Scale Insects on Urban Trees Benefit Spiders, Other Natural Enemies in Plants Below
By Caleb Wilson, Ph.D.
Last year, I shared findings from my research on infestations of scale insects in urban trees. Because scales are eaten by many arthropod predators and used as hosts by many parasitoid wasp species (collectively referred to as “natural enemies”), we studied the natural enemy communities found within urban trees infested with scales relative to uninfested trees. In short, our findings, published in October 2022 in Environmental Entomology, suggest that tolerating scales on urban trees can conserve natural enemies both within trees and in shrubs below them. By supporting large natural enemy communities in plants below them, scale-infested trees may also prevent pest outbreaks on plants growing near them.
Since completing that original study, my advisor Steven Frank, Ph.D., at North Carolina State University and I conducted two follow-up studies within this system to better understand 1) what kinds of predators are supported by scale-infested trees and 2) do scale-infested trees support natural prey removal (also known as “biological control”) in shrubs growing below them?
To answer the first question, we examined the community of spiders that we found in scale-infested and uninfested tree canopies and in holly shrubs below these trees. Spiders were the most abundant natural enemy group we collected in trees and shrubs, and spiders are an ecologically diverse group of predators that use a variety of strategies to capture and kill their prey. Because spiders are important predators of many landscape pests, identifying environmental factors that conserve spiders, such as pest densities in trees, will inform sustainable pest management practices in urban landscapes.
Spiders are often separated into different “guilds” based on their different prey-capture strategies. For example, orb-web weaving spiders create distinct orb-shaped webs that are often oriented vertically to capture prey. In comparison, sheet-web weaving spiders create small, flat webs that are oriented horizontally, while active hunting spiders do not create webs at all but rather chase and capture their prey.
Knowing which spider guilds are abundant in scale-infested trees will indicate which spiders benefit from the recruiting of natural enemies associated with scale insects. This is important because researchers have documented that certain spider families or guilds are often rarer in cities compared to rural areas. If scale-infested urban trees support these otherwise uncommon spider guilds, scale-infested trees may be important for conserving spiders that would otherwise be sensitive to urban development.
So, what did we find? We found that scale-infested trees hosted more orb-web weaving spiders relative to uninfested trees, while holly shrubs under infested trees hosted significantly more orb-web weavers, space-web weavers, and active hunting spiders relative to shrubs under scale-uninfested trees.
Our results suggest that tolerating scales on urban trees can help conserve orb-web weaving, space-web weaving, and hunting spiders over other guilds. More broadly, our results indicate that scales, and likely other tree pests as well, have understudied potential for spider conservation in cities.
Our second follow-up study assessed if insect prey were more likely to be removed from holly shrubs underneath scale-infested trees relative to shrubs under uninfested trees. The prey we used for these experiments were crapemyrtle aphids (Tinocallis kahawaluokalani), dead Drosophila adults (a mixture of D. suzukii and D. melanogaster), and caterpillars (Helicoverpa zea and Spodoptera frugiperda). On large, planted holly shrubs underneath infested and uninfested trees we recorded removal of all three prey types. These shrubs comprised two different species: Ilex vomitoria, a native species, and I. cornuta, an exotic holly species.
We ran similar experiments that measured the removal of Drosophila and caterpillar prey on potted holly shrubs underneath both tree types. We ran prey-removal experiments on planted and potted hollies so that we could compare our results in shrubs that ground-dwelling predators were able to access (planted hollies) and in potted hollies that ground-dwelling predators could not access. To keep ground-dwelling predators out of potted hollies, we coated the outside of pots with Fluon and we treated the soil in these pots with permethrin.
In this case, we found that Drosophila adults in planted hollies and caterpillars in potted hollies were more likely to be removed underneath infested trees relative to uninfested trees. In all other experiments we found no effect of tree type on prey removal. We also found that caterpillars were more likely to be removed from native Ilex vomitoria shrubs relative to exotic I. corntua shrubs.
In addition to supporting natural enemies in plants below them, scale-infested trees can also support natural pest regulation in shrubs below them. However, this effect can be influenced by the type of insect prey present within shrubs, as well as what species of shrub is present.
What should we take away from these studies? First, scale-infested trees have the potential to conserve natural enemies both within their canopies and in shrubs below them. Second, scales and the diverse arthropod communities found in close association with them, conserve orb-web weaving, space-web weaving, and active hunting spiders. Third, by conserving natural enemies, scale-infested trees also support biological control of pests in plants below them.
Scale insects appear to have understudied conservation potential for natural enemies in urban landscapes, and these conservation benefits also have the potential to prevent pest issues in nearby plants. Although scales are often considered pests in urban trees, our work indicates that scales are important for conserving natural enemies and their biological control services.
Journal of Insect Conservation
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: email@example.com.