After Chinese plant biologists noticed that urban trees showed less leaf damage from insect pests than rural trees, they decided to take it a step further. They did so in their home city of Beijing, and with two species of trees common in the Chinese capital: the honey tree (also called the Japanese pagoda tree) and the green ash. They collected thousands of leaves from hundreds of trees and looked for an explanation for why some trees were more visited by leaf-eating insects than others.
The researchers found that the amount of nighttime light the trees were exposed to was a crucial factor. Trees of both species showed less nutritional damage the more light they were lit at night. The researchers noted that trees with brighter light had stiffer leaves. This may make them less attractive to insects. The increased stiffness is thought to be due to the additional light, which allows the trees to photosynthesize for longer and thus collect more energy.
However, increased leaf stiffness is not the only possible explanation. It is also possible that insects prefer to avoid night light, which makes it easier for their natural enemies to prey on. The researchers still want to investigate this avenue.
This research may be good news for urban trees, but it’s not so good news for urban insects. Insect populations are under pressure in densely populated areas, which can have implications for the wider ecosystem.
