Evidence of a direct chemical plant defense role for maltol against spruce budworm

This study examines the direct chemical defensive role of maltol, a previously identified secondary metabolite found in balsam fir, Abies balsamea (L.) Mill. (Pinaceae), that was detected during herbivory of spruce budworm, Choristoneura fumiferana (Clemens) (Lepidoptera: Tortricidae). Although used extensively in many industries, in addition to being found in multiple plant species, its functional role in plants remains unknown. The objectives of this study were to quantify the amount of free maltol and its potential conjugated form, maltol glucoside, in various foliage age classes and to evaluate whether constitutive foliage levels of maltol have an impact on spruce budworm fitness in maltol supplementation assays. Gas chromatography-mass spectrometry (GC-MS) analysis of balsam fir foliage showed that maltol is produced in all foliage age classes tested; however, concentrations were significantly higher in older foliage. Liquid chromatography-mass spectrometry-mass spectrometry (LC-MS-MS) analysis showed that maltol also exists in balsam fir in its glucosylated form, a unique discovery in conifers. Similar to maltol, maltol glucoside is also present in current and 1-year-old balsam fir foliage and in significantly higher concentration in older foliage. We investigated the impact of maltol-treated diet on spruce budworm fitness. Maltol additions that reflected constitutive foliage concentrations caused a significant reduction in larval development rate and pupal mass, whereas higher concentrations were required to cause significant mortality. These results suggest that maltol may be an important component of a direct defense strategy in balsam fir against spruce budworm herbivory.