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How to Manage Pests

UC Pest Management Guidelines

| More pests | More crops | About guidelines |

Beet armyworm larva.

Lettuce

Beet Armyworm

Scientific Name: Spodoptera exigua

(Reviewed 8/07, updated 8/07)

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 hairless.

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, and often damage can't be seen from above. In the San Joaquin Valley, fall populations are much more damaging than spring populations.

MANAGEMENT

Biological Control
Many natural enemies attack beet armyworms. Among the most common parasites are the wasps, Hyposoter exiguae and Chelonus insularis, and the tachinid fly, Lespesia archippivora. Viral diseases also kill significant numbers.

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

Organically Acceptable Methods
Cultural and biological control and sprays of Bacillus thuringiensis and the Entrust formulation of spinosad are acceptable for use on organically certified crops, but spinosad is very detrimental to populations of syrphid flies.

Monitoring and Treatment Decisions
Start monitoring before seedlings emerge by checking for egg masses and young larvae in surrounding weeds. If populations 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.

Treat if you find an average of one second or third instar larva per 10 plants. Most insecticides are more effective against young larvae than against eggs, so wait until the majority of eggs have hatched before treating. On older plants, after thinning and before heading, treat if more than one larva is found on every two plants. Treat just before heading if this threshold is exceeded. For armyworm control, the best time to apply insecticide is at dawn or dusk (twilight hours).

Common name Amount/Acre** R.E.I.+ P.H.I.+
(trade name)   (hours) (days)
When choosing a pesticide, consider information relating to the impact on natural enemies and honey bees and environmental impact.
 
A. BACILLUS THURINGIENSIS ssp. AIZAWAI#
  (various products) 0.5–1.5 lb 4 0
  MODE OF ACTION GROUP NUMBER1: 11.B1
  COMMENTS: Not harmful to natural enemies. Will also control loopers and imported cabbageworm.
 
B. METHOMYL*
  (Lannate) SP 0.25–1 lb 48 0.25–0.5 lb: 7
        over 0.5 lb: 10
  ...OR...
  (Lannate) LV 0.75–3 pt 48 0.75 to 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.
 
C. PERMETHRIN*
  (Pounce) 3.2EC 4–8 oz 12 1
  MODE OF ACTION GROUP NUMBER1: 3
  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: A pyrethroid (Group 3)1 insecticide.
  COMMENTS: Do not apply more than 2 lb a.i./acre/season. Do not graze treated areas or feed crop refuse to livestock. Do not use if leafminers are present.
 
D. ZETA-CYPERMETHRIN*
  (Mustang) 1.5EW 3.41–4.26 oz 12 1
  MODE OF ACTION GROUP NUMBER1: 3
  COMMENTS: Do not exceed 0.3 lb a.i./acre/season. Do not use if leafminers are present. For use on head lettuce only.
 
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 of its negative impact on syrphid fly larvae.
 
F. METHOXYFENOZIDE
  (Intrepid) 2F Label rates 4 7
  MODE OF ACTION GROUP NUMBER1: 18A
 
G. THIODICARB*
  (Larvin) 3.2 Label rates 48 14
  MODE OF ACTION GROUP NUMBER1: 1A
  COMMENTS: Do not use if leafminers are present.
 
H. INDOXACARB
  (Avaunt) 3.5 oz 12 3
  MODE OF ACTION GROUP NUMBER1: 22
  COMMENTS: Use to control low level populations.
 
I. EMAMECTIN BENZOATE*
  (Proclaim) 2.4–4.8 oz 48 7
  MODE OF ACTION GROUP NUMBER1: 6
 
 
**  Mix with enough water to provide complete coverage.
+ 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 to harvest. In some cases the REI exceeds the PHI. The longer of two intervals is the minimum time that must elapse before harvest.
* Permit required from county agricultural commissioner for purchase or use.
# 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 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: Lettuce
UC ANR Publication 3450
Insects and Other Arthropods
E. T. Natwick, UC Cooperative Extension, Imperial Co.
W. E. Chaney, UC Cooperative Extension, Monterey Co.
Acknowledgment for contributions to the insects and mites section:
N. C. Toscano, Entomology, UC Riverside

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