Behavioural responses of psyllids (Hemiptera: Aphalaridae) to olfactory and visual cues of their eucalypt hosts

Submission note: A thesis submitted in fulfilment of the requirements for the degree of Doctor of Philosophy by published work to the Department of Ecology, Environment and Evolution, School of Life Sciences , College of Science, Health and Engineering, La Trobe University.

This thesis was a recipient of the Nancy Millis Award for theses of exceptional merit.

Psyllids are small Hemiptera belonging to the suborder Sternorrhyncha. They are a diverse taxon (around 3,850 species) which includes a number of serious pest species. In natural ecosystems, nymphs of the majority of species develop on only one or a narrow range of hosts, i.e. they exhibit high host specificity. In this thesis, I consider the sensory modalities that mediate host specificity in four Eucalyptusfeeding psyllid species; they include Ctenarytaina bipartita Burckhardt et al., C. eucalypti (Maskell), Anoeconeossa bundoorensis Taylor et al. and Glycaspis brimblecombei Taylor (Aphalaridae: Spondyliaspidinae). Owing to the uniqueness of host plant volatiles (HPVs), olfactory cues are generally considered to provide the most reliable cues governing insect host specificity. Surprisingly therefore, the absence of positive chemotactic responses to host HPVs in Y-tube bioassays suggests that olfaction does not play a prominent role in host finding (Chapter I). In contrast, strong inter-specific preferences for colour stimuli suggests that psyllid visual systems have undergone adaptive “spectral tuning” to facilitate the location of host leaves and other modules of suitable age within host canopies. Intriguingly, attraction to “red” was observed despite the widely presumed absence of long wavelength receptors in Hemiptera (Chapter II). Behavioural and morphological measurements of eyes, to infer the visual acuity of different species, revealed trade-offs between spatial resolution and light sensitivity (Chapter III). Differences in host module (leaves versus stems) usage and microhabitat preferences for feeding and oviposition are suggested to have driven variable requirements for acute vision. Finally, bioassays with live host leaves sandwiched between glass (to prevent escape of HPVs) demonstrated that psyllids discriminated between leaves based on their age using visual cues only. Variation in the composition of free amino acids with leaf age, associated with changes in leaf colour, suggest that psyllid colour preferences are linked to their nutritional requirements (Chapter IV). My findings reveal that vision and, to a lesser extent olfaction, are involved in host selection rather than host finding. Visual cues are often considered to provide pre-alighting cues to insect herbivores (e.g. aphids) but my findings indicate they are functionally important for psyllids when in contact with the host.