How to Manage Pests

UC Pest Management Guidelines

Strawberry

Corn Earworm

Scientific Name: Helicoverpa (=Heliothis) zea

(Reviewed 6/08, updated 5/10)

In this Guideline:


DESCRIPTION OF THE PEST

Adult corn earworms are grayish brown moths with a wingspan of about 1.5 inches. The corn earworm can be found in all strawberry-growing areas but is primarily a problem in coastal southern California. There, adults emerge from overwintering pupae in large numbers each spring, often early March to mid-April. Each female produces between 500 and 3000 spherical eggs with rows of ridges along the sides.

The eggs, which are usually laid singly on the undersides of younger leaves, are initially white, but then develop a brown ring near the top before hatching. When temperatures are warm, eggs may hatch within 2 or 3 days. A newly hatched corn earworm has a black head and rows of dark-colored tubercles and bristles along the body; older larvae exhibit a wide variation in color, ranging from green, pink, or brown to nearly black. The time needed to complete a generation is temperature dependent but often takes about 1 month.

DAMAGE

Corn earworms damage strawberries by burrowing into fruit. Although there are several generations each season, only larvae of the first generation attack southern California strawberries in spring. Entrance holes made by early instar larvae are not visible, and the fruit must be cut to determine their presence. Larvae typically feed in the air pocket at the fruit's center; mature fruit containing large larvae appear seedy and develop a shrunken surface with one or more brown patches. Contamination of the fruit prevents it from being marketed as fresh or processed fruit; federal tolerance currently requires downgrading to juice stock if a single 7 mm or larger larva is found per 44 pounds of fruit (about 1,100 berries).

MANAGEMENT

Management of corn earworm is occasionally necessary in South Coast strawberries, especially following a mild winter. Corn earworm becomes more of a problem as the season progresses, especially in April and later and when temperatures start to warm. They can be especially problematic when fruit are directed to processing because of lengthened harvest intervals and lack of insecticides being applied for other pests. Monitor for healthy and parasitized eggs in spring to determine the need for treatment.

Biological Control
A number of predaceous insects and parasites will feed on corn earworm eggs. A tiny parasitic wasp, Trichogramma pretiosum, has been found developing in Helicoverpa eggs on strawberries, but the percent parasitization from natural populations appears to be low. Trichogramma can be purchased from commercial sources for augmentative release. The frequency of release and release rates to effect control, however, have not been determined on strawberries. If Trichogramma are purchased for release, check for the quality of the emerging adults. The minute pirate bug is a predator that has been observed to feed on corn earworm eggs. While both of these biocontrol agents can provide some pest suppression, the very low tolerance for insect contamination in strawberries makes this control option less attractive when populations are high.

Cultural Control
Planting a very early maturing sweet corn cultivar at the edges of strawberry fields may reduce strawberry contamination by the corn earworm. Female moths strongly prefer to oviposit on corn silk, so silking must coincide with the period of strawberry fruit susceptibility. Planting corn at different times may be necessary to extend the period when the corn is silking.

Organically Acceptable Methods
Biological and cultural control methods and sprays of the Entrust formulation of spinosad and Bacillus thuringiensis are acceptable for use on organically certified strawberries.

Monitoring and Treatment Decisions
Monitor the first generation of this pest in South Coast strawberries. Use Texas-style Heliothis pheromone traps to monitor emergence and flight activity of moths beginning in late February and early March. Begin surveying strawberries or trap crops for eggs when 10 or more adults are trapped in a period of 1 week. If unparasitized eggs are found in the strawberry field, consider spraying. Most insecticides are more effective against early instars, so detecting hatch is important. On average, it takes 147 degree-days greater than 55oF for the larvae to develop from newly hatched larvae to fourth instars. For heavy infestations, treatments may need to be repeated at 10- to 14-day intervals, depending on the residual activity of the product applied.

Common name Amount/Acre R.E.I.+ P.H.I.+
(trade name)   (hours) (days)

  Calculate impact of pesticide on air quality
The following materials are listed in order of usefulness in an IPM program, taking into account efficacy and impact on natural enemies and honey bees. When choosing a pesticide, also consider information relating to environmental impact. Not all registered pesticides are listed. Always read label of product being used.
 
A. METHOXYFENOZIDE
  (Intrepid 2F) 6–12 fl oz 4 3
  MODE OF ACTION GROUP NUMBER1: 18
 
B. METHOMYL
  (Lannate) LV
  (Lannate) SP
  MODE OF ACTION GROUP NUMBER1: 1A
  COMMENTS: Use may result in mite problems. Do not apply more than 4.5 lb a.i./acre/crop. Strawberries have been voluntarily withdrawn from the Lannate label. It is legal to use older product that lists strawberries on its label until it is gone.
 
C. SPINETORAM
  (Radiant SC) 6–10 fl oz 4 1
  MODE OF ACTION GROUP NUMBER1: 5
  COMMENTS: Rotate to an insecticide with a different mode of action after two successive applications of either spinetoram or spinosad to help delay the development of resistance to Group 5 insecticides. The use of this product may best be reserved for control of western flower thrips because the options are more limited for this pest.
 
D. SPINOSAD
  (Entrust)# 1.25–1.5 oz 4 1
  (Success) 6 fl oz 4 1
  MODE OF ACTION GROUP NUMBER1: 5
  COMMENTS: Most effective against younger larvae. Rotate to an insecticide with a different mode of action after two successive applications. Maintaining proper pH of the spray tank water is critical for maximum efficacy.
 
E. BACILLUS THURINGIENSIS ssp. KURSTAKI#
  (Various products) Label rates 4 0
  MODE OF ACTION GROUP NUMBER1: 11
  COMMENTS: Most effective against newly hatched larvae and not very effective against large larvae and those that have already entered the fruit to feed. Carefully time treatments to egg hatch. Because residual activity is short, it may be necessary to repeat applications at 4- to 7-day intervals during extended periods of peak egg hatch.
 
+ 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.
# Acceptable for use on 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: Strawberry
UC ANR Publication 3468

Insects and Mites

  • F. G. Zalom, Entomology, UC Davis
  • M. P. Bolda, UC Cooperative Extension, Santa Cruz County
  • S. K. Dara, UC Cooperative Extension, Santa Barbara County
  • S. Joseph, UC Cooperative Extension, Monterey County
Acknowledgment for contributions to the Insects and Mites:
  • P. A. Phillips, UC IPM Program and UC Cooperative Extension, Ventura County
  • N. C. Toscano, Entomology, UC Riverside

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