How to Manage Pests

UC Pest Management Guidelines

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 retinaculum (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. 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 midafternoon 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.

PUBLICATION

UC IPM Pest Management Guidelines: Lettuce
UC ANR Publication 3450
Insects and Other Arthropods
E. T. Natwick, UC Cooperative Extension, Imperial Co.
W. E. Chaney, UC Cooperative Extension, Monterey Co.
Acknowledgment for contributions to the insects and mites section:
N. C. Toscano, Entomology, UC Riverside

Top of page