How to Manage Pests

UC Pest Management Guidelines

Lettuce

Loopers

Scientific Names:
Cabbage looper: Trichoplusia ni
Alfalfa looper: Autographa californica

(Reviewed 4/17, updated 4/17)

In this Guideline:


DESCRIPTION OF THE PESTS

Loopers arch their backs as they crawl along in a "looping" manner. Cabbage loopers usually have a narrow, white stripe along each side and several narrow lines down the back. Eggs are dome-shaped, and laid on the undersurfaces of older leaves. Adult moths have brown, mottled forewings marked in the center with a small silver figure 8.

DAMAGE

Young larvae feed primarily on the undersides of lower leaves, skeletonizing them. High numbers of loopers can damage seedlings severely enough to kill them or slow growth enough to inhibit uniform maturing of the crop. Older larvae may burrow into the head from the top.

Management

Biological Control

Loopers have many natural enemies that often keep their numbers below economic thresholds. Maximize the use of these natural enemies by limiting treatments, especially between thinning and heading. In some areas, an important biological control agent is nuclear polyhedrosis virus that occurs naturally in the field. Bodies of loopers killed by the virus are dark, soft, and shapeless with their body contents often spilling onto the leaves.

Another important natural enemy in southern California, the parasitoid wasp, Trichogramma pretiosum, which attacks looper eggs and eggs of other caterpillars such as corn earworm and tobacco budworm . Eggs parasitized by Trichogramma turn black as the immature parasitoid matures inside the egg and are easy to distinguish from non-parasitized eggs that remain white except for a black spot that appears just before hatching. Parasites that attack looper caterpillars include the tachinid fly, Voria ruralis, and three wasps, Hyposoter exiguae, Copidosoma truncatellum, and Microplitis brassicae.

Organically Acceptable Methods

Use biological control and sprays of Bacillus thuringiensis, and the Entrust formulation of spinosad in organically certified crops. Note that spinosad is detrimental to beneficial syrphid flies.

Monitoring and Treatment Decisions

Look for signs of looper feeding: plants with holes in outer leaves, feces, and caterpillars feeding on edge of hole.

  • Check fields twice a week once seedlings emerge. 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.
  • Check at least 25 plants for caterpillars in each quadrant of a 40- to 80-acre field twice a week. Fields smaller than 40 acres may require fewer samples. In fields where the crop is heading, stop at five different locations in each quadrant and sample five plants at each location.
  • Apply insecticide on seedlings or small plants if numbers of medium-size to large loopers are large enough to stunt growth. Spray well-established plants only if you find an average of more than one-half larvae per plant. For cabbage looper control, the best time to apply insecticide is in the afternoon.
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 oz 4 1
  MODE-OF-ACTION GROUP NUMBER1: 18A
 
B. BACILLUS THURINGIENSIS ssp. AIZAWAI#
  (various products) Label rates 4 0
  MODE-OF-ACTION GROUP NUMBER1: 11.B1
  COMMENTS: Not harmful to natural enemies.
 
C. CHLORANTRANILIPROLE
  (Coragen) 3.5–5 fl oz 4 1
  MODE-OF-ACTION GROUP NUMBER1: 28
 
D. ACEPHATE  
  (Orthene 97) 0.66–1 lb 24 21
  MODE-OF-ACTION GROUP NUMBER1: 1B
  COMMENTS: Use on head lettuce only. Do not apply after first head begins to form. Do not apply more than 2.125 lb per crop cycle. Do not feed to livestock or allow animals to graze on treated areas. This insecticide will also control aphids.
 
E. SPINOSAD
  (Entrust)# 1–2 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 beneficial syrphid fly larvae.
 
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
  . . . 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: Will also give fair control of aphids. 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: 3
  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 finished spray/acre by aircraft, 15 gal/acre with ground equipment. Do not use if leafminers are present.
  . . . or . . .
  (Ambush 25) 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.
* 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 group numbers are assigned by IRAC (Insecticide Resistance Action Committee).

[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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