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.
Highlights
Shoot feeding by sucking insects is generally accepted as an adaptation for the utilisation of the highest quality modules available on a host, often at a specific time
C. eucalypti nymphs were more abundant than C. bipartita nymphs (Figure 1)
Nymphs of C. eucalypti were most abundant in buds but could be found up to three leaf pairs distant from the bud (Figures 1ac)
Summary
Shoot feeding by sucking insects is generally accepted as an adaptation for the utilisation of the highest quality modules available on a host, often at a specific time. Peaks in their abundance coincide with flushes in host plant growth [1,2] Populations of these “flush feeders” exhibit their greatest abundance on high quality plant modules [3,4]. Aphid feeding declined on hosts directing resources into tuber maturation which is a time when the phloem concentration of glutamine decreases [7]. This nutritionally driven change in behaviour and its consequence for aphid reproductive performance has populationlevel significance, e.g. populations crash when host quality becomes unsuitable in mid-summer [8]. Temporal variation in the nutritional quality of eucalypts has been implicated in the onset of seasonal dormancy (“oligopause”) in Ctenarytaina eucalypti [13]
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