An evaluation of some natural enemies of Spodoptera exigua on sugarbeet in northern California

Abstract

Eggs and small to medium-sizedlarvae of Spodoptera exigua (Hubner)(Lepidoptera: Noctuidae) are exploited by acomplex of natural enemies in spring-plantedsugarbeet fields in northern California. Fieldstudies revealed that predation on sentinel eggmasses ranged from ∼20 to 100%egg mass;predation rate was lowest in fields previouslytreated with methomyl and highest innon-treated fields. Predators typicallydestroyed all of the eggs in a given egg mass;percentage predation per egg mass was densityindependent (spatial context). Survival of eggs(to neonate larvae) in cages that excludedpredators ranged from ∼80 to $>$90%. Theegg-predator guild consisted of adults andnymphs of Orius tristicolor(White)(Anthocoridae), Nabis americoferusCarayon (Nabidae), Lygus hesperus Knight(Miridae), and Geocoris punctipes (Say)(Geocoridae); larvae of Chrysoperlacarnea (Stephens) (Chrysopidae); and adults ofCollops vittatus (Say) (Melyridae). Laboratory evaluation revealed that largelarvae of C. carnea and the adults of theother species (except for O. tristicolor)could consume 100 eggs of S. exigua in a48 h period. The parasite guild associatedwith small and medium-sized larvae consisted ofthree species: Hyposoter exiguae(Viereck) and Pristomerus spinator (F.)(Ichneumonidae), both larval endoparasites; andChelonus insularis Cresson (Braconidae),an egg-larval endoparasite. Parasitization in field samples ranged from ∼30 to ∼65%. Smalland medium-sized larvae were also infected witha nuclear polyhedrosis virus (NPV); rates ofNPV infection ranged from 0 to ∼35% in fieldsamples. These results are consistent withanecdotal evidence that natural enemies,primarily generalist predators, are largelyresponsible for maintaining populations ofS. exigua at relatively low levels innontreated sugarbeet fields.