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UC Pest Management Guidelines


Beet armyworm larva.

Cole Crops

Beet Armyworm

Scientific Name: Spodoptera exigua

(Reviewed 6/07, updated 10/10)

In this Guideline:


DESCRIPTION OF THE PEST

Beet armyworms vary in color but are usually a shade of olive green with many fine, wavy, light colored stripes down the back and sides. Often there is a dark spot on the side of the body just above the second true leg. The body surface is smooth and almost hairless. Mature larvae may be up to 1.5 inches long. Adult moths lay their eggs in scale-covered cottony masses on leaf surfaces. When eggs first hatch, the tiny larvae feed in groups near the egg mass, skeletonizing or completely consuming leaves. As they grow older, larvae disperse and move toward the center of the plant. Beet armyworms build up as weather warms and are most common on late summer and fall crops.

DAMAGE

Beet armyworms can destroy seedlings, consume large portions of leaves, or stunt growth by feeding on buds. However, serious economic damage to cole crops is uncommon.

MANAGEMENT

Cultural and biological controls help suppress armyworm populations. Disc fields immediately following harvest to kill larvae and pupae. Destroy weeds along field borders; armyworms often migrate from these areas into newly planted fields. Seedlings are very susceptible to armyworm damage. Fields should be monitored frequently for beet armyworm from planting until heading.

Biological Control
Many natural enemies attack beet armyworms. Among the most common parasites are the wasps Hyposoter exiguae and Chelonus insularis, and the tachinid fly Lespesia archippivora. Viral diseases may also be important.

Organically Acceptable Methods
Biological control and sprays of Bacillus thuringiensis and the Entrust formulation of spinosad are organically acceptable management tools.

Monitoring and Treatment Decisions
Start monitoring for beet armyworm even before seedlings emerge. Check for egg masses and young larvae in pigweeds, lambsquarters, nettleleaf goosefoot, and other weeds surrounding the field. If populations are high on weeds, watch especially carefully for infestations on crop seedlings. Pheromone traps are also available for monitoring adult flights in order to predict egg laying.

Once seedlings emerge, check them at least twice a week for armyworm egg masses and young larvae. Treat if you find one second or third instar larva for every 10 plants. Sample for armyworms at the same time you are sampling for loopers and cabbageworms and include them in your total counts for caterpillars. It is not usually necessary to treat older plants between thinning and heading. Treat just before heading if caterpillars are in the field. Beet armyworms are more difficult to control with insecticides than loopers and cabbageworms, so be sure to make note of their presence in your monitoring records. There are reports of insecticide resistance to certain materials in certain areas. The more broad-spectrum insecticides (endosulfan, methomyl) adversely affect natural enemies in most cases.

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. BACILLUS THURINGIENSIS ssp. AIZAWAI#
  (various products) Label rates 4 0
  MODE OF ACTION GROUP NUMBER1: 11.B1
  COMMENTS: This material is most effective against newly hatched larvae so proper treatment timing is essential.
 
B. EMAMECTIN BENZOATE*
  (Proclaim) 3.2–4.8 oz 12 14
  MODE OF ACTION GROUP NUMBER1: 6
 
C. INDOXACARB
  (Avaunt) 2.5–3.5 oz 12 3
  MODE OF ACTION GROUP NUMBER1: 22
  COMMENTS: Do not apply more than 14 oz/acre/crop. Add a wetting agent to improve coverage. Minimum interval between sprays is 3 days.
 
D. SPINETORAM
  (Radiant) SC 5–10 fl oz 4 1
  MODE OF ACTION GROUP NUMBER1: 5
  COMMENTS: Toxic against some natural enemies (predatory thrips, syrphid fly larva, beetles) when sprayed and 5 to 7 days after. Control improved with addition of an adjuvant.
 
E. SPINOSAD
  (Entrust)# 1.25–3 oz 4 1
  (Success) 6 oz 4 1
  MODE OF ACTION GROUP NUMBER1: 5
  COMMENTS: Toxic against some natural enemies (predatory thrips, syrphid fly larva, beetles) when sprayed and 5 to 7 days after.
 
F. CHLORANTRANILIPROLE
  (Coragen) 3.5–5 fl oz 4 3
  MODE OF ACTION GROUP NUMBER1: 28
  COMMENTS: Foliar application; use with an effective adjuvant for best performance.
 
G. FLUBENDIAMIDE
  (Synapse) WG 2 oz 12 1
  MODE OF ACTION GROUP NUMBER1: 28
  COMMENTS: Do not apply more than 4 oz/acre (0.06 lb a.i./acre)/crop season.
 
H. METHOXYFENOZIDE
  (Intrepid) 2F 8 fl oz 4 1
  MODE OF ACTION GROUP NUMBER1: 18A
  COMMENTS: For early season applications only to young crop and small plants.
 
I. CRYOLITE
  (Cryolite) 96W 8–16 lb 12 7
  MODE OF ACTION GROUP NUMBER1: 9A
  COMMENTS: Use on broccoli, Brussels sprouts and cauliflower. Must be ingested by the insect. Apply when young caterpillars are present. Can be used in an insecticide resistance management program.
 
J. METHOMYL*
  (Lannate) LV 0.75–3 pt 48 see comments
  (Lannate) 90SP 0.25–1 lb 48 see comments
  MODE OF ACTION GROUP NUMBER1: 1A
  COMMENTS: Preharvest interval is 3 days for broccoli, Brussels sprouts, and cauliflower and 1 day for cabbage. See label for other cole crops.
 
+ 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 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 to help prevent the development of resistance. 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 Group numbers are 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: Cole Crops
UC ANR Publication 3442
Insects and Mites
E. T. Natwick, UC Cooperative Extension, Imperial County
Acknowledgments for contributions to Insects and Mites:
W. J. Bentley, UC IPM Program, Kearney Agricultural Center, Parlier
W. E. Chaney, UC Cooperative Extension, Monterey County
N. C. Toscano, Entomology, UC Riverside

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