Taking an environmentally sensitive approach to pest management


Integrated Pest & Crop Management



AUTHOR

Wayne C. Bailey
University of Missouri
Division of Plant Sciences
(573) 882-2838
baileyw@missouri.edu

True Armyworm in Grass Pastures, Wheat and Corn

Wayne C. Bailey
University of Missouri
(573) 882-2838
baileyw@missouri.edu

Published: June 6, 2011

True Armyworm in Grass Pastures, Wheat and Corn. Wheat, tall fescue, grass pastures, and occasionally field corn are all hosts of true armyworm (Mythimna unipuncta formerly Pseudaletia unipuncta). Problems with true armyworm larvae are occurring in some grass pastures in south central and southwestern regions of the state and in wheat and corn in scattered fields throughout Missouri. This pest is most active in grass pastures where larvae were first reported about three weeks ago and continue to cause problems in some areas. The major damage in tall fescue and other grass pastures is defoliation 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.

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. 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 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. 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. 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.

Several corn fields have reached the economic threshold for true armyworm this past week. Larvae of this pest can severely damage corn when high 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 cause excessive defoliation and plant mortality.

Table 1. Insecticides Control of True Armyworm in Tall Fescue and Grass Pastures
Chemical Name Trade Name Rate of formulated material per acre Placement/Comments
malathion Malathion
several Products
see specific labels foliar broadcast
zeta-cypermethrin *Mustang Max **2.8 to 4.0 fl oz/acre foliar broadcast
carbaryl Sevin XLR Plus 1 to 1 1/2 quarts/acre foliar broadcast
spinosad Success 3 to 6 fl oz./acre foliar broadcast
spinosad Tracer 4SC
Voliam xpress
1.0 to 3.0 fl oz/acre foliar broadcast

 

Table 2. Insecticides Control of True Armyworm in Wheat
Chemical Name Trade Name Rate of formulated material Placement/Comments REI Hours Pre-Harvest Intervals Days
cyfluthrin *Baythroid XL 1.8 to 2.4 fl oz foliage
1st & 2nd instars only
12 30 (grain)
3 (grazing or forage)
methomyl *Lannate SP 1/4 to 1/2 lb foliage 48 7 (grain)
10 (grazing or feeding)
zeta-cypermethrin *Mustang Max 1.76 to 4.0 fl oz foliage 12 14 (grain. forage, hay)
chlorpyrifos *Nufos 4E 1 pt foliage 24 28 (grain or straw)
14 (forage or hay)
microencapsulated
methyl parathion
*Penncap-M 2 to 3 pt foliage 48 15 (harvest or graze)
carbaryl Sevin 80S 1 1/4 to 1 7/8 lb foliage 12 21 (grain or straw)
7 (hay or forage)
spinosad Tracer naturalyte 1.5 to 3.0 fl oz foliage 4 21 (grain or straw)
14 (forage or hay)
chlorpyrifos +
bifenthrin
*Stallion 9.25 to 11.75 fl oz foliage 24 14 (grazing) 28 (straw)
cyfluthrin *Tombstone Helios 1.8 to 2.4 fl oz foliage 12 30 (grain)
7 (grazing)
lambda-cyhalothrin *Warrior II with Zeon 1.28 to 1.92 fl oz foliage 24 30 (grain or straw)
7 (hay or forage)
* Designates a restricted-use pesticide. Use is restricted to certified applicators only.
Read the label to determine appropriated insecticide rates. Be sure to follow all directions, precautions and restrictions.

 

Table 3. Insecticides Control of True Armyworm in Corn
Chemical Name Trade Name Rate of formulated material Placement/Comments REI Hours Pre-Harvest Intervals Days
permethrin *Ambush 25WP 6.4 to 12.8 fl oz foliage
12 30 (grain or stover), 0 (forage)
permethrin *Ambush Insecticide 6.4 to 12.8 fl oz foliage 12 30 (grain or stover), 0 (forage)
permethrin *multiple products see specific label foliage 12 see specific label
esfenvalerate *Asana XL 5.8 to 9.6 fl oz foliage 12 21 (grain)
cyfluthrin *Baythroid XL
(for 1st & 2nd instars)
1.6 to 2.8 fl oz foliage 12 21 (grain or fodder)
0 (green forage)
flubendiamide *Belt SC 2.0 to 3.0 fl oz foliage 12 1 (green forage and silage)
28 (grain or stover)
bifenthrin *Brigade 2EC 2.1 to 6.4 fl oz foliage 12 30 (grain, fodder, graze)
chlorpyrifos +
gamma-cyhalothrin
*Cobalt 13 to 26 fl oz foliage 24 21 (grain or ears)
14 (graze or silage harvest)
deltamethrin *Delta Gold 1.5EC 1.5 to 1.9 fl oz foliage 12 21 (grain, fodder)
12 (cut forage or graze)
zeta-cypermethrin +
bifenthrin
*Hero 4.0 to 10.3 fl oz foliage 12 30 (grain, stover, graze)
60 (forage)
methoxyfenozide Intrepid 2F 4.0 to 8.0 fl oz foliage 4 21 (grain)
methomyl *Lannate SP 1/4 to 1/2 lb foliage 48 0 (ears), 3 (forage), 21 (fodder)
methomyl *Lannate LV 3/4 to 1 1/2 pt foliage 48 0 (ears), 3 (forage), 21 (fodder)
chlorpyrifos *Lorsban Advanced 1 to 2 pt foliage 24 21 (grain, ears ,forage, fodder)
chlorpyrifos *Lorsban 4E 1 to 2 pt foliage 24 21 (grain, ears ,forage, fodder)
zeta-cypermethrin *Mustang Max 3.2 to 4.0 fl oz foliage 12 30 (grain, stover) 60 (forage)
chlorpyrifos *Nufos 4E 1 to 2 pt foliage 24 21 (grain or ears)
microencapsulated
methyl parathion
*Penncap-M 2 to 3 pt foliage 48 12 (grain, forage, graze)
carbaryl Sevin 4F 2 to 4 pt foliage 12 48 (grain or fodder)
14 (harvest or graze forage)
chlorpyrifos +
bifenthrin
*Stallion 9.25 to 11.75 fl oz foliage 24 30 (grain, stover) 60 (forage)
cyfluthrin *Tombstone Helios 1.6 to 2.8 fl oz foliage 12 21 (grain or fodder), 0 (forage)
spinosad Tracer 4SC 1.0 to 3.0 fl oz foliage 1 28 (grain), 3 (fodder or forage)
lambda-cyhalothrin *Warrior II 1.28 to 1.92 fl oz foliage 24 21 (grain), 1 (graze, forage)
21 (treated feed or fodder)
* Designates a restricted-use pesticide. Use is restricted to certified applicators only.
Read the label to determine appropriated insecticide rates. Be sure to follow all directions, precautions and restrictions.
   About IPM     Contact Us    Subscribe     Unsubcribe

Copyright © 2018 — Curators of the University of Missouri. All rights reserved. DMCA and other copyright information. An equal opportunity/access/affirmative action/pro-disabled and veteran employer.

Printed from: https://ipm.missouri.edu
E-mail: IPM@missouri.edu

REVISED: November 10, 2011