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Urban Heat Boosts Some Pest Populations 200-Fold, Killing Red Maples

New research from North Carolina State University shows that urban “heat islands” are slowly killing red maples in the southeastern United States. One factor is that researchers have found warmer temperatures increase the number of young produced by the gloomy scale insect – a significant tree pest – by 300 percent, which in turn leads to 200 times more adult gloomy scales on urban trees.

Scale insects on a tree. Photo credit: Adam Dale. Click to enlarge.

Scale insects on a tree. Photo credit: Adam Dale. Click to enlarge.

“We’d been seeing higher numbers of plant-eating insects like the gloomy scale in cities, and now we know why,” says Adam Dale, a Ph.D. student at NC State and lead author of two papers describing the work. “These findings also raise concerns about potential pest outbreaks as temperatures increase due to global climate change.”

Gloomy scales (Melanaspis tenebricosa) suck sap from trees, removing nutrients and energy. This reduces tree growth and can eventually kill trees.

The researchers focused specifically on the abundance of gloomy scales on red maple (Acer rubrum) trees at 26 sites in Raleigh, North Carolina. “We wanted to look at the most important pest species of the most common tree species in urban areas of the southeastern United States,” says Dr. Steve Frank, an assistant professor of entomology at NC State and senior author of the papers.

Urbanization reduces the amount of vegetation in a habitat and increases impervious surfaces such as roads and rooftops. This can diminish predator and parasitoid communities and their ability to control pests.  However, it also makes cities hotter than rural areas. The researchers collected data on a wide variety of ecological variables that could affect gloomy scale populations, including habitat characteristics, the temperature at each tree site, and the abundance of predators and parasitoids.

“Temperature was the most important predictor of gloomy scale abundance – the warmer it was, the more scale insects we found,” Dale says. “The other variables we looked at had no significant effect on gloomy scale abundance.” The researchers also found that temperatures were related to the amount of impervious surfaces in the area, including streets, sidewalks, and parking lots. In short, the higher the percentage of impervious surfaces, the warmer the area.

Gloomy scales reproduce only once per year, giving birth to live young. So Dale collected gloomy scales at each site he was monitoring and dissected them to see how many young they were about to produce.

“At the coolest sites – 18.26 degrees Celsius (64.87 Fahrenheit) – the females were producing approximately 20 young,” Dale says. “At the warmest sites – 20.12 degrees Celsius (68.22 Fahrenheit) – the females were producing around 60 young. That supports the differences we saw in scale insect abundance on the trees. Populations at the warmest sites were over 200 times more abundant than those at the coolest sites.”

This image highlights the impact of scale insects on tree health. The tree on the left is infested with scales, while the tree on the right is not. Photo credit: Adam Dale. Click to enlarge.

This image highlights the impact of scale insects on tree health. The tree on the left is infested with scales, while the tree on the right is not. Photo credit: Adam Dale. Click to enlarge.

“We don’t know all of the variables that contribute to the higher abundance, but higher reproduction rates are clearly part of the picture,” Frank says. “When we look at abundance, we’re looking at an accumulation of generations of scales.”

The researchers also found a second factor in urban heat’s adverse impact on red maples. Specifically, the researchers found that higher temperatures increase stress on red maples by making it harder for them get water from their roots to their leaves.

To get a sense of the overall impact of heat on red maples, the researchers evaluated the condition of 2,780 trees in Raleigh, North Carolina, and compared the condition against a heat map of the city.

“This work tells us that urban planners and foresters may need to change the way they decide which trees to plant, and select trees that are better suited to hotter conditions,” Dale says.

“This also tells us that we need to plant more trees and vegetation in cities, increasing shade on impervious surfaces and limiting the ‘heat island’ effect,” Frank says. “It would also make sense to choose trees that are less susceptible to scales and other pest species.”

The research on scale insect abundance is published in “Urban warming trumps natural enemy regulation of herbivorous pests,” which is forthcoming from the journal Ecological Applications. The research on the overall impact of urban heat on red maples is in “The effects of urban warming on herbivore abundance and street tree condition,” which was published in PLOS ONE on July 23.

