How to Manage Pests

UC Pest Management Guidelines


Western Yellowstriped Armyworm

Scientific Name: Spodoptera praefica

(Reviewed 1/17, updated 1/17)

In this Guideline:

Description of the Pest

Western yellowstriped armyworm may be abundant in alfalfa fields in the Central Valley from June to early September.

The caterpillar is usually black, with two prominent stripes and many narrow bright ones on each side. At maturity it is approximately 1.5 to 2 inches long. Eggs are laid in clusters on the upper side of leaves and covered with a gray, cottony material. Eggs hatch in a few days and larvae reach full size in 2 to 3 weeks. Larvae pupate on or just under the soil surface. Adults are brown moths that primarily fly at night but may be encountered flying up as you walk through the field.

There are at least five generations per year in the low desert and four generations in the Central Valley.


Armyworms skeletonize leaves, leaving veins largely intact.


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 management methods that 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, and lacewings. The parasitic wasp, Hyposoter exiguae, is believed to be 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 can also be important.

Cultural Control

Fields may be cut to avoid damage.

Organically Acceptable Methods

Biological and cultural control methods, as well as sprays of Bacillus thuringiensis (e.g., Xentari, Agree), are acceptable for use on an organically certified crop.

Monitoring and Treatment Decisions (View photos for identification of caterpillars)

In early summer start sweeping fields with adequate plant height 2 to 3 times per week to monitor for caterpillars; monitoring can be discontinued after September. 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 caterpillars and leafhoppers 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 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. Not all registered pesticides are listed. Always read the label of the product being used.
  (Intrepid 2F) 4–8 fl oz 4 0(forage);
7(fodder and hay)
  COMMENTS: Make no more than one application per cutting. Not for use in alfalfa grown for seed or for sprouts for human consumption.
  (Coragen) 3.5–5 fl oz 4 0
  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.
  (Steward EC) 6.7–11.3 fl oz 12 7
  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."
  (Xentari, Agree) Label rates 4 0
  COMMENTS: Apply when larvae are small (in first or second instar). Does not harm beneficial insects. Repeat the 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.
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 a1B 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).
* Permit required from county agricultural commissioner for purchase or use.
# Acceptable for use on an organically grown crop.



[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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