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How to Manage Pests

UC Pest Management Guidelines


Larva of the armyworm, Pseudaletia unipuncta.

Small Grains

Armyworms

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

(Reviewed 2/07, updated 2/09)

In this Guideline:


DESCRIPTION OF THE PESTS

The armyworm is pale green when young; as they mature they become greenish brown to black with a yellowish stripe along each side. Young larvae move like loopers, arching their body into a loop as they crawl. Western yellowstriped armyworms are black with yellow or orange stripes along the side. Mature larvae of both species may reach 2 inches in length.

DAMAGE

Larvae of both species cause damage by eating leaves. Entire leaves may be consumed or damage may consist of notches chewed out of the leaves giving them a tattered look. Damage may occur when larvae hatch from eggs laid in the cereal crop or from larvae migrating into the cereal crop from an adjacent field.

MANAGEMENT

Biological Control
Armyworms are attacked by a parasitic wasp, Hyposoter sp. Parasitized worms can be identified by pulling the larvae apart and looking for the green parasite larvae that pop out. Hyposoter is usually not active in early spring when cereals may be attacked by armyworms but growers should check for its presence. Virus diseases of armyworms may also be important natural control agents. Diseased caterpillars first appear yellowish and limp, and after death hang from plants as shapeless, dark tubes from which the disintegrated body contents ooze.

Organically Acceptable Methods
Biological and cultural controls and sprays of Bacillus thuringiensis.

Cultural Control
If larvae are moving into the cereal crop from an adjacent crop, some control may be obtained by plowing a deep, wide ditch between the two fields and keeping it filled with water until the migration stops.

Monitoring
Examine fields periodically for the presence of armyworms. Larvae hatching from eggs laid in the field may be found throughout the field. Those migrating in from an adjacent crop will most likely be found at the edge of the field.

Management Decisions
No economic threshold levels have been established for armyworms. Fields should be treated if armyworm numbers are sufficient to cause defoliation. Small caterpillars, less than 0.5 inch long, are easier to kill than larvae over 0.5 inch in length.

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 relating to natural enemies and honey bees as well as the environmental impact. Not all registered pesticides are listed. Always read label of product being used.
 
A. METHOMYL*
  (Lannate SP) 0.25–0.5 lb 48 7
  MODE OF ACTION GROUP NUMBER1: 1A
  COMMENTS: May be used on barley, rye, oats, and wheat. Do not graze within 10 days. Do not apply more than 1.8 lb a.i./acre/crop. Highly toxic to honey bees if bees are present at treatment time or within a day after.
 
B. METHYL PARATHION*
  (Penncap-M) 2–3 pt 5 days 15
  MODE OF ACTION GROUP NUMBER1: 1B
  COMMENTS: May be used on barley, oats, and wheat. This product is highly toxic to foraging bees, young hive bees, and brood. Do not apply to any field when bees are present or in the surrounding vicinity. Do not graze within 15 days.
 
C. BACILLUS THURINGIENSIS ssp. KURSTAKI#
  (various products) Label rates 4 0
  MODE OF ACTION GROUP NUMBER1: 11.B2
  COMMENTS: Effective only on larvae less than 0.5 inches long. This material can be applied at any time with reasonable safety to bees.
 
+ 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 these 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: Small Grains
UC ANR Publication 3466
Insects and Mites
C. G. Summers, Entomology, UC Davis/Kearney Agricultural Center, Parlier
L. D. Godfrey, Entomology, UC Davis
Acknowledgment for contributions to Insects and Mites:
D. Gonzalez, Entomology, UC Riverside

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