Eucalypt-feeding psyllids use colour to locate the choicest leaves: what looks good must also be good
Creators: name: Farnier, Kevin
name: Dyer, Adrian G.
name: Steinbauer, Martin J.
Publisher: name: F1000Research
Year: 2017

description: Most leaf sucking insects exhibit a strong preference for yellow and light green wavelength radiations. These preferences for specific parts of the light spectrum appear to be linked to the reflectance of their host leaves which varies with leaf ontogeny and physiology. Consequently, insects appear to have tuned their visual systems to differentiate low from high quality host leaves and thereby, facilitate their location of nutrient rich foliage. In contrast to yellow-green leaves which generally indicate nutrient-rich and/or sustainable food sources, "red" leaves are commonly associated with high levels of chemical defence or may announce imminent leaf abscission. Therefore, most herbivorous insects appear to avoid or not to be attracted to long wavelengths radiations ("red").<br><br><i>Glycaspis brimblecombei</i> and <i>Anoeconeossa bundoorensis </i>are<i> </i>two species of <i>Eucalyptus</i>-feeding psyllids which have <i>Eucalyptus camaldulensis </i>as a common host. <i>E. camaldulensis</i> presents the characteristic to exhibit vivid red anthocyanin-rich young leaves whereas adult fully-expanded leaves present a dark green colouration. Given that both psyllid species are know to feed preferentially on young expanding foliage, this study aimed to investigate whether these were able to discriminate between young and old host leaves based on host visual cues.<br><br>We used behavioural assays to assess the attraction of the two psyllid species in the presence of "young" red and "older" green leaf squares sandwiched between two glass cover slits, sealed with silicone grease (to prevent the leakage of odour cues) in a dual choice arena.<br>In parallel, we investigated the variations in leaf colours with leaf ontogeny by measuring changes in the reflectance and foliar pigment concentrations with leaf age. Finally, we analysed amino acids concentrations of host leaves at different developmental stages as an indication of nutritional quality.<br><br>Our results show that <i>A. bundoorensis </i>was significantly more attracted to "red" young leaves whereas <i>G. brimblecombei </i>did not discriminate between young "red" and older "green" host leaves. Leaf redness in <i>E. camaldulensis</i> young leaves appeared to be as much the consequence of greater levels of foliar anthocyanins as that of lower levels of chlorophylls compared to those found in older leaves. Expectedly, our analyses of foliar amino acid confirmed marked nutritional differences between young and older host leaves.<br><br>Our study provides a first evidence of attraction to long wavelength (red) reflecting leaves in herbivorous insects. It provides a strong evidence for vision-driven discrimination of leaf age and physiology in psyllids. More importantly, it suggests that the attraction to red may be a lot more widespread than initially thought, especially in plant taxa, such as eucalypt, which are highly reliant on anthocyanins to prevent photodamage in young leaves. Lastly, our work raised the question of the visual mechanisms underlying the attraction to "red" in hemipterans; for which evidence for a specific long-wavelength receptor is still lacking.