How to Manage Pests

UC Pest Management Guidelines

Alfalfa

Beet Armyworm

Scientific Name: Spodoptera exigua

(Reviewed 1/17, updated 1/17)

In this Guideline:


Description of the Pest

The adult beet armyworm is a small, mottled gray- or dusky-winged moth. The moths are nocturnal (fly mostly at night) but may be seen by taking sweep net samples in the field.

Females deposit pale greenish or pinkish, striated eggs on the upper side of the alfalfa leaves in small or large masses covered with white cottony material. The eggs hatch in a few days, and the tiny caterpillars begin feeding on the plant. Heavy feeding on the tips of plant stems can cause flagging as terminal leaves turn white. The smooth-skinned caterpillars become full grown in about 2 to 3 weeks and are about 1.25 inches long. They range from olive green to almost black in color down the middle of the back with a yellow stripe on each side of the body.

Armyworms are common pests in the Central Valley and desert valleys from June through September. There are at least 5 generations per year in the low desert and four in the Central Valley. The final generation may overwinter as large larvae or pupae.

Damage

Armyworms skeletonize foliage, leaving veins largely intact. First- and second- instar larvae tend to feed in clusters around the egg mass from which they hatch. This frequently causes a tattered appearance to the terminals. This whitish appearance caused by the feeding is known as "whitecaps" and can be seen across a field. As the larvae mature and move to more stems, the areas of "whitecaps" tend to coalesce and the entire field takes on a tattered look.

Management

Populations of armyworms are frequently controlled by natural enemies and are more or less cyclic, occurring in large numbers only every few years. Early harvest, border cutting, and biological control are important components of a management program that will prevent damage from armyworms.

Biological Control

Natural enemies can provide good control of armyworms in many fields.

Predators include bigeyed bugs, spiders, minute pirate bugs, damsel bugs, assassin bugs, and lacewings.

The parasitic wasp, Hyposoter exiguae, is the most important of at least 10 parasites attacking this pest. Sample for parasitism by pulling the heads from older caterpillars and squeezing the body contents out toward the head end. Hyposoter larvae are a light, translucent green color.

Viral diseases of armyworms are also important natural control agents under certain conditions of temperature and humidity. Diseased caterpillars first appear yellowish and limp. After death they hang from plants as shapeless forms oozing the disintegrated body contents.

Cultural Control

Border-strip harvesting is a useful method for preserving natural enemies because it helps retain parasite larvae in the field. For more details, see BORDER-STRIP HARVESTING. Early cutting will give satisfactory control if the infestation appears late in the cutting cycle.

Organically Acceptable Methods

Biological and cultural controls, as well as sprays of Bacillus thuringiensis, are acceptable for use on an organically certified crop.

Monitoring and Treatment Decisions

In early summer start sweeping fields with adequate plant height 2 to 3 times per week to monitor for caterpillars and continue through fall. Divide each field into 4 sections and take 5 sweeps per section with a 15-inch diameter sweep net, for a total of 20 sweeps. For information on sampling, see SAMPLING WITH A SWEEP NET.

Combine monitoring of armyworms with monitoring for alfalfa caterpillar as described in ALFALFA CATERPILLAR AND ARMYWORM MONITORING. Count and record the number of healthy and parasitized caterpillars caught in your sweep net on a monitoring form (PDF).

If cutting is not practical or not scheduled soon after monitoring, apply a pesticide if there is an average of:

  • 10 or more nonparasitized alfalfa caterpillars per sweep,
  • 15 or more nonparasitized armyworms per sweep, or
  • 10 or more nonparasitized alfalfa caterpillars and armyworms combined per sweep.
Common name Amount per acre** REI‡ PHI‡
(Example trade name)   (hours) (days)
Calculate impact of pesticide on air quality
Bee precaution pesticide ratings
The following materials are listed in order of usefulness, taking into account efficacy and impact on natural enemies and honey bees. When choosing a pesticide, also consider environmental impact. Not all registered pesticides are listed. Always read label of product being used.
 
A. METHOXYFENOZIDE      
  (Intrepid 2F) Label rates 4 0 (forage)
        7 (fodder and hay)
  MODE-OF-ACTION GROUP NUMBER1: 18
  COMMENTS: Make no more than one application per cutting. Not for use in alfalfa grown for seed or for sprouts for human consumption.
 
B. CHLORANTRANILIPROLE      
  (Coragen) 3.5–5 fl oz 4 0
  MODE-OF-ACTION GROUP NUMBER1: 28
  COMMENTS: Make no more than one application per cutting. To reduce the development of resistance, do not make more than two applications of any group 28 insecticides in a crop year.
 
C. INDOXACARB      
  (Steward EC) 6.7–11.3 fl oz 12 7
  MODE-OF-ACTION GROUP NUMBER1: 22A
  COMMENTS: Make no more than one application per cutting. Steward EC can be used for alfalfa grown for seed, but seeds cannot be used for sprouts intended for human consumption or livestock feed. All seed must be tagged: "Not for human or animal use.".
 
D. BACILLUS THURINGIENSIS ssp. AIZAWAI#
  (Xentari, Agree WG) Label rates 4 0
  MODE-OF-ACTION GROUP NUMBER1: 11A
  COMMENTS: Apply when larvae are small (in first or second instar). Does not harm beneficial insects. Repeat pesticide application as necessary.
 
** See label for dilution rates.
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 an organically grown crop.
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).

[Precautions]

PUBLICATION

[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Alfalfa
UC ANR Publication 3430

Insects and Mites

L. D. Godfrey, Entomology, UC Davis
E. T. Natwick, UC Cooperative Extension, Imperial County
P. B. Goodell, Entomology, Kearney Agricultural Center, Parlier
R. F. Long, UC Cooperative Extension, Yolo County
V. M. Barlow, UC Cooperative Extension, Riverside County and UC IPM Program

Acknowledgment for contributions to Insects and Mites:
D. R. Haviland, UC Cooperative Extension, Kern County
M. Rethwisch, UC Cooperative Extension, Riverside County (Blythe)
C. G. Summers, Entomology, Kearney Agricultural Center

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