Scale insects support natural enemies in both landscape trees and shrubs below them

Scale insects are frequently abundant on urban trees. Although scales can
worsen tree condition, some tree species tolerate moderate scale
densities. Scales are prey for many natural enemies. Therefore,
scale-infested trees may conserve natural enemies in their canopies and in
nearby plants. We examined if scale-infested oaks—Quercus phellos
L.—hosted more natural enemies than scale-uninfested oaks—Q. acutissima
Carruth. and Q. lyrata Walter in Raleigh, NC. USA. We also tested if
natural enemies were more abundant in holly shrubs (Ilex spp.) planted
below scale-infested compared to scale-uninfested oaks. We collected
natural enemies from the canopies of both tree types and from holly shrubs
planted below these trees. To determine if tree type affected the
abundance of natural enemies that passively dispersed to shrubs, we
created hanging cup traps to collect arthropods as they fell from trees.
To determine if enemies became more abundant on shrubs below
scale-infested compared to scale-uninfested trees over short time scales,
we collected natural enemies from holly shrubs below each tree type at
three to six-day intervals. Scale-infested trees hosted more natural
enemies than scale-uninfested trees and shrubs below scale-infested trees
hosted more natural enemies than shrubs under scale-uninfested trees.
Natural enemy abundance in hanging cup traps did not differ by tree type;
however, shrubs underneath scale-infested trees accumulated more natural
enemies than shrubs under scale-uninfested trees in six to nine days.
Tolerating moderate pest densities in urban trees may support natural
enemy communities, and thus biological control services, in shrubs below
them.