In some areas an important biological control agent is a nuclear polyhedrosis virus that occurs naturally in the field. Bodies of loopers killed by the virus are dark, soft, and shapeless with their body contents often spilling onto the leaves. Another important natural enemy in southern California, the tiny parasitic wasp, Trichogramma pretiosum. Eggs parasitized by Trichogramma turn black as the parasite matures inside and are easy to distinguish from normal eggs that remain white except for a black spot that appears just before hatching. Other parasites that attack looper caterpillars include the tachnid fly, Voria ruralis, and three wasps, Hyposoter exiguae, Copidosoma truncatellum, and Microplitis brassicae.
Check fields twice a week once seedlings emerge.
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. From germination to thinning and from thinning to head formation, check 25 plants randomly every few yards as you follow the standard zigzag sampling path through the field. Fields smaller than 40 acres may require fewer samples. Treat seedlings or small plants if populations of medium-sized to large loopers are large enough to stunt growth.