Effects of elevated CO 2 on an insect omnivore: A test for nutritional effects mediated by host plants and prey

Abstract

Elevated atmospheric CO 2 has generally been found to have negative impacts on the performance of insect herbivores via negative effects on plant quality. Yet, reduced food quality may lead to an increase in crop damage when pests compensate by consuming more plant tissue. The flow-on effects of elevated CO 2 level on higher trophic levels have rarely been investigated. We tested the effects of elevated CO 2 on the behavior and performance of the omnivorous bug Oechalia schellenbergii (Heteroptera: Pentatomidae) and its prey, a polyphagous chewing herbivorous pest ( Helicoverpa armigera; Lepidoptera: Noctuidae), feeding on pea ( Pisum sativum) foliage. We hypothesized that elevated CO 2 would impose negative nutritional effects on the omnivore by lowering the quality of plants, the prey, or both. Plants grown at elevated CO 2 were significantly larger, with reduced N content, than plants grown at ambient CO 2. H. armigera larvae feeding on elevated CO 2-grown plants were significantly smaller than those grown on ambient-grown plants, but prey N content did not differ between CO 2 treatments. The omnivore required prey to complete its development, and performed best on a mixed plant-prey diet, regardless of CO 2 level. Bugs that failed to reach adulthood were slower to develop on the elevated CO 2–prey diet; those that successfully eclosed were not affected by CO 2 treatment in terms of development time, adult weight, adult longevity, hatch rate or pre-oviposition time. The bugs did not display compensatory feeding when offered prey of similar size from different CO 2 treatments. The bugs performed best when fed larvae from the elevated-CO 2 treatment apparently because these prey were smaller and thus easier to subdue. Taken together, results indicate that elevated CO 2 may benefit generalist predators through increased prey vulnerability, which would put pest species under higher risk of predation.