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UC Pest Management Guidelines


Larva of the armyworm, Pseudaletia unipuncta.

Corn

Armyworms

Scientific names:
Armyworm: Mythimna (= Pseudaletia) unipuncta
Beet armyworm: Spodoptera exigua
Western yellowstriped armyworm: Spodoptera praefica

(Reviewed 1/06, updated 8/08)

In this Guideline:


DESCRIPTION OF THE PESTS

Eggs of the armyworm and beet armyworm are pale green to pink, striated, and deposited in a mass that is covered with a white cottony material. At first, the tiny, newly hatched larvae feed in a colony, skeletonizing the first few leaves on which they feed. The larvae are about 1.25 inches long when full grown. Armyworms are greenish brown in color with several longitudinal stripes, while beet armyworm are mottled olive green to almost black.

Eggs of the western yellowstriped armyworm are similar to those of the beet armyworm. The egg masses are larger and covered by a gray cottony material. Newly hatched larvae also feed in colonies; mature larvae attain a length of 1.5 to 2.0 inches and are black with a prominent yellowish stripe and several narrow bright ones on each side of the body. An intense black spot is usually visible on each side of the first legless segment behind the head.

DAMAGE

Newly hatched armyworms feed in colonies and skeletonize leaves. Larvae that are half grown or more will feed singly on leaves, in the ear, or on the tassels. Damage by army­worm to the ears resembles that caused by corn earworm. Because of the damage to tassels and ears, tolerance for armyworm damage in sweet corn is very low.

MANAGEMENT

Armyworms are often attracted to fields with barnyardgrass. When they are done feeding on barnyardgrass, larvae move to corn so it is important to control this weed.Specific treatment thresholds have not been established for armyworms on field corn, but treatment is seldom necessary. Sweet corn, however, has a greater potential for damage and may require treatment. While these pests may be present any time from June through September, populations are usually most damaging in late summer. For sweet corn, pheromone traps may be useful to determine the timing of moth flights and subsequent larval infestations. In those rare instances where control measures are required, the beet armyworm is more difficult to control than the western yellowstriped armyworm and may require the higher treatment rate of the materials recommended below. Insecticide applications will be most effective if applied against small larvae. Frequently, spot treatments are sufficient.

Organically Acceptable Methods
Controlling weeds, especially barnyardgrass, and applying sprays of Bacillus thuringiensis are organically acceptable management tools.

Common name Amount/Acre** R.E.I.+ P.H.I.+
(trade name)   (hours) (days)

  Calculate impact of pesticide on air quality
The following materials are listed in order of usefulness in an IPM program, taking into account efficacy, information related to natural enemies and honey bees and environmental impact. Not all registered pesticides are listed. Always read label of product being used.
 
A. INDOXACARB
  (Avaunt) 2.5–3.5 oz 12 Sweet corn: 3
        Fodder/stover: 35
  MODE OF ACTION GROUP NUMBER1: 22
  COMMENTS: For use on sweet corn only. Can only be used before silking. Do not apply more than 14 oz product/acre/crop and allow at least 3 days between sprays.
 
B. METHOXYFENOZIDE
  (Intrepid) 2F 4–8 oz 4 Field: 21
        Sweet corn: 3 (ears/forage)
        21 (dry fodder)
  MODE OF ACTION GROUP NUMBER1: 18A
 
C. METHOMYL*
  (Lannate SP) 0.25–0.5 lb 48 see label
  MODE OF ACTION GROUP NUMBER1: 1A
  COMMENTS: Do not apply more than 2.25 lb a.i./acre/crop for field and popcorn, and 6.3 lb a.i./acre/crop for sweet corn. Phytotoxicity may occur on certain sweet corn varieties.
 
D. BACILLUS THURINGIENSIS ssp. AIZAWAI#
  (various products) Label rates 4 0
  MODE OF ACTION GROUP NUMBER1: 11.B1
  COMMENTS: This material may be less effective than broad-spectrum insecticides, but it does not destroy natural enemies of corn earworm. Control is maximized by thorough coverage and by making applications when larvae are small.
 
E. SPINETORAM
  (Radiant) SC 3–6 fl oz 4 see comments
  MODE OF ACTION GROUP NUMBER1: 5
  COMMENTS: For control of beet armyworms and armyworms; not for western yellowstriped armyworm. Preharvest interval for sweet corn and seed corn harvested for grain is 1 day and 3 days when harvested for forage and fodder; for field corn, teosinte, and popcorn it is 28 days for grain harvest and 3 days for forage and fodder.
 
F. ESFENVALERATE*
  (Asana XL) 5.8–9.6 fl oz 12 Seed corn/Popcorn: 1
        Field: 21
  MODE OF ACTION GROUP NUMBER1: 3
  COMMENTS: Do not exceed 0.25 lb a.i./acre/season on field corn and seed corn or 0.5 lb a.i./acre/season on popcorn.
 
G. PERMETHRIN*
  (Pounce) 3.2EC 4–8 oz 12 Grain or fodder: 30
Sweet corn: 1
  MODE OF ACTION GROUP NUMBER1: 3
  COMMENTS: For field corn, popcorn, field corn grown for seed, and sweet corn. Apply before brown silk stage.
 
** Mix with sufficient water to obtain full coverage.
+ Restricted entry interval (R.E.I.) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (P.H.I.) is the number of days from treatment to harvest. In some cases the REI exceeds the PHI. The longer of two intervals is the minimum time that must elapse before harvest.
* Permit required from county agricultural commissioner for purchase or use.
# Acceptable for use on organically grown produce.
1 Modes of action are important in preventing the development of resistance to pesticides. Rotate chemicals with a different mode-of-action group number, and do not use products with the same mode-of-action group number more than twice per season. For example, the organophosphates have a group number of 1B; chemicals with a 1B group number should be alternated with chemicals that have a group number other than 1B. Mode of action is assigned by IRAC (Insecticide Resistance Action Committee). For additional information, see their Web site at http://www.irac-online.org/

[Precautions]

PUBLICATION

[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Corn
UC ANR Publication 3443
Insects and Mites
L. D. Godfrey, Entomology, UC Davis
S. D. Wright, UC Cooperative Extension, Tulare County
C. G. Summers, Kearney Agricultural Center, Parlier
C. A. Frate, UC Cooperative Extension, Tulare County
Acknowledgement for contributions to Insect and Mites:
M. J. Jimenez, UC Cooperative Extension, Tulare County

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