Scale Insects Threaten City Trees

Troublesome bugs known as scale insects (pictured) may be tiny, but they can take down an oak tree. Considered pests, the creatures tend to flock to cities where they damage and sometimes kill trees by sucking out their sap.

A new study shows that scale insects, a waxy-looking parasite that damages or kills the plants and trees they attach to, increase in populous regions because they have adapted to the heat of urban areas. The pests threaten to become a bigger problem as climate change nudges temperatures up, scientists say.

"We now have a better understanding of why trees in urban areas are infested by so many of these pests," study researcher Steve Frank, an assistant professor of entomology at North Carolina State University, said in a statement. "And if climate change causes temperatures to rise in forests, as we expect, we may see scale insects becoming a much bigger problem for ecosystem health."

Frank and his colleagues analyzed the Raleigh, N.C., population of the oak lecanium scale insect (Parthenolecanium quercifex), which feed exclusively on oaks. The team collected scale insect egg sacs from both hot and cool zones and incubated them in hot and cool greenhouses. The egg sacs from the warmest urban zones produced almost four times as many insects as the egg sacs from cooler urban zones.

By referencing at temperature maps of the city, the researchers found that the scale populations were as much as 800 percent higher in the warm zones. The team looked at the prevalence of parasites and fertility rates among the insects from both hot and cold zones of the city to try to explain this trend, but they did not find any significant differences. The scientists concluded that the bugs have adapted in response to urban warming.

"The scale insects in the hotter urban zones appear to have adapted or acclimated to the higher temperatures in urban environments," said study researcher Emily Meineke, a doctoral student at NC State. "Theoretically, that adaptation would also allow them to take advantage of warmer temperatures that may result from climate change."

The urban heat island effect can make temperatures several degrees warmer in a city than its surrounding rural areas, as clusters of roads and buildings are very good absorbers of the sun's rays. Rising numbers of "urban" scale insects could spell trouble for city trees, which provide some environmental benefits like cooling through shade and carbon sequestration. If global temperatures continue to rise, scale insects could spread and infest more rural trees, the researchers warn.

Their findings were detailed in a study published online last week in the journal PLOS ONE.

h/t: LiveScience.com