Why a Little Bit of This Tree Pest is a Good Thing in Urban Landscapes
By Caleb Wilson, Ph.D.
Urban trees often host higher densities of scale insects than trees in rural areas. Scales are tiny dome-shaped insects that feed on sap that they extract from plants with their hidden straw-like mouth parts. Because scales are minute and nondescript, they often go unnoticed as they feed on plants. However, when scales are abundant on trees they can cause leaf dieback in tree branches and, in extreme cases, tree death.
The good news, however, is that scales are usually only a problem on trees in highly urbanized locations with little surrounding vegetation—such as a tree in a paved parking lot. In moderately urbanized locations such as yards or parks with surrounding vegetation such as turfgrass, shrubs, and neighboring trees, scales typically do not become dense enough to worsen tree health.
So, why should we care about these largely unnoticeable insects in our yards if they aren’t killing our trees? The answer is that scales may help conserve insect predators and parasitoids (collectively referred to as natural enemies), which could help prevent pest problems elsewhere in our yards.
Scales are eaten by a multitude of arthropods such as lady beetles, lacewing larvae, and spiders. Additionally, many parasitoid wasp species use scales as hosts in which to lay their eggs and feed their larvae. Many scale species also excrete honeydew as they feed, which is eaten as a supplemental food source by natural enemies and other non-predatory insects as well. Thus, scales may help support natural enemies by directly serving as prey or hosts for parasitoids, and scales could support natural enemies indirectly by providing honeydew for other arthropods that insect predators might consume.
If scales support natural enemy communities in trees, they might also support natural enemy communities in nearby plants. Natural enemies attracted to the high scale densities found in urban trees may also forage for food in plants below these trees. Therefore, a shrub planted below a tree with many scales might be visited regularly by natural enemies which, could prevent pest issues from arising within the shrub. To investigate these dynamics, my advisor Steven Frank, Ph.D., at North Carolina State University and I conducted a study that was published last week in Environmental Entomology.
First, to determine the ability of scale insects to support natural enemy communities, we collected arthropods from the canopies of scale-infested willow oaks (Quercus phellos) and uninfested sawtooth (Q. acutissima) and overcup (Q. lyrata) oaks in Raleigh, North Carolina, in 2019 and 2020. Willow oaks are commonly planted urban trees in the southeast that can host high densities of oak lecanium scale (Parthenolecanium quercifex), European fruit lecanium scale (P. corni), and many other species. Sawtooth and overcup oaks are also commonly planted urban species that host few scales in their canopies.
We found that scale-infested oaks hosted more natural enemies than uninfested oaks in both years. Specifically, scale-infested oaks had more spiders, parasitoids, and lady beetles than uninfested oaks.
To see if shrubs growing below scale-infested trees had more natural enemies than shrubs underneath uninfested trees, we then collected arthropods from shrubs below both tree types in 2020 and 2021. We collected arthropods from landscape yaupon holly (Ilex vomitoria) and Chinese holly (I. cornuta) shrubs. We also ran an accumulation experiment in which we resampled holly shrubs at three- and six-day intervals to see if natural enemies became more abundant on shrubs under scale-infested trees compared to uninfested trees over short time scales.
We found that in both 2020 and 2021 natural enemies were more abundant in shrubs below scale-infested trees compared to uninfested trees. Namely, spiders, predatory hemipterans (e.g. assassin bugs), and ants were more abundant in shrubs below scale-infested trees. In our accumulation experiment, natural enemies became more abundant in shrubs below scale-infested trees compared to uninfested trees in six to nine days.
So, what should we take away from these results? First, it is important to recognize that scales, even though they can be pests of urban trees, help support natural enemy communities both within trees and in nearby plants. This means that tolerating scales on trees helps ensure that natural enemy communities are available to consume other insect pests on nearby plants.
Second, our results indicate that the natural enemy communities in trees and shrubs are linked. Treating a tree with pesticides could kill off natural enemies that would otherwise help manage nearby pests. In other words, treating a tree with pesticides could alleviate pest problems within the tree but could result in pest outbreaks in shrubs beneath the tree as natural enemies are killed off. When treating trees with pesticides, it may be better to treat them in the winter, when natural enemies are not active. If a tree needs to be treated during the summer, it may be better to use a pesticide such as horticultural oil that can kill scales but is less likely to kill natural enemies than a broad-spectrum pesticide might.
Above all, our results suggest that urban landscapes should be viewed as ecosystems in which natural enemy communities in trees and shrubs are linked and that management decisions in either location has the potential to affect arthropod communities in the other.
Caleb Wilson, Ph.D., is a postdoctoral research associate in the Department of Entomology at Michigan State University and a recent doctoral graduate from North Carolina State University. Email: firstname.lastname@example.org.