Deeper ridges and a more hemispherical shape distinguish corn earworm eggs from looper eggs. Earworms often migrate into lettuce from surrounding crops. 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 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.
Naturally occurring pathogens, including a nuclear polyhedrosis virus, often kill earworms. 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. In fields where the crop is heading, stop at five different locations in each quadrant and sample five plants at each location.