Evidence of a direct chemical plant defense role for maltol against spruce budworm Martin Williams, Eldon Eveleigh, Glen Forbes, Rosanna Lamb, Lucas Roscoe & Peter Silk* Canadian Forest Service – Atlantic Forestry Centre, Natural Resources Canada, PO Box 4000, Fredericton, NB, E3B 5P7, Canada Accepted: 26 April 2019 Key words: aglycone, balsam fir, constitutive, glycosylation, herbivore, insect–plant interaction, secondary metabolites, volatiles, Choristoneura fumiferana, Lepidoptera, Tortricidae Abstract 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 bud- worm 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 spectrome- try–mass spectrometry (LC-MS-MS) analysis showed that maltol also exists in balsam fir in its gluco- sylated 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. Introduction One of the most destructive insect defoliators in Canada is the spruce budworm (SBW), Choristoneura fumiferana (Clemens) (Lepidoptera: Tortricidae). Population out- breaks of this defoliating pest of spruce [Picea glauca (Moench) Voss, Picea mariana (Mill.) BSP, and Picea rubens Sarg.] and balsam fir [Abies balsamea (L.) Miller] (all Pinaceae) usually happen every 30–40 years (Royama, 1984) and impact forest productivity through tree mortal- ity and diminished growth and yield due to defoliation (MacLean, 2016). Use of insecticides such as Bacillus thuringiensis Berliner var. kurstaki (Btk) and MIMIC â (in- sect growth regulator, tebufenozide) have been the most prevalent options used to minimize tree mortality or yield loss (Cadogan et al., 1998; Carisey et al., 2004). Although effective against SBW, economic and ecological impacts on non-target organisms such as natural enemies, as well as potential resistance development in target insects (Ferr  e & van Rie, 2002; Janmaat & Myers, 2003) from repeated long-term use are important concerns. Consequently, the development of cost-effective and ecologically safe man- agement techniques for SBW is important. A potentially effective approach may involve exploiting host plant-derived secondary metabolites such as ter- penoids, alkaloids, flavonoids, and phenolics, as well as defensive proteins (Howe & Jander, 2008) that are pro- duced either constitutively, induced following herbivory, or both. In conifers, some of these compounds are well known, and they can impact herbivore fitness, including that of SBW. For example, monoterpenes are naturally produced secondary metabolites in conifers, and some *Correspondence: E-mail: peter.silk@canada.ca © 2019 Her Majesty the Queen in Right of Canada, Entomologia Experimentalis et Applicata © 2019 The Netherlands Entomological Society Entomologia Experimentalis et Applicata 167: 755–762, 2019 755 DOI: 10.1111/eea.12822