Data from: Phloem-feeding insects creat parasitoid free space for caterpillars

Seemingly small ecological changes can have large, ramifying effects that defy expectations. Such are keystone effects in ecosystems. Phloem-feeding insect herbivores can act as keystone species by altering community structure and species interactions via plant-mediated or ant-mediated mechanisms. Plant responses triggered by phloem-feeders can disrupt tri-trophic interactions induced by leaf-chewing herbivores, while ants that tend phloem-feeders can deter or prey on other arthropods. Here, we investigate how phloem-feeding herbivores change caterpillar-parasitoid interactions on Quercus alba (white oak) trees in natural forests. We factorially manipulated the presence of phloem-feeding insects as well as ant access on Q. alba branches over multiple years and sites, and measured parasitism rates of co-occurring caterpillars. While 19.3% of caterpillars were parasitized when phloem-feeders were removed, the presence of phloem-feeders completely suppressed parasitism of caterpillars (0%). This stark pattern was consistent across the diverse community of phloem-feeders and caterpillars. Our manipulation of ant access had no effect on parasitism of caterpillars, implicating a plant-mediated mechanism. We further assessed the mechanistic hypothesis that phloem-feeders suppress plant emission of caterpillar-induced volatile compounds, which could disrupt host location behavior by parasitoids of caterpillars. Phloem-feeders indeed reduced concentrations of four volatile compounds, consistent with the putative plant volatile-mediated mechanism. Given the important role of parasitoids in controlling herbivore populations, this keystone effect of phloem-feeders offers novel insight into community dynamics in forests and potentially other terrestrial ecosystems.