The Ecological Applications study’s findings are also consistent with an earlier study from Frank’s lab that found another scale insect species is more abundant at warmer temperatures due to increased survival rates.

“This work makes us think that increasing urbanization and rising temperatures associated with global climate change could lead to increases in scale insect populations, which could have correspondingly negative effects on trees like the red maple,” Dale says.

The work was supported by the Department of the Interior’s Southeast Climate Science Center, which is based at NC State and provides scientific information to help land managers respond effectively to climate change. The project was also supported by Agriculture and Food Research Initiative Competitive Grant number 2013-02476 from the USDA National Institute of Food and Agriculture.

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Note to Editors: The study abstracts follow.

“The effects of urban warming on herbivore abundance and street tree condition”

Authors: Adam G. Dale and Steven D. Frank, North Carolina State University

Published: July 23, PLOS ONE

Abstract: Trees are essential to urban habitats because they provide services that benefit the environment and improve human health. Unfortunately, urban trees often have more herbivorous insect pests than rural trees but the mechanisms and consequences of these infestations are not well documented. Here, we examine how temperature affects the abundance of a scale insect, Melanaspis tenebricosa (Comstock) (Hemiptera: Diaspididae), on one of the most commonly planted street trees in the eastern U.S. Next, we determine how both pest abundance and temperature are associated with water stress, growth, and condition of 26 urban street trees. Although trees in the warmest urban sites grew the most, they were more water stressed and in worse condition. Our analyses indicate that high temperatures were associated with water stress, while visible declines in tree condition were best explained by scale-insect infestation rather than temperature. To test the broader relevance of these results, we extend our analysis to a database of more than 2700 Raleigh, US street trees. Plotting these trees on a Landsat thermal image of Raleigh, we found that warmer sites had over 70% more trees in poor condition than those in cooler sites. Our results support previous studies linking warmer urban habitats to greater pest abundance and extend this association to show its effect on street tree condition. Our results suggest that street tree condition and ecosystem services may decline as urban expansion and global warming exacerbate the urban heat island effect. This presents a gloomy outlook for urban forests and emphasizes the need for management tools. Existing urban tree inventories and thermal maps could be used to identify species that would be most suitable for urban conditions.

“Urban warming trumps natural enemy regulation of herbivorous pests”

Authors: Adam G. Dale and Steven D. Frank, North Carolina State University

Published: forthcoming, Ecological Applications

DOI: 10.1890/13-1961.1

Abstract: Trees provide ecosystem services that counter negative effects of urban habitats on human and environmental health. Unfortunately, herbivorous arthropod pests are often more abundant on urban than rural trees, reducing tree growth, survival, and ecosystem services. Previous research where vegetation complexity was reduced has attributed elevated urban pest abundance to decreased regulation by natural enemies. However, reducing vegetation complexity, particularly the density of overstory trees, also makes cities hotter than natural habitats. We ask how urban habitat characteristics influence an abiotic factor, temperature, and a biotic factor, natural enemy abundance, in regulating the abundance of an urban forest pest, the gloomy scale, (Melanaspis tenebricosa). We used a map of surface temperature to select red maple trees (Acer rubrum) at warmer and cooler sites in Raleigh, NC. We quantified habitat complexity by measuring impervious surface cover, local vegetation structural complexity, and landscape scale vegetation cover around each tree. Using path analysis, we determined that impervious surface (the most important habitat variable) increased scale insect abundance by increasing tree canopy temperature, rather than by reducing natural enemy abundance or percent parasitism. As a mechanism for this response, we found that increasing temperature significantly increases scale insect fecundity and contributes to greater population increase. Specifically, adult female M. tenebricosa egg sets increased by approximately fourteen eggs for every 1° C increase in temperature. Climate change models predict that the global climate will increase by 2-3° C in the next 50-100 years, which we found would increase scale insect abundance by three orders of magnitude. This result supports predictions that urban and natural forests will face greater herbivory in the future, and suggests that a primary cause could be direct, positive effects of warming on herbivore fitness rather than altered trophic interactions.

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