This paper reports complex lipids and sterols in the leaves of Eucalyptus macrorhyncha, the dedicated host of an unnamed species of Glycaspis (Synglycaspis) (hereafter referred as Synglycaspis), exploring their role in Synglycaspis's specificity to leaves of E. macrorhyncha. Seeking an answer to the above, we evaluated the levels of lipids and sterols in the leaves of Eucalyptus rossii and Eucalyptus dives the chosen “closest” relatives of E. macrorhyncha from Orange region in the South-eastern Highlands Bioregion of New South Wales. Low levels of mono- and di-glycerides in the leaves of E. macrorhyncha appear to be a key factor in regulating the choice of E. macrorhyncha by Synglycaspis. An LC–MS/MS analysis revealed incidence of sitosterol, ergosterol, and stigmasterol. Sitosterol and certain undetermined sterols with apparent molecular weights corresponding to 354, 382, and 440 g mol ^{– 1} occur maximally in the leaves of E. macrorhyncha. Increased levels of ergosterol in galls housing the second, third, and fourth instars and its decreased levels in galls with fifth instars of Synglycaspis indicate the production of this sterol particularly during gall growth and possible utilization by the developing instars of the gall-inducing species of Synglycaspis. The 440 g mol ^{– 1} sterol was highly represented in young leaves of E. macrorhyncha as against in the leaves of E. rossii and E. dives, and this sterol could be a key factor in the preference of young leaves of E. macrorhyncha by the gravid female of Synglycaspis for oviposition and ensuing gall development.