Combining ecological methods and molecular gut-content analysis to investigate predation of a lepidopteran pest complex of Brassica crops

In South East Queensland, Australia, Brassica crops are typically attacked by Crocidolomia pavonana Fabricius (Lepidoptera: Crambidae) in late summer and autumn (February to May) and by Plutella xylostella L. (Lepidoptera: Plutellidae) and Pieris rapae L. (Lepidoptera: Pieridae) from late autumn to the end of spring (May to November). Physical exclusion studies were used to assess the impact of predators on pest populations in two cabbage [Brassica oleracea L. var. capitata cv. Warrior (Brassicaceae)] crops grown sequentially between April and September 2006. The studies were complemented by molecular gut-content analysis of predators simultaneously hand-collected from the study crops by destructive sampling of cabbage plants. In May 2006, exclusion studies showed that 57% (95% CI=40-74%) of the C.pavonana study cohort was lost to predation. Spiders represented 81% of all predators collected and Lycosidae collected from the soil surface beneath sampled plants dominated (61% of predators); 23% of Lycosidae contained C.pavonana DNA, whereas 5% contained P.xylostella DNA. In August 2006, exclusion studies showed that 88% (95% CI=83-93%) of the P.xylostella cohort was lost to predation. Spiders represented 99% of all predators collected; soil surface dwelling Lycosidae (51% of predators) dominated, followed by Linyphiidae (20%) and foliar-dwelling spiders (16%; Theridiidae, Salticidae, Clubionidae/Miturgidae, and Oxyopidae combined); 12% of Lycosidae and 38% of foliar-dwelling spiders contained P.xylostella DNA. Life tables for P.xylostella that were constructed for cohorts exposed to predators and cohorts from which predators had been physically excluded showed that eggs and neonates suffered the highest rates of predation. In a concurrent study, P.xylostella that were naturally recruited to the field population suffered similar high levels of early-stage mortality.