Analysis of the predator community of a subterranean herbivorous insect based on polymerase chain reaction

The identity and impact of trophic linkages within subterranean arthropod communities are challenging to establish, a fact that hinders the development of conservation biological control programs of subterranean herbivores. Diabrotica virgifera (the western corn rootworm) is a severe agricultural pest that lives subterraneously during its pre-imaginal stages and succumbs to high levels of pre-imaginal mortality from unknown agents. The guts of 1500 field-collected arthropod predators were analyzed for D. virgifera-specific DNA sequences using quantitative polymerase chain reaction (qPCR). These gut analyses were used to generate relative and taxon-specific prey consumption indices for the major predator taxa and to determine relative consumption levels during D. virgifera egg and larval stages by predator feeding guilds. Laboratory feeding assays were used to determine the meal size consumed during 5 min and digestion rates of D. virgifera DNA of four predators abundant in D. virgifera-infested cornfields. More than 17 taxa consumed D. virgifera in the field. Harvestmen and small rove beetles were the most abundant predators captured, and the most frequent predators within the community to consume D. virgifera. The largest proportions of individual species' populations testing positive for D. virgifera DNA were found in ground beetles (Scarites quadriceps and Poecilus chalcites) and spiders, wolf spiders, and predaceous mites. Because of the longer duration of the egg stage, significantly more predators consumed D. virgifera eggs than larvae, but a similar proportion of the predator community fed on eggs and larvae. Predators with sucking mouthparts had a higher consumption index than chewing predators. Laboratory assays confirmed that sucking predators consume more D. virgifera DNA during 5 min than the chewing predators, and all four predators digested this DNA at a similar rate. This research substantiates that a diverse community of soil-dwelling and subterranean predators contribute to the high level of mortality incurred by D. virgifera in cornfields (similar to 99% pre-adult mortality). Moreover, qPCR is a useful tool for describing trophic relationships within subterranean food webs, a crucial step in determining the relative contributions of a diverse predator community to the population dynamics of an herbivorous arthropod.