How to Manage Pests

UC Pest Management Guidelines

Lettuce

Armyworm

Scientific Name: Mythimna unipuncta

(Reviewed 4/17, updated 4/17)

In this Guideline:


Description of the Pest

Armyworm larvae vary in color but are usually dark green or gray, with three thick stripes running down each side. First instar larvae move by "looping", but older larvae move the same way as other caterpillars. Adult moths fly at night, have a wingspan of about 1.5 inches (about 35–45 mm), and a single white spot in the middle of their buff-colored forewing.

Damage

Armyworms occasionally mass migrate from grasses when their food source is being depleted. They 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

Biological Control

Many natural enemies attack armyworms. Among the most common are the parasitoid wasps (Hyposoter exiguae, Chelonus insularis, Meteorus autographae, Cotesia marginiventris, and Trichogramma spp.), the tachinid fly, Lespesia archippivora, predaceous bugs (minute pirate, bigeyed, damsel, assassin), spiders (Araneae: Lycosidae and Phalangiidae), and beetles e.g., ground beetles (Carabidae). 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

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

Begin 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 for selecting the appropriate insecticide.

Most insecticides are more effective against young larvae than against eggs, so wait until the majority of eggs have hatched before treating. Apply insecticides 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 insecticides 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 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. 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.
 
B. CHLORANTRANILIPROLE
  (Coragen) 3.5–5 fl oz 4 1
  MODE-OF-ACTION GROUP NUMBER1: 28
 
C. 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.
 
D. 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.
 
E. 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: Do not use if leafminers are a problem. Caused leaf area reductions of nearly 38% in seedlings of the Mesa variety.
 
F. INDOXACARB
  (Avaunt) 3.5 oz 12 3
  MODE-OF-ACTION GROUP NUMBER1: 22
 
G. EMAMECTIN BENZOATE*
  (Proclaim) 2.4–4.8 oz 12 7
  MODE-OF-ACTION GROUP NUMBER1: 6
 
** 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

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