Do Yellow Leaves Signal Autumn or Insects?

Does your tree need some insect control? Is it turning yellow because it’s autumn, or because it’s infested?

That’s the question researchers tackled in “Autumn tree colors as a handicap signal,” published in The Royal Society journal. Based at Oxford’s Department of Zoology, the scientists knew that sometimes leaves turn yellow as a defensive sign against infestations, as brighter colors dissuade insects from invading.


In autumn, how can we tell the difference between a tree’s SOS and its normal transition to cool-weather colors?

Previously, only two studies have remotely touched on this conundrum, and they were both fairly specific (one looked at only fruit trees). Just like you’d apply more tree insect control with a larger infestation, trees turn a brighter, more vibrant warning color more quickly with bigger pest attacks.

The team hypothesized that trees changing colors and offering more garish displays would encourage pests to seek out other trees with more pleasing colors.

Colors and Tree Insect Control

Some homeowners think that autumn marks the end of pests and the need for tree insect control, but it can be prime time for some insects, like aphids. Tree insect control in autumn can help safeguard your trees through the following spring, giving you a head start.

However, researchers are quick to point out that not all yellowing trees are infested, and it’s a common, healthy and natural response for many trees.

The researchers looked at over 260 species of trees and compared the degree of autumnal coloration to the degree with which they were infested with aphids. Aphids were chosen because they’re the major source of complaints when homeowners seek out tree insect control.

It’s easy to find trees with aphid infestations, plus they are one of the most well-documented pests during autumn months. As a “single weather host species,” aphids were also identified as the most likely pest to encourage autumnal coloration.

Red (and Yellow) Flags

Field guides were used to identify the trees, excluding hybrids and cultivated trees, to ensure purity. Scores were assigned to each tree based on their colors and aphid diversity, with variables like leaf size, climate, fruit/flower colors, etc.

Results showed that the degree of yellowing was directly related to how many autumnal aphids had infested the trees. Redness also appeared when infestations were particularly dense.

Summary points include the fact that autumn coloration is “expensive” for trees, and their last line of defense. It can also be used as a true cry for help that homeowners can address with tree insect control and the aid of tree services experts. Aphids are damaging, discriminating and largely depend on color to select their host.

If yellowing leaves make you suspect you need tree insect control, contact Reliable Tree Care today.