Agriculture: Lettuce Pest Management Guidelines

Corn Earworm and Tobacco Budworm

Description of the Pest

Corn earworm eggs are white when first laid, but soon develop a dark red or brown ring around the top. Before hatching the eggs darken as the larvae develop inside. Deeper ridges and a hemispherical shape distinguish corn earworm eggs from looper eggs. Larvae have discrete rows of tubercles with one or two protruding hairs along their backs. As larvae get older they develop distinct stripes, but the overall color is variable.

The tobacco budworm is similar in appearance and life history to the corn earworm but can be distinguished by the presence of tiny spines on the tubercle at the base of hairs on the 8th abdominal segment (tobacco budworm) and the presence of retinaculum (tooth) on the mandible of a tobacco budworm when viewed under a microscope.

Earworms often migrate into lettuce from surrounding crops. Tobacco budworm can be found in lettuce in California's southern desert.

Damage

Corn earworms and tobacco budworms can destroy lettuce seedlings by feeding on the crown. They also bore into heads of maturing lettuce where they are difficult to control.

Management

Biological Control

If not disrupted by pesticide applications, the corn earworm's natural enemies can frequently reduce its numbers to tolerable levels, particularly between thinning and heading, when plants are not so vulnerable to damage. Common natural enemies in southern California include the egg parasitoid, Trichogramma pretiosum. Eggs parasitized by it turn black and are easy to distinguish from non-parasitized eggs. Other natural enemies include the parasitoid Hyposoter exiguae, which also attacks beet armyworms and loopers, and such general predators as minute pirate bugs (Orius spp.) and bigeyed bugs (Geocoris spp.). Naturally occurring pathogens, including a nuclear polyhedrosis virus, often kill earworms.

Organically Acceptable Methods

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

Monitoring and Treatment Decisions

As soon as seedlings emerge, check for presence of Helicoverpa and Heliothis eggs to determine if they are parasitized, hatched, or about to hatch. If they have hatched, look for caterpillars. If you find a significant number of eggs and caterpillars on seedlings, apply insecticide after eggs have hatched.

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.

Consider applying insecticides

  • between thinning and heading: if you find an average of more than one larva for each two plants. Further applications may be necessary.
  • once heads form: if you find an average of one larva in every 25 plants. Apply insecticide right after egg hatch and before larvae enter the heads.

The best time to apply insecticides to control budworms is during mid-afternoon in the southern desert.

Common name Amount per acre** REI‡ PHI‡
(Example trade name) (hours) (days)
Not all registered pesticides are listed. 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. Always read the label of the product being used.
A. CHLORANTRANILIPROLE
(Coragen) 3.5–5 fl oz 4 1
MODE-OF-ACTION GROUP NUMBER1: 28
B. 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.
C. INDOXACARB
(Avaunt) 3.5 oz 12 3
MODE-OF-ACTION GROUP NUMBER1: 22
COMMENTS: Use to control low numbers.
D. EMAMECTIN BENZOATE*
(Proclaim) 2.4–4.8 oz 12 7
MODE-OF-ACTION GROUP NUMBER1: 6
E. BACILLUS THURINGIENSIS ssp. KURSTAKI#
(various products) 0.5–1.5 lb 4 0
MODE-OF-ACTION GROUP NUMBER1: 11A
COMMENTS: Not harmful to natural enemies. Also helps control loopers and imported cabbageworm but is only partially effective for control of corn earworm and tobacco budworm.
F. POLYHEDRAL OCCLUSION BODIES (OBS) OF THE NUCLEAR POLYHEDROSIS VIRUS OF HELICOVERPA ZEA
(Gemstar LC)# 4–10 fl oz 4 0
MODE-OF-ACTION GROUP NUMBER1:
COMMENTS: Product was not yet tested by UC experts to control corn earworm and tobacco budworm pest on lettuce in all California lettuce producing areas.
G. ZETA–CYPERMETHRIN*
(Mustang) 2.39–4.26 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.
H. 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 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.
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.
# Acceptable for organically grown produce.
Text Updated: 04/17
Treatment Table Updated: 04/17