Agriculture: Strawberry Pest Management Guidelines

Beet Armyworm

Spodoptera exigua

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 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 (late winter in Southern California). Young larvae feed on foliage and crowns before attacking berries. Damage most commonly occurs in Southern California and Santa Maria growing areas, but damage can be serious in any region if larvae feed on 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 adult females 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 numbers can become greater in previously infested second-year plantings and damage fruits in spring. Larger armyworms feed directly into the berries, while smaller armyworms often feed on the shoulder of the berry beneath the calyx sepals.

Management

As with lygus and cutworm management, weed control in and around fields is an important aspect of managing armyworms. Insecticides may be necessary in Southern California if beet armyworm numbers are high around the time of transplanting. At other times, consider the level of parasitism and mortality from disease before making the decision to spray for beet armyworm.

Biological Control

Young beet armyworms can be heavily parasitized by the ichneumonid parasitic wasp, 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 numbers.

Organically Acceptable Methods

Use cultural and naturally occurring biological controls, and sprays of Bacillus thuringiensis ssp. aizawai or the Entrust formulation of spinosad on organically certified strawberries.

Monitoring and Treatment Decisions

In Southern California and the Santa Maria growing area, 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 moth 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 spraying to determine if armyworms will be controlled by natural enemies.

Common name Amount per acre REI‡ PHI‡
(Example trade name) (hours) (days)
Not all registered pesticides are listed. The following are ranked with the pesticides having the greatest IPM value listed first—the most effective and least harmful to natural enemies, honey bees, and the environment are at the top of the table. When choosing a pesticide, consider information relating to air and water quality, resistance management, and the pesticide's properties and application timing. Always read the label of the product being used.
 
A. METHOXYFENOZIDE
  (Intrepid 2F) 6–12 fl oz 4 3
  MODE-OF-ACTION GROUP NUMBER1: 18
 
B. 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 insecticide may best be reserved for control of western flower thrips because the options are more limited for this pest. Toxic to bees; do not spray directly or allow to drift onto blooming crops or weeds where bees are foraging.
 
C. SPINOSAD
  (Entrust)# 1.25–2 oz 4 1
  (Success) 4–6 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. Toxic to bees; do not spray directly or allow to drift onto blooming crops or weeds where bees are foraging.
 
D. BACILLUS THURINGIENSIS ssp. KURSTAKI#
  (Deliver) 0.5–1.5 lb 4 0
  MODE-OF-ACTION GROUP NUMBER1: 11A
  COMMENTS: Spray when armyworms are still small. To be effective, Bacillus thuringiensis must be applied no later than the second instar stage.
 
E. BACILLUS THURINGIENSIS ssp. AIZAWAI#
  (Agree WG, Xentari) 0.5–2 lb 4 0
  MODE-OF-ACTION GROUP NUMBER1: 11A
  COMMENTS: Spray when armyworms are still small. To be effective, Bacillus thuringiensis must be applied no later than the second instar stage.
 
F. DIAZINON*
  (Diazinon AG600 WBC) 12.75 fl oz/100 gal water 72 (3 days) 5
  MODE-OF-ACTION GROUP NUMBER1: 1B
  COMMENTS: Do not allow this insecticide to run off into surface waters. Highly toxic to bees; do not spray directly or allow to drift onto blooming crops or weeds where bees are foraging.
Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (PHI) 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 insecticides 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; insecticides with a 1B group number should be alternated with insecticides that have a group number other than 1B. Mode-of-action group numbers for insecticides and miticides (un=unknown or uncertain mode of action) are assigned by IRAC (Insecticide Resistance Action Committee).
Text Updated: 07/18
Treatment Table Updated: 07/18