Insect herbivory may cause changes in the visual properties of leaves and affect the camouflage of herbivores to avian predators
​Blue tit was used as a model predator to test how birds see herbivore-mediated changes in leaves. (Photograph by T-M. Koski)

​Visual properties of leaves and the camouflage of larvae against leaves

The leaves of control and defoliated trees as well as the larvae were photographed and the photos were converted to spectral sensitivities of blue tit by using avian visual model. Based on these values, we calculated hue (i.e. description of colour channels that best explained variation in colour) and contrast of leaves as well as the difference in contrast between larvae and leaves. In addition, we applied JND (just noticeable difference) discrimination model to test whether birds 1) discriminate the leaves of control trees from the leaves of defoliated trees and 2) larvae against the leaves based on colour and/or luminance.
We found that trees had a systemic response to herbivore-damage, as concentration of chlorophyll a was lower in the undamaged leaves of herbivore-damaged trees compared to control trees. In addition, the leaves of defoliated trees may appear greener or more yellowish to birds, because hue values were significantly higher in these leaves compared to leaves of control trees. The leaves of herbivore-damaged trees also had significantly higher contrast compared to control trees. However, based on JND discrimination model, these changes are subtle and likely not obvious cues to birds. Surprisingly, the ‘reduction in camouflage’ hypothesis was not supported, as the difference in colour and contrast of larvae against leaves were greater when larvae were on control leaves, indicating that the camouflage of larvae may be slightly better when the larvae are on defoliated trees.
Our results indicate that herbivory may influence interactions between plants, insect herbivores and their avian predators by causing systemic changes in plants, thus supporting recent behavioural studies that have shown responses of birds to herbivore-mediated systemic changes in plants.
Tuuli-Marjaani Koski