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How to Manage Pests

UC Pest Management Guidelines

| More pests | More crops | About guidelines |

Western yellowstriped armyworm larva.

Alfalfa

Western Yellowstriped Armyworm

Scientific Name: Spodoptera praefica

(Reviewed 11/06, updated 11/06)

In this Guideline:

DESCRIPTION OF THE PEST

This pest may be abundant in alfalfa fields in the Central Valley. 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. This pest may be abundant at any time from June to early September.

DAMAGE

Armyworms skeletonize leaves, leaving veins largely intact.

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 in avoiding 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 controls, 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.

If cutting is not practical or not scheduled soon after monitoring, treat if there is an average of ten or more nonparasitized alfalfa caterpillars per sweep, fifteen or more nonparasitized armyworms per sweep, or 10 or more nonparasitized alfalfa caterpillars and armyworms combined per sweep.

Common name Amount/Acre** R.E.I.+ P.H.I.+
(trade name)   (hours) (days)
 
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.
 
A. INDOXACARB
  (Steward) 1.25 SC 6.7–11.3 fl oz 12 7
  MODE OF ACTION: An oxadiazine (Group 22)1 insecticide.
  COMMENTS: Make no more than one application per cutting. Not for use in alfalfa grown for seed or for alfalfa sprouts for human consumption. Do not apply more than 45 fl oz/acre/crop season.
 
B. METHOMYL*
  (Lannate) 90SP 0.5 lb 48 0
  MODE OF ACTION: A carbamate (Group 1A)1 insecticide.
  COMMENTS: Do not graze or feed livestock for 7 days after application. Do not apply more than 3.6 lb a.i./acre/crop. Do not apply when bees are present.
 
C. CARBARYL*
  (Sevin) 80SP 1.25–1.63 lb 12 7
  MODE OF ACTION: A carbamate (Group 1A)1 insecticide.
  COMMENTS: Do not apply when bees are present or more than once per cutting.
 
D. BACILLUS THURINGIENSIS ssp. AIZAWAI#
  (Xentari, Agree) Label rates 4 0
  MODE OF ACTION: A microbial (Group 11.B1)1 insecticide.
  COMMENTS: Apply when larvae are small (in first or second instar). Does not harm beneficial insects. Repeat treatment as necessary.
 
 
**  See label for dilution rates.
+ 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 until the field can be grazed or cut. In some cases the R.E.I. exceeds the P.H.I.; the longer of these two intervals is the minimum time that must elapse before harvest may take place.
* Permit required from county agricultural commissioner for purchase or use.
# Acceptable for use on an organically grown crop.
1 Modes of action are important in preventing the development of resistance to pesticides. 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. 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 is 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: Alfalfa
UC ANR Publication 3430
Insects and Mites
C. G. Summers, Entomology, Kearney Agricultural Center, Parlier
L. D. Godfrey, Entomology, UC Davis
M. Rethwisch, UC Cooperative Extension, Riverside County (Blythe)
D. R. Haviland, UC Cooperative Extension, Kern County
P. B. Goodell, Entomology, Kearney Agricultural Center, Parlier
R. F. Long, UC Cooperative Extension, Yolo County

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Statewide IPM Program, Agriculture and Natural Resources, University of California
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