Corn Earworm and Tobacco Budworm

Corn earworm eggs are white when first laid, but soon develop a dark red or brown ring around the top. Before hatching the eggs darken as the larvae develop inside. Deeper ridges and a 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 get older they develop distinct stripes, but the overall color is variable.

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.

Earworms often migrate into lettuce from surrounding crops. Tobacco budworm can be found in lettuce in California's southern desert.

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.

If not disrupted by pesticide applications, the corn earworm's natural enemies can frequently reduce its numbers 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 parasitoid, Trichogramma pretiosum. Eggs parasitized by it turn black and are easy to distinguish from non-parasitized eggs. Other natural enemies include the parasitoid 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.

Use biological and cultural controls and sprays of Bacillus thuringiensis, Entrust formulation of spinosad, or Gemstar on organically certified crops. Note that spinosad is detrimental to beneficial syrphid flies and parasitoids.

As soon as seedlings emerge, check for presence of Helicoverpa and Heliothis eggs to 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, apply insecticide 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, but species identification is important in choosing an insecticide. 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.

Consider applying insecticides

• between thinning and heading: if you find an average of more than one larva for each two plants. Further applications may be necessary.
• once heads form: if you find an average of one larva in every 25 plants. Apply insecticide right after egg hatch and before larvae enter the heads.

The best time to apply insecticides to control budworms is during mid-afternoon in the southern desert.