Corn Earworm

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 can produce 500 to 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.

Corn earworms damage strawberries by burrowing into fruit. Although there are several generations each season, only larvae of the overwintering 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 of corn earworm may become 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 is 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 insecticides.

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 biological control agents can provide some pest suppression, the very low tolerance for insect contamination in strawberries makes this control option less attractive when earworm numbers are high.

Planting early maturing sweet corn cultivars at the edges of strawberry fields as "trap crops" may reduce strawberry contamination by the corn earworm. Female moths strongly prefer to lay eggs 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, and the corn must be removed following silking to destroy trapped larvae.

Use biological and cultural control methods and sprays of the Entrust formulation of spinosad and Bacillus thuringiensis on organically certified strawberries.

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, insecticide applications may need to be repeated at 10- to 14-day intervals, depending on the residual activity of the product applied.