Lygus feed on celery by piercing cell membranes, according to the University of California IPM experts.
This activity affects the subsequent cell division in the area of feeding. The result is sunken, callused, elongated lesions, frequently just below the first node of the celery stalk. Inner petioles may show black spots, twisting, or other distortion, especially if immature lygus are present.
Natural enemies, especially parasitic wasps that attack eggs or nymphs, help control lygus populations. However, in native vegetation where populations of lygus build, natural enemies are not effective at preventing large numbers of adults from migrating into celery.
The most important predators of lygus bugs are big-eyed bugs, which feed on eggs and young nymphs. Lygus adults are more difficult for predators to capture than nymphs because of their quick movements.
Lygus bugs frequently move into crop fields in spring when weeds and native vegetation dry down. Consequently, large numbers of lygus can move into celery fields in a short period of time. Determining the level of lygus infestation is difficult because the insects are cryptic and their activity cycle during any day varies greatly. Lygus become inactive and secretive during temperature extremes and windy conditions.
Because of the severity of the damage to celery, the threshold, while not specifically established, is very low. Sweep nets and keeping a careful eye out for adults flying in front of your movement through the field are the only ways to check for lygus.