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How to Manage Pests

UC Pest Management Guidelines

| More pests | More crops | About guidelines |

Alfalfa

Beet Armyworm

Scientific Name: Spodoptera exigua

(Reviewed 11/06, updated 11/06)

In this Guideline:

Description of the pest
Damage
Management

DESCRIPTION OF THE PEST

The adult beet armyworm is a small, mottled gray- or dusky-winged moth. The moths fly mostly at night but may be seen flying up as you walk through 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 stalks 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 may be 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 a 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 is very visible 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 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 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. Diseased caterpillars first appear yellowish and limp. After death they hang from plants as shapeless, dark tubes 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.

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.

INDOXACARB

(Steward) 1.25 SC

6.7–11.3 fl oz

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 sprouts for human consumption. Do not apply more than 45 fl oz/acre/crop season.

METHOMYL*

(Lannate) 90SP

0.5 lb

MODE OF ACTION: A carbamate (Group 1A)¹ insecticide.

COMMENTS: Restricted reentry interval: 48 hours. 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.

BACILLUS THURINGIENSIS ssp. AIZAWAI#

(Xentari, Agree)

Label rates

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/.

PUBLICATION

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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