How to Manage Pests

UC Pest Management Guidelines

Strawberry

Beet Armyworm

Scientific Name: Spodoptera exigua

(Reviewed 6/08, updated 5/10)

In this Guideline:


DESCRIPTION OF THE PEST

The beet armyworm adult is a gray and brown moth that lays its masses of round, pale-colored eggs beneath a covering of hairlike fluff collected from their wings. Newly hatched armyworms are often green in color and feed in groups, skeletonizing the undersides of leaves. Older beet armyworm larvae are green and smooth skinned with light stripes lengthwise along their sides. They commonly have a black spot on their side above the second leg.

DAMAGE

Moths from overwintering larvae lay eggs in spring, and young larvae feed on foliage and crowns before attacking berries. Greatest damage can occur in southern California and Santa Maria growing areas if larvae feed in the crowns of newly transplanted strawberry plants. Feeding at this time can kill the young transplants. Damage also can occur to summer-planted strawberries. Fall populations of armyworm moths will often fly into strawberry fields to lay eggs. Newly hatched armyworms feed on foliage, skeletonizing the upper or lower leaf surfaces next to their egg mass. Beet armyworm populations can build within second-year plantings and damage fruit in spring in southern California and later elsewhere. Larger armyworms feed directly into the berries; smaller armyworms will often feed on the shoulder of the berry beneath the calyx sepals.

MANAGEMENT

As with lygus and cutworm management, weed control is an important aspect of managing armyworms. Treatments may be necessary in southern California if beet armyworm populations are high around the time of transplanting. At other times, evaluate the level of parasitism and mortality from disease before making a decision to treat for beet armyworm.

Biological Control
Young beet armyworms can be heavily parasitized by the ichneumonid parasite, Hyposoter exiguae. This parasite can easily be monitored in the armyworm populations by simply pulling young worms apart and looking for the parasite larva inside. In addition, armyworms often become diseased with a virus that can cause high mortality; larvae turn black when killed by the virus. High natural mortality translates to few mature larvae surviving to cause further damage.

Cultural Control
Because adult moths are attracted to weeds for egg laying, good weed control helps minimize armyworm populations.

Organically Acceptable Methods
Cultural and naturally occurring biological controls, and sprays of Bacillus thuringiensis ssp. aizawai or the Entrust formulation of spinosad are acceptable for use on organically certified strawberries.

Monitoring and Treatment Decisions
In southern California and the Santa Maria growing areas, plants are most vulnerable to beet armyworms soon after transplanting when larval feeding in the crown can kill the young transplants. Monitor beet armyworms flights with pheromone traps just before and after transplanting. If trap catches indicate a lot of beet armyworm activity, examine young strawberry plants for egg masses and time treatments to egg hatch.

At other times of the year and in other areas, if large numbers of predators, parasites, or virus are present, delay treating to determine if the armyworms might be controlled by the natural enemies.

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 and impact on natural enemies and honey bees. When choosing a pesticide, also consider information relating to environmental impact. Not all registered pesticides are listed. Always read label of product being used.
 
A. SPINOSAD
  (Entrust)# 1.25–1.5 oz 4 1
  (Success) 6 fl oz 4 1
  MODE OF ACTION GROUP NUMBER1: 5
  COMMENTS: Most effective against younger larvae. Rotate to an insecticide with a different mode of action after two successive applications. Maintaining proper pH of the spray tank water is critical for maximum efficacy.
 
B. BACILLUS THURINGIENSIS ssp. AIZAWAI#
  (Xentari) 0.5–2 lb 4 0
  MODE OF ACTION GROUP NUMBER1: 11
  COMMENTS: Treat when armyworms are still small. To be effective, Bt must be applied no later than the 2nd instar.
 
C. METHOXYFENOZIDE
  (Intrepid 2F) 6–12 fl oz 4 3
  MODE OF ACTION GROUP NUMBER1: 18
 
D. SPINETORAM
  (Radiant SC) 6–10 fl oz 4 1
  MODE OF ACTION GROUP NUMBER1: 5
  COMMENTS: Rotate to an insecticide with a different mode of action after two successive applications of either spinetoram or spinosad to help delay the development of resistance to Group 5 insecticides. The use of this product may best be reserved for control of western flower thrips because the options are more limited for this pest.
 
E. DIAZINON* 12.75 fl oz/
100 gal water
3 days 5
  MODE OF ACTION GROUP NUMBER1: 1B
  COMMENTS: Do not allow this material to run off into surface waters.
 
F. CHLORPYRIFOS*
  (Lorsban 4E) 1 qt 24 21
  MODE OF ACTION GROUP NUMBER1: 1B
  COMMENTS: Do not allow this material to run off into surface waters.
 
+ 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.
# Acceptable for use on organically grown produce.
* Permit required from county agricultural commissioner for purchase or use.
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: Strawberry
UC ANR Publication 3468

Insects and Mites

  • F. G. Zalom, Entomology, UC Davis
  • M. P. Bolda, UC Cooperative Extension, Santa Cruz County
  • S. K. Dara, UC Cooperative Extension, Santa Barbara County
  • S. Joseph, UC Cooperative Extension, Monterey County
Acknowledgment for contributions to the Insects and Mites:
  • P. A. Phillips, UC IPM Program and UC Cooperative Extension, Ventura County
  • N. C. Toscano, Entomology, UC Riverside

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