An evaluation of some natural enemies of Nezara viridula in northern California

Natural enemies associated with eggs of Nezara viridula (L.) (Heteroptera: Pentatomidae) were evaluated by placing sentinel egg masses in weeds and cultivated tomato and bean crops in northern California. Egg predation was generally less than 10% and normally involved predators with chewing mouth parts. Predators seldom destroyed an entire egg mass, typically eating <40 eggs per exploited mass. Laboratory evaluation of >25 species of potential arthropod predators revealed that few fed on N. viridula eggs to any extent; however, numerous species fed on N. viridula nymphs. Five species of egg parasites were recovered from sentinel egg masses: Trissolcus basalis (Wollaston), Gryon obesum Masner and Telenomus podisi Ashmead (Scelionidae); and Ooencyrtus californicus Girault and O. johnsoni (Howard) (Encyrtidae). The major egg parasite was T. basalis, the only exotic member of the parasite guild; it typically parasitized 100% of the eggs in an exploited egg mass. The results indicate that parasitization of eggs and predation of small nymphs can be important biotic mortality factors for N. viridula populations in northern California. It is suggested that a combination of factors - viz., egg parasitization, nymphal predation, regional shortage of overwintering sites, and local shortages of suitable hosts - maintains this exotic pest at relatively low levels in the region.