How to Manage Pests

UC Pest Management Guidelines

Lettuce

Beet Armyworm

Scientific Name: Spodoptera exigua

(Reviewed 4/17, updated 4/17)

In this Guideline:


Description of the Pest

Beet armyworms lay their eggs in distinctive cottony masses on leaf surfaces. Newly hatched beet armyworms are small green worms that often feed in groups. Older beet armyworms vary in color, but usually have many fine, wavy, light-colored stripes down the back and a broader stripe down each side. The body appears smooth and hairless. The adult moth is mottled gray and brown with a wingspan of about 1 inch. The life cycle takes about a month in warm weather, and there are three to five generations a year.

DAMAGE

These worms feed in the crown of the plant and can severely stunt or kill seedlings. The potential for damage is reduced between thinning and head formation. Once heads form, beet armyworms may cause serious damage by boring in from the bottom of the head which means that damage cannot be seen from above. In the San Joaquin Valley, fall populations are much more damaging than spring populations. In the central coast, beet armyworm damage starts to appear in late summer. In the low desert valleys, beet armyworm starts as a stand establishment pest during September through October and is a pest on developing lettuce heads in November and December.

Management

Biological Control

Many natural enemies attack beet armyworms. Among them, parasitoids are most common and they include the wasps, Hyposoter exiguae and Chelonus insularis, and the tachinid fly, Lespesia archippivora. Viral disease (NPV or nucleopolyhedrovirus) also kill significant numbers.

Cultural Control

Disc fields immediately following harvest to kill larvae and pupae. Destroy weeds along field borders.

Organically Acceptable Methods

Use biological and cultural controls and sprays of Bacillus thuringiensis and the Entrust formulation of spinosad on organically certified crops. Note that spinosad is detrimental to beneficial syrphid flies and parasitoids.

Monitoring and Treatment Decisions

Start monitoring before seedlings emerge by checking for egg masses and young larvae in surrounding weeds. If numbers are high on weeds, watch carefully for larvae on seedlings.

  • From germination to head formation, check at least 25 plants in each quadrant of a 40- to 80-acre field twice a week. In fields where the crop is heading, stop at five different locations in each quadrant and sample five plants at each location. Fields smaller than 40 acres may require fewer samples.
  • Check for egg masses and young larvae. Loopers, cabbageworms, armyworms, corn earworms, tobacco budworms, cutworms, and other caterpillars that feed on leaves and heads of lettuce can be assessed together, but species identification is important in choosing an insecticide.

Apply insecticide when the majority of eggs have hatched because most insecticides are less effective against insect eggs. Apply insecticide if you find:

  • an average of one second or third instar larva per 10 young plants.
  • more than one larva on every two older plants (after thinning and before heading).

Apply insecticide just before heading if this threshold is exceeded. For beet armyworm control, the best time to apply insecticide is at dawn or dusk (twilight hours).

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.
 
A. METHOXYFENOZIDE
  (Intrepid 2F) 4–8 fl oz 4 1
  MODE-OF-ACTION GROUP NUMBER1: 18A
  COMMENTS: Do not apply more than 16 fl oz per acre per application or 64 fl oz of Intrepid 2F (1 lb a.i.)/acre per season.
 
B. BACILLUS THURINGIENSIS ssp. AIZAWAI#
  (various products) 0.5–1.5 lb 4 0
  MODE-OF-ACTION GROUP NUMBER1: 11A
  COMMENTS: Not harmful to natural enemies. Will also control loopers and imported cabbageworm.
 
C. POLYHEDRAL OCCLUSION BODIES (OBS) OF THE NUCLEAR POLYHEDROSIS VIRUS OF SPODOPTERA EXIGUA
(Spod-X LC)# 1.7–3.4 fl oz 4 0
MODE-OF-ACTION GROUP NUMBER1: –
 
D. CHLORANTRANILIPROLE
  (Coragen) 3.5–5 fl oz 4 1
  MODE-OF-ACTION GROUP NUMBER1: 28
 
E. SPINOSAD
  (Entrust)# 1.25–2.5 oz 4 1
  (Success) 4–8 oz 4 1
  MODE-OF-ACTION GROUP NUMBER1: 5
  COMMENTS: Not recommended when lettuce aphid is present because it harms syrphid fly larvae and parasitoids.
 
F. INDOXACARB
  (Avaunt) 3.5 oz 12 3
  MODE-OF-ACTION GROUP NUMBER1: 22
  COMMENTS: Use to control low numbers.
 
G. EMAMECTIN BENZOATE*
  (Proclaim) 2.4–4.8 oz 12 7
  MODE-OF-ACTION GROUP NUMBER1: 6
 
H. METHOMYL*
  (Lannate SP) 0.25–1 lb 48 0.25–0.5 lb: 7
over 0.5 lb: 10
  (Lannate LV) 0.75–3 pt 48 0.75–1.5 pt: 7
over 1.5 pt: 10
  MODE-OF-ACTION GROUP NUMBER1: 1A
  COMMENTS: Will also control loopers. Do not use if leafminers are present. Caused leaf area reductions of nearly 38% in seedlings of the Mesa variety.
 
I. ZETA-CYPERMETHRIN*
  (Mustang) 3.41–4.26 oz 12 1
  MODE-OF-ACTION GROUP NUMBER1: 3A
  COMMENTS: Do not exceed 0.3 lb a.i./acre per season. Do not use if leafminers are present. For use on head lettuce only.
 
J. PERMETHRIN*
  (Pounce 25WP) 6.4–12.8 oz 12 1
  MODE-OF-ACTION GROUP NUMBER1: 3A
  COMMENTS: Apply a minimum of 5 gal of finished spray/acre by aircraft, 15 gal/acre with ground equipment. Do not use if leafminers are present.
  . . . or . . .
  (Ambush) 6.4–12.8 oz 12 1
  MODE-OF-ACTION GROUP NUMBER1: 3A
  COMMENTS: Do not apply more than 2 lb a.i./acre per season. Do not graze treated areas or feed crop refuse to livestock. Do not use if leafminers are present.
 
** Mix with enough water to provide complete coverage.
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 organically grown produce.
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).
* Permit required from county agricultural commissioner for purchase or use.

[Precautions]

PUBLICATION

[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Lettuce
UC ANR Publication 3450

Insects and Other Arthropods

E. T. Natwick, UC Cooperative Extension, Imperial County
S. V. Joseph, UC Cooperative Extension, Monterey County
S. K. Dara, UC Cooperative Extension, Santa Barbara County

Acknowledgment for contributions to Insects and Mites:
N. C. Toscano, Entomology, UC Riverside

Top of page


Statewide IPM Program, Agriculture and Natural Resources, University of California
All contents copyright © 2017 The Regents of the University of California. All rights reserved.

For noncommercial purposes only, any Web site may link directly to this page. FOR ALL OTHER USES or more information, read Legal Notices. Unfortunately, we cannot provide individual solutions to specific pest problems. See our Home page, or in the U.S., contact your local Cooperative Extension office for assistance.

Agriculture and Natural Resources, University of California

Accessibility   /PMG/r441300711.html revised: June 15, 2017. Contact webmaster.