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How to Manage Pests

UC Pest Management Guidelines


Larva of the armyworm, Pseudaletia unipuncta.

Lettuce

Armyworm

Scientific Name: Mythimna (= Pseudaletia) unipuncta

(Reviewed 8/07, updated 10/10)

In this Guideline:


DESCRIPTION OF THE PEST

Armyworm larvae are variable in color but are usually dark green or gray, with three thick stripes running down each side. First instar larvae loop, but older larvae move the same way as other armyworms.

DAMAGE

Armyworms can damage lettuce seedlings by feeding in the crown and chewing away the midrib and growing point. Once heads form, these caterpillars can bore into them, making the heads unmarketable.

MANAGEMENT

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

Organically Acceptable Methods
Cultural 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, but species identification is important in choosing an insecticide.

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)

  Calculate impact of pesticide on air quality
When choosing a pesticide, consider information relating to the impact on natural enemies and honey bees and environmental impact. Not all registered pesticides are listed. Always read label of product being used.
 
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. CHLORANTRANILIPROLE
  (Coragen) 3.5–5 fl oz 4 1
  MODE OF ACTION GROUP NUMBER1: 28
 
C. FLUBENDIAMIDE
  (Synapse WG) 2–3 oz 12 1
  MODE OF ACTION GROUP NUMBER1: 28
  COMMENTS: Do not apply more than 9 oz/acre (0.135 lb a.i./acre)/crop season.
 
D. 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 0.75–1.5 pt: 7
        over 1.5 pt: 10
  MODE OF ACTION GROUP NUMBER1: 1A
  COMMENTS: Do not use if leafminers are a problem. Caused leaf area reductions of nearly 38% in seedlings of the Mesa variety.
 
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. INDOXACARB
  (Avaunt) 3.5 oz 12 3
  MODE OF ACTION GROUP NUMBER1: 22
 
G. 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 County
Acknowledgment for contributions to Insects and Mites:
W. E. Chaney, UC Cooperative Extension, Monterey County
N. C. Toscano, Entomology, UC Riverside

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