Settling down to dine on a dioecious Gondwanan relict: why are Aacanthocnema dobsoni nymphs more abundant at the base of branchlets?

Abstract

Female herbivorous insects often dictate the distributions of offspring through oviposition site selection. The optimal oviposition hypothesis proposes that sites selected should benefit offspring because they provide the best diets, but distributions can reflect adult responses to non-dietary factors. On its dioecious host [Allocasuarina verticillata (Lam.) L. Johnson (Casuarinaceae)], the largely sessile nymphs of Aacanthocnema dobsoni (Froggatt) (Hemiptera: Triozidae) are most abundant at the bases of the equisetoid, photosynthetic branchlets and can also be associated with chlorosis. We asked whether the distribution of nymphs could be explained by the nutritional quality of hosts (at tree and branchlet levels) and investigated the nutritional implications of chlorosis. Binary choice oviposition bioassays revealed that female psyllids exhibited a weak preference for the apical (younger) halves of branchlets of male and female trees over basal (older) halves. In the absence of nymphs, basal and apical halves of branchlets of both sexes had comparable concentrations of amino N. The severity of chlorosis of individual female branchlets was positively related to the density of nymphs. Along entire female branchlets, chlorosis was associated with significant reductions in the concentrations of certain amino acids (including glutamic acid). For basal halves of female branchlets, chlorosis was associated with increased concentrations of a different suite of amino acids, i.e., evidence of nutritional enhancement. Fifth instars from female branchlets were slightly larger than those from male branchlets but nymphal morphology is constrained by branchlet diameter. The conformation of the host (especially the spaces between the scale-like leaves) appears to dictate oviposition site selection more than nutritional quality. Nutritional enhancement is suggested to explain the absence of a strong linkage between preference and performance. Aggregation of adults (and eggs) at the base of branchlets could arise from heightened risk of dislodgement by wind at the ends of branchlets and enhanced mate finding in less exposed locations.