Are Trees in Species-Rich Urban Plantings Less Susceptible to Pest Damage?
By Caleb J. Wilson, Ph.D.
Urban landscapes are characterized by extensive impervious surface cover in the form of roads, parking lots, and buildings. These surfaces absorb sunlight and re-emit this energy as heat, which makes urban areas warmer than their rural surroundings. This warming effect also favors the proliferation of certain sap-sucking insect pests such as scale insects in urban trees.
In the eastern United States, gloomy scale (Melanaspis tenebricosa) is a pest on urban red maple trees but rarely a problem for red maples in forests. Gloomy scales on hot urban trees produce more eggs than their counterparts in forests, especially if host trees are simultaneously water stressed, which they often are. Gloomy scales are targeted by multiple parasitoid wasp species and are consumed by many generalist insect predators such as lady beetles and lacewing larvae. However, these natural enemies (the collective term for insect predators and parasitoids) rarely reduce gloomy scale populations below damaging densities on urban red maples.
Due to many factors associated with scale biology, it is also difficult and rarely effective to manage species like gloomy scale with chemical insecticides. Therefore, to protect the health and ecosystem services provided by urban trees, it is necessary to identify how landscaping practices can enhance predation and parasitism of scale insects on urban trees. One such practice might be increasing the number of other tree species growing around urban red maples.
Why Tree Diversity Supports Pest Resistance
Trees in species-rich forests often suffer less damage from insect pests compared to trees in forests dominated by a few common species. Many hypotheses have been proposed to explain these findings. Two that have gained prominence among scientists are “the resource concentration hypothesis” and “the enemies hypothesis,” both coined by Richard B. Root, Ph.D., in a landmark publication in Ecological Monographs in 1973.
The resource concentration hypothesis posits that plants in species-rich settings are less likely to be eaten by herbivorous insects because it is harder for herbivores to locate their hosts. Because tree species produce volatiles that attract and repel different insect taxa, forests with many tree species likely contain a diverse mixture of volatiles from multiple species that could confuse an herbivore trying to detect volatiles from a specific host species. Alternatively, the presence of many species in close proximity might disrupt visual signals that insect herbivores use to locate their preferred host.
In contrast, the enemies hypothesis suggests that, where there are more plant species, there are more resources that can support natural enemy communities. These resources—such as pollen and nectar, ideal microclimate conditions, alternative prey, and ideal nesting habitat—could support natural enemy population growth and thus natural predation and parasitism of insect pests.
Does Diversity in Nearby Trees Protect Urban Maples From Scale Infestations?
In urban settings, red maples (Acer rubrum) are often planted in rows with few other tree species surrounding them. Increasing the diversity of tree species in such locations could improve gloomy scale biological control by natural enemies. To test this, we designed a study to see if red maples surrounded by many other tree species were less likely to be infested with gloomy scales and if this effect was attributable to increased biological control by natural enemies.
To do so, we recorded all tree species present within a 25-meter radius of 95 red maple trees in the city of Raleigh, North Carolina. We also used geographic information system (GIS) software to measure the percent canopy cover around trees. We then recorded gloomy scale density and natural enemy abundance in these trees. We quantified generalist insect predation in 30 trees by placing small note cards that had 10 dead fruit fly adults in the canopy of all trees and recording how many were removed after 24 hours. Finally, we quantified gloomy scale parasitism by recording how many scales out of 50 were parasitized in 27 red maples. Our results were published in September in the journal Urban Forestry & Urban Greening.
We found that red maples surrounded by many tree species tended to host lower scale densities relative to red maples surrounded by few tree species. We also found that scale density tended to be the lowest on red maples surrounded by high canopy cover and many tree species compared to red maples surrounded by low canopy cover and many tree species. In other words, it is better to plant red maples in locations where there are many trees represented by many species rather than locations with either few trees or few tree species.
Does this mean natural enemies are more abundant in diverse settings and that they are eating all the gloomy scales? Surprisingly, we found that red maples surrounded by many tree species had fewer parasitoids, while generalist predator abundance did not change relative to maples surrounded by few tree species. Parasitoids appear to become less abundant when red maples have fewer scales in them, while generalist predators are unaffected and likely switch to feeding on other prey. Given these findings, it is perhaps unsurprising that we found no beneficial effect of tree diversity on generalist predation of fruit flies nor on gloomy scale parasitism. However, we found that red maples surrounded by many tree species and extensive canopy cover had fewer scales per natural enemy—or, put another way, there were more natural enemies per individual scale in these trees. Individual scales may therefore be more likely to be consumed or parasitized in red maples in diverse settings.
Resource Concentration, Natural Enemies, or Both?
In sum, red maples in diverse settings have fewer scales, but not because of greater parasitism or predation by natural enemies. So, if natural enemies aren’t the reason for lower scale densities, what else is going on here? The resource concentration hypothesis might be the answer.
Gloomy scales are passively dispersed from trees by wind when immature scales (termed “crawlers”) first emerge from their mothers and search for a location to feed. Crawlers floating in the wind from one red maple may be less likely to land on a nearby red maple if it is surrounded by many different tree species. Therefore, tree diversity might protect red maples from gloomy scale infestation by slowing the rate at which scales colonize new red maples.
Meanwhile, since trees produce shade that cools nearby areas, red maples surrounded by many trees, regardless of species identity, may host fewer scales due to cooling effects. The gloomy scales found on these shaded red maples likely produce fewer offspring than scales on red maples surrounded by little tree cover. Over time, red maples surrounded by many trees would accrue additional scales at a slower rate compared to trees with few neighbors.
All of this is to say that multiple ecological mechanisms could explain why red maples surrounded by many trees and many tree species tend to host few gloomy scales. Scale crawlers may be less likely to land on a red maple in an area of diverse tree species, and cooler temperatures in these trees would not favor scale reproductive output. Finally, the greater ratio of natural enemies to scales in these trees could also increase the probability that a predator or parasitoid might encounter and kill scales.
More Evidence for Diversifying Urban Tree Plantings
A small but growing body of research has found that complex vegetation cover and plant diversity can support natural enemy conservation and their biological control services in urban landscapes. Our study indicates that diversifying urban tree plantings may be a sustainable and effective cultural management strategy for otherwise difficult-to-manage pests such as scale insects.
By supporting healthy red maples, tree diversification may reduce the need to treat trees with insecticides to manage scales and reduce the possibility of off-target effects on pollinators, predators, and other insect species. Diversifying urban tree plantings may therefore support insect conservation, tree health, and the many ecological services provided by urban trees.
Urban Forestry & Urban Greening
Caleb J. Wilson, Ph.D., is an assistant professor in the Department of Entomology at the University of Kentucky. Email: email@example.com.