Thermal tool to evaluate essential oil composition of different Eucalyptus genotypes in relation to Glycaspis brimblecombei susceptibility (Hemiptera: Aphalaridae)

Eucalyptus spp. (Myrtaceae) is economically important worldwide and is native from Australia. Glycaspis brimblecombei (Hemiptera: Aphalaridae) is an insect that causes damages in eucalyptus plantations. Field observations show that highly infested plants (susceptible), less infested plants (less susceptible), and non-infested plants (resistant) can coexist at the same eucalyptus cultivation area. The aim of this study was to compare the preferences of Glycaspis brimblecombei with Eucalyptus spp. essential oil composition, evaluating the thermal behavior and searching for repellent compounds using chromatography. Simultaneous thermogravimetric and differential thermal analysis (TGA–DTA) results suggest that the less volatile essential oils were obtained from more resistant eucalyptus, with total mass loss temperature between 168 and 176 oC. This behavior is due to the ratio between monoterpenes and sesquiterpenes in essential oils. Furthermore, D-limonene, caryophyllene, geranyl acetate, and other compounds may be correlated to the resistance against the insect. Gas chromatography–mass spectrometry (GC/MS) showed that susceptible samples had the highest concentrations of eucalyptol (77.0% and 49.8%) and lower concentration (3 and 9%) and variability of sesquiterpenes (7 and 10 compounds); additionally, they did not present β-guaiene, calamenene, α-copaene, α-terpinyl acetate, 1,4-cadinadiene, D-limonene, and caryophyllene in their essential oil composition. The eucalyptus resistance against Glycaspis brimblecombei is related to concentration and variability of sesquiterpenes, as well as the presence of specific compounds.