Urban tree pests can support biological control services in landscape shrubs

Scale insects are common tree pests in urban ecosystems. Although severe
scale infestations can worsen tree condition, trees can tolerate moderate
scale densities. Scale insects are prey for many arthropod natural enemies
that also feed on plant pests throughout urban landscapes. Because
scale-infested trees support natural enemy communities, they may support
biological control services on nearby plants and function analogously to
banker plants in greenhouse production systems. In this study, we tested
if sentinel insect prey were more likely to be removed on shrubs below
scale-infested trees compared to scale-uninfested trees. We conducted
several biological control experiments from 2019–2021 using fruit flies,
aphids, and caterpillars in potted and planted holly shrubs below
scale-infested and scale-uninfested oak trees. We found that caterpillars
in potted shrubs and fruit flies in planted landscape shrubs were more
likely to be removed underneath scale-infested trees compared to
scale-uninfested trees. Caterpillars were also more likely to be removed
from landscape Ilex vomitoria shrubs compared to I. cornuta shrubs. In all
other experiments, we found no effect of scale infestation status or shrub
species on prey removal. Our results suggest that scale-infested trees can
support biological control services in shrubs below them but that this
effect can vary depending on prey and shrub species. The natural enemy
communities in urban trees and shrubs appear to be linked and tolerating
tree pests can favor conservation biological control services in urban
landscapes.