How to Manage Pests

UC Pest Management Guidelines

DESCRIPTION OF THE PEST

Other names for the corn earworm are tomato fruitworm and cotton bollworm. Adult corn earworms are grayish brown moths with a wingspan of about 1.5 inches. In coastal southern California, the only strawberry-producing region where the corn earworm is a problem, adults emerge from overwintering pupae in large numbers each spring, often about 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. In the warm temperatures common in southern California, eggs may hatch within 2 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 winter strawberries. 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 whole 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. 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. 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
Plant a very early maturing sweet corn cultivar around strawberry fields to provide significant reductions in strawberry contamination by the earworm. Female moths strongly prefer to oviposit on corn and only lay eggs on strawberries if corn or other preferred hosts are not readily available.

Organically Acceptable Methods
Biological and cultural control methods and sprays of Bacillus thuringiensis and the Entrust formulation of spinosad 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 in early April. Begin surveying strawberries or trap crops for eggs when flight activity peaks (i.e., when 10 or more adults are trapped in 1 week). If unparasitized eggs are found in the strawberry field, consider spraying. Outbreaks of corn earworms often occur in years when warm air currents associated with El Niño conditions allow moths to migrate from the south; extra monitoring is advised during these years until harvest is complete.

PUBLICATION

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 Co.
P. A. Phillips, UC IPM Program/UC Cooperative Extension Ventura Co.
Acknowledgment for contributions to the Insects and Mites:
N. C. Toscano, Entomology, UC Riverside

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