City Heat Boosts Tree-Killing Scale Insect Populations
Big cities with large expanses of concrete, asphalt, and buildings are usually warmer than the suburbs or countrysides that surround them, a phenomenon known as the urban heat island effect. Now new research from North Carolina State University shows that these urban heat islands 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.
“We’d been seeing higher numbers of plant-eating insects like the gloomy scale in cities, and now we know why,” said Adam Dale, a PhD student at NC State and lead author of two papers describing the work that were published in the journal Ecological Applications and in PLOS ONE. “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,” said Dr. Steve Frank, an assistant professor of entomology at NC State and senior author of the papers.
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,” Dr. Dale said. “The other variables we looked at had no significant effect on gloomy scale abundance.”
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 — the females were producing approximately 20 young,” he said. “At the warmest sites — 20.12 degrees Celsius — 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.”
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 said. “It 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.”
“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,” said Frank. “It would also make sense to choose trees that are less susceptible to scales and other pest species.”
Their 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.
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