Shoot feeding as a nutrient acquisition strategy in free-living psylloids

Abstract

Shoot feeding by sucking insects is accepted as an adaptation to feeding where plant nutrients are most concentrated and/or of higher quality. Psylloids are an important hemipteran taxon, most of which are free-living and comprise many shoot feeding species, whose nutritional ecology has been largely ignored. I conducted a longitudinal study of Ctenarytaina eucalypti (Maskell) and C. bipartita Burckhardt et al. (Aphalaridae) feeding on eucalypts to document how within-plant (ontogenic) variation in nutritional quality, in particular of free amino acids, determines host suitability and hence the distribution and abundance of nymphs. Nymphs were most abundant within developing apical buds but were not more abundant on branchlets of greater vigour (indicated by rate of extension). Nymphs could be found up to two (C. bipartita) to three (C. eucalypti) alternate leaf pairs distant from apical buds but infrequently and in low numbers; they were never found on older, fully expanded leaves. The position of a leaf on a branchlet (indicative of age) determined its nutritional quality. Younger leaves had higher water contents, lower chlorophyll contents and differed in amino acid (essential and non-essential) composition compared to older leaves. The abundance of C. eucalypti nymphs on expanding leaves and in buds was positively correlated with the concentrations of methionine, valine and threonine in E. globulus leaves at the same or comparable position on a branchlet. The abundance of C. bipartita nymphs was positively correlated with foliar leucine concentrations. Shoot feeding by these two psyllids facilitates access to more concentrated, better quality plant nutrients but may not entirely explain the adaptive significance of their behaviour. The humid microclimate created by the architecture of the hosts' apical buds protects eggs and nymphs from desiccation and is suggested to have had a significant influence on the evolution of host utilisation strategies of psyllids within this genus.

Figure 1. Distribution and abundance of psyllid nymphs.

A, B, C. C. eucalypti on E. globulus in October, November and December 2011, respectively. D, E, F. C. bipartita on E. kitsoniana in October, November and December 2011, respectively. Similarities of means, determined by post-hoc tests, indicated by superscripted letters above bars in individual figures. Note different scale on y-axes for each species of psyllid and the position of the bud at each survey.

Figure 2. Vigour of eucalypt hosts of psyllid nymphs.

A. Rate of extension of E. globulus branchlets. B. Rate of extension of E. kitsoniana branchlets. Similarities of means, determined by post-hoc tests, indicated by superscripted letters above bars in individual figures.

Figure 3. Abundance of psyllid nymphs versus foliar amino acid concentrations.

A. Methionine and C. eucalypti. B. Valine and C. eucalypti. C. Threonine and C. eucalypti. D. Leucine and C. bipartita. Nymphal counts are for the October survey. Figures show regression lines and associated 95% confidence intervals.