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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.

Determining relative strengths of trophic links is critical for ranking predators for conservation biological control. Molecular gut-content analysis enables ranking by incidence of prey remains in the gut, but differential digestive rates bias such rankings toward predators with slower rates. This bias can be reduced by indexing each predator’s half-life to that of the middle-most half-life in a predator complex. We demonstrate this with data from key species in the predator complex of Colorado potato beetle (CPB), Leptinotarsa decemlineata (Say) (Coleoptera: Chrysomelidae), comprising adults and immatures of four taxonomically diverse species. These animals display order-of-magnitude variation in detectability half-life for the cytochrome oxidase I DNA sequence of a single CPB egg: from 7.0 h in larval Coleomegilla maculata (DeGeer) (Coleoptera: Coccinellidae) to 84.4 h in nymphal Perillus bioculatus (Fabricius) (Hemiptera: Pentatomidae). The raw species-specific incidence of L. decemlineata DNA in the guts of 351 field-collected predators ranged from 11 to 95%, ranking them as follows: C. maculata adults