719-621 66222-104
n y
Spinosad Lambda-Cyhalothrin
OMRI
Lambda-Cy EC Insecticide
70506-121
y
Lambda-Cyhalothrin
LambdaStar EC
71532-20-91026
y
Lambda-Cyhalothrin
LambdaStar 1CS
71532-25-91026
y
Lambda-Cyhalothrin
LambdaStar Plus
71532-29-91026
y
Lambda-Cyhalothrin
Kendo Insecticide
74530-38
y
Lambda-Cyhalothrin
This list is not inclusive of all products registered and available. All additional label directions and restrictions must be followed (e.g., buffers, crop rotation, maximum annual application rates, time to grazing, PHI, retreatment intervals, time to slaughter) June 15, 2017
Last updated June 2017. Any reference to commercial products, trade names, or brand names is for information only, and no endorsement or approval is intended. Issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, in cooperation with the United States Department of Agriculture. University of Vermont Extension, Burlington, Vermont. University of Vermont Extension, and U.S. Department of Agriculture, cooperating, offer education and employment to everyone without regard to race, color, national origin, gender, religion, age, disability, political beliefs, sexual orientation, and marital or familial status.
