Eucalyptus species, native to Australia, Indonesia, the Philippines, and New Guinea, are the most widely planted hardwood timber species in the world. The trees, moved around the globe as seeds, escaped the diverse community of herbivores found in their native range. However, a number of herbivore species from the native range of eucalypts have invaded many Eucalyptus-growing regions in North America, Europe, Africa, Asia, and South America in the last 30 years. In addition, there have been shifts of native species, particularly in Africa, Asia, and South America, onto Eucalyptus. There are risks that these species as well as generalist herbivores from other parts of the world will invade Australia and threaten the trees in their native range. The risk to Eucalyptus plantations in Australia is further compounded by planting commercially important species outside their endemic range and shifting of local herbivore populations onto new host trees. Understanding the mechanisms underlying host specificity of Australian insects can provide insight into patterns of host range expansion of both native and exotic insects.

Most innate responses to color stimuli lead herbivorous insects to orient to “green” and “yellow” surfaces. Early research showed that aphid orientation to plants is influenced by foliar pigments and leads them to alight on leaves of a specific physiological state regardless of whether or not it is their actual host. In this study, we quantified the color preferences of four psyllids specialized on young to recently expanded leaves of different Eucalyptus hosts presenting distinct between (inter-specific) and within canopy (ontogenic) optical characteristics. Color preferences of Ctenarytaina eucalypti and C. bipartita were similar to those observed in aphids with more frequent selection of yellow and green stimuli, consistent with the coloration of their host leaves. However, attraction of Anoeconeossa bundoorensis and Glycaspis brimblecombei to a red stimulus contrasts strongly with the literature for hemipteran and herbivorous insects generally for which attraction to red is peculiar. Interestingly, both red-attracted species occur on the same host eucalypt, which expresses anthocyanic (red) young leaves. Our experiments demonstrate that these two species are sensitive to long wavelength radiation. Behavioral work and modeling of putative “aphid-like” photoreceptors were conducted to investigate whether achromatic vision mediates perception of “red”. Our results do not provide strong evidence for an intensity-dependant type of attraction. Nevertheless, the current knowledge of photoreceptors in Hemiptera identifies the achromatic as the most likely mechanism for detecting long wavelengths. Thus, our findings highlight the need for physiological work with Psylloidea to dissect the mechanisms responsible for such atypical responses. We discuss the ecological implications of our work in relation to red foliar pigments in expanding leaves of perennial plants which differs greatly from the thoroughly studied aphid-autumnal leaves system involving senescing foliage.