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UC Pest Management Guidelines


Color variations of tomato fruitworm (also called corn earworm and cotton bollworm).

Lettuce

Corn Earworm and Tobacco Budworm

Scientific Names:
Corn earworm: Heliothis zea
Tobacco budworm: Heliothis virescens

(Reviewed 8/07, updated 10/10)

In this Guideline:


DESCRIPTION OF THE PESTS

Corn earworm eggs are white when first laid, but soon develop a dark red or brown ring around the top. Before hatching they darken as the larvae develop inside. Deeper ridges and a more 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 mature they develop distinct stripes, but the overall color is variable. Earworms often migrate into lettuce from surrounding crops. 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 retinaculom (tooth) on the mandible of a tobacco budworm when viewed under a microscope. Tobacco budworm occurs in lettuce in the southern desert areas of California.

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 populations 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 parasite, Trichogramma pretiosum. Eggs parasitized by it turn black and are easy to distinguish from normal eggs. Other natural enemies include the parasite 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
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
As soon as seedlings emerge, check for Heliothis eggs and 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, treat 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.

Between thinning and heading, if you find an average of more than one larva for each two plants, treat. Further applications may be necessary. Once heads form, treat if you find an average of one larva in every 25 plants.

Try to treat 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. In the Imperial Valley, the tobacco budworm has developed resistance to certain insecticides that still control the earworm, so correct identification may be important. Use the photos and illustrations available online to correctly identify the two species if resistance is suspected.

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. CHLORANTRANILIPROLE
  (Coragen) 3.5–5 fl oz 4 1
  MODE OF ACTION GROUP NUMBER1: 28
 
B. 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.
 
C. PERMETHRIN*
  (Pounce 25W) 6.4–12.8 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 25WP) 6.4–12.8 oz 12 1
  MODE OF ACTION GROUP NUMBER1: 3
  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) 2.39–4.26 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. INDOXACARB
  (Avaunt) 3.5 oz 12 3
  MODE OF ACTION GROUP NUMBER1: 22
  COMMENTS: Use to control low level populations.
 
G. EMAMECTIN BENZOATE*
  (Proclaim) 2.4–4.8 oz 12 7
  MODE OF ACTION GROUP NUMBER1: 6
 
H. BACILLUS THURINGIENSIS ssp. KURSTAKI#
  (various products) 0.5–1.5 lb 4 0
  MODE OF ACTION GROUP NUMBER1: 11.B2
  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.
 
** 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 Group numbers are 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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