Cities tend to warm faster than their rural surroundings, a trend that may undermine natural control systems for plant-killing pests, according to a recent study.

Insects such as the emerald ash borer and scale insects are ravaging forests and crops throughout the United States, causing billions of dollars of damage and losses. Their effects are especially devastating for cities, which have fewer trees but rely on them to regulate temperatures with their canopies and by circulating moisture.

Losing the shade and other cooling attributes can make cities even warmer, leading to increases in heart attacks, kidney stones and breathing trouble (ClimateWire, July 10).

Rising average temperatures and milder winters are often favorable to these pests, expanding their habitats and accelerating their reproduction.

Many of these pests have predators or parasites that benefit from warming, as well, but a study published last month in the journal Biology Letters showed that the overall effect could still be increases of harmful insects.

In this study, researchers examined scale insects, Parthenolecanium quercifex, across 60 sites in Raleigh, N.C. The insect preys on oak trees, drinking sap and destroying the plant's ability to move water.

"They are the laziest insects that exist," said Emily Meineke, lead author of the report and a graduate student in entomology at North Carolina State University. "Their strategy is to plant themselves in a place and not move. They essentially will anchor themselves to a plant with their mouthparts."

Their waste then accumulates near trees, providing a home for harmful molds. As a result, an infestation can quickly cripple or kill even the mightiest oak.

"In the hotter parts of the city, scale insects were many times more abundant on the trees, and up to 12 times more prevalent," Meineke said. "One of the reasons scales are more abundant where it's warmer is that the interactions with its enemies have changed."

A tree-killing pest outproduces its enemies
The scale insect's biggest enemies are parasitoid wasps. Wasps attack and lay their eggs inside scale insects. When the eggs hatch, the larvae devour the scale from the inside, either killing it or slowing down how much it can reproduce.

The researchers found that the parasitoid numbers tracked with scale insect populations, but in warmer parts of the city, the wasps couldn't keep up. Higher temperatures created a mismatch where scale insects would reproduce faster than their parasites.

"The take-home is that the warming reduces the parasitory effects on egg count," Meineke said. "A natural check in the system is being reduced."

Michael Raupp, an entomology professor at the University of Maryland, College Park, who was not involved in this study, said the findings were not surprising and that they confirm the hypothesis that the net effect of warming will be more insect pests in the short term. "It's like a little microcosm of what's going on at a global scale," he said.

He noted that these kinds of mismatches between parasites and hosts are occurring among other organisms and that the effect isn't just a result of differing numbers. "The other piece of this puzzle is insects have periods in their lives where they're more susceptible to predators and parasites than others," Raupp said. "What this translates into is this window of vulnerability becomes shorter."

One of the emerging concerns is the mismatch in symbiotic relationships, like flowers and bees, according to Raupp. Warmer temperatures may lead to pollinators emerging before plants bloom, leading to declines in both populations.

Meineke said her study suggests that mitigating urban heat islands could be an effective method to reduce the harm from scale insect pests. She said she is now interested in quantifying services from trees, such as shade and moisture circulation, and then observing how they change under duress from pests.

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC., 202-628-6500