Insect herbivory may cause changes in the visual properties of leaves and affect the camouflage of herbivores to avian predators
Behavioral Ecology and Sociobiology
Reason for embargo
‘Cry for help’ hypothesis predicts that attraction of predators with chemical or visual cues can decrease insect damage of plants. Visual cues involve changes in photosynthetic activity and the reflectance of leaves, and there is some evidence that birds may use these changes as foraging cues. However, changes in the visual properties of leaves have not been quantified and it is not known how birds see these changes. We also presented and tested a new ‘reduction in camouflage’ hypothesis (not mutually exclusive with ‘cry for help’) stating that herbivore-mediated changes in leaf colour can increase the conspicuousness of herbivore against leaves. To define changes in the visual properties of leaves, their detectability to birds, and whether these changes affect the conspicuousness of herbivore, we manipulated the level of herbivory in silver birch trees (Betula pendula) with autumnal moth (Epirrita autumnata) larvae, and used blue tit (Cyanistes caeruleus) vision models to images of leaves and larvae. Hue, luminance (lightness), contrast, light transmission, chlorophyll content, photosynthetic activity and water content of the leaves were compared between herbivore-damaged and control trees. The leaves of herbivore-damaged trees had a decreased chlorophyll a concentration, increased contrast and they reflected more longer wavelengths. However, these changes are likely not obvious to birds. In contrast to our expectation, there were only minor differences in conspicuousness of larvae against the leaves of damaged trees, which may be very subtle to predator vision. Nevertheless, according to visual models, larvae should be easily detectable to birds from both herbivore-damaged and control trees.
This study was financially supported by University of Turku Graduate School (T-MK) and by Academy of Finland (grant 218086 to TL; grants 259075 and 271832 to ET).
This is the author accepted manuscript. The final version is available from Springer Verlag via the DOI in this record.
Vol. 71: 97