How to Manage Pests
UC Pest Management Guidelines
Scientific Name: Helicoverpa (=Heliothis) zea
(Reviewed 6/08, updated 5/10)
In this Guideline:
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
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 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 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.
A number of predaceous insects and parasites will feed on corn
earworm eggs. A tiny parasitic
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.
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.
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.
|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.
||6–12 fl oz
||MODE OF ACTION GROUP NUMBER1: 18
||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.
||6–10 fl oz
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.
||6 fl oz
||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.
||BACILLUS THURINGIENSIS ssp. KURSTAKI#
||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.
UC IPM Pest Management Guidelines:
UC ANR Publication
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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