If you’re swatting at more insects this summer in the city, you might blame the pavement.
Sidewalks, along with other urban fixtures like parking lots and buildings, are contributing to an imbalance between two of nature’s rivals: pest insects and the predatory bugs that normally keep each other’s population in check.
It’s the heat these surfaces absorb that’s causing the issue. Cities are inherently warmer than rural areas because they have fewer plants to provide shade.
The more asphalt, concrete and brick – the more heat, and in some cases, the more pest insects, like P. quercifex, which thrive in warmer conditions.
“It’s a scale insect that I’m betting we have on every willow oak tree in Raleigh,” said Emily Meineke, a Ph.D. student at N.C. State.
Scale insects are a species of small juice-suckers that have a protective, husk-like shell. They’re prey for parasitoid wasps.
Meineke, along with other NCSU researchers, is studying the relationship between the tiny tree huggers and their natural enemies and how both adapt to the rising heat in urban environments as more green landscapes succumb to development.
In 2014, the team tracked the life cycles of both kinds of insects in cooler and warmer portions of Raleigh, using gauges to measure air temperatures. Cooler sections had more trees; hotter zones had higher concentrations of pavement, asphalt and concrete.
The results confirmed their suspicions about changes in the wasp-vs.-scale-insect population balance.
“Urban warming makes places hotter, and in those hotter spots the development of scale insects speeds, but the development of the wasp doesn’t,” Meineke said.
Hot and bothered
Scientists call it phenological mismatch – when one species’ life stages no longer sync with another species that depends on it in some way. Honeybees and flowers provide a classic example of this phenomenon. When flowers bloom before honeybees are ready to pollinate, fewer seeds are spread, resulting in fewer plants.
What causes one species to speed up while the other remains at its regular pace under the same conditions is a bit of a mystery that researchers are still working on. Meineke: “That’s the million-dollar question.”
In the case of P.quercifex and parasitoid wasps, researchers believe the wasps seek to escape the heat by finding shelter in bark crevices, while the scale insects – which are stationary and fixed to tree branches by tiny mouth parts – are able withstand hotter temperatures.
Scientists caution that the overabundance of pest insects will bring far-reaching implications. For one thing, they’re affecting the trees.
Trees in urban environments have many benefits – they manage storm water, prevent erosion, reduce air and noise pollution, and save energy.
P. quercifex is the oak tree’s No.1 pest, and oaks are some of the most prevalent trees in North Carolina – not just in the state’s forest, but within its cities’ limits, too.
“Willow oaks seem to get the most of them (P. quercifex), and there are a lot of willow oaks in Charlotte,” said Steve Frank, an associate professor of entomology at N.C. State. “I have been there and they all have P. quercifex scale.”
But it’s not just the scale insects on oak trees affected by the increasing city heat. As more green spaces make way for urban development, exotic bugs that are cold-intolerant, like fire ants, have begun to appear in warm cities further north.
The normal range of fire ants now is as far north as North Carolina – except that you can find them in Virginia Beach, Va., according to Frank. “You can find them in certain warm cities as far north as New Jersey, where the urban environment is creating a refuge for these exotic species.”
What can be done?
If higher temperatures are causing the problems, then the solution is to reduce the heat.
Cutting back on impervious surfaces like concrete will help. So will installing reflective and green roofs that reduce the amount of area absorbing heat. But the biggest fix comes from planting more trees and maintaining the health of existing ones in cities.
“If you’ve a got downtown plaza with no trees, you can shade that and actually change the temperature to a large extent,” Frank said. “The essence of it is that cities need to invest in maintaining the green infrastructure that they have, and to increase it.”