Urban tree isolation affects the abundance of its pests and their natural enemies

The proximity to transportation hubs and the large movement of goods and people in cities make trees especially prone to invasive insect pests. Urban tree arrangements, in isolated tree sites or in forest patches, may alter their susceptibility to insect herbivory. Surprisingly little is known about how urban tree spatial arrangements influence pest abundance and top-down regulation by natural enemies. We sampled the abundance of an invasive pest of Eucalyptus, the psyllid Glycaspis brimblecombei, and of two of its natural enemies, the specialist exotic parasitoid Psyllaephagus bliteus and the generalist native predator Anthocoris nemoralis over two years. We measured insect abundances in isolated trees versus monospecific planted forest patches in 17 pairs of sites, each within a Portuguese city. The abundance of the exotic pest was three-fold higher in forest patches than in isolated trees. The parasitism rate was similar to 30 times higher in forest patches (11.5 %) than in isolated trees (0.4 %). The interdependence among insect species abundances also differed between spatial arrangements. In isolated trees, predator and parasitoid abundances depended markedly on prey abundance. In forest patches, we found a marked dependence of the predator on the composition of the surrounding landscape. Our study adds empirical data indicating that urban tree isolation matters for pest abundance and regulation by its enemies. In similar systems, avoiding dense monospecific patches of exotic trees and increasing the compositional heterogeneity of the landscape are promising paths to maintain the sustainability of urban trees and their environmental and societal benefits.