Spruce Budworm (Lepidoptera: Tortricidae) Oral Secretions II: Chemistry

Abstract

As sessile organisms, plants have evolved different methods to defend against attacks and have adapted their defense measures to discriminate between mechanical damage and herbivory by insects. One of the ways that plant defenses are triggered is via elicitors from insect oral secretions (OS). In this study, we investigated the ability of second-instar (L2) spruce budworm [SBW; Choristoneura fumiferana (Clemens)] to alter the volatile organic compounds (VOCs) of four conifer species [Abies balsamea (L.) Mill., Picea mariana (Miller) B.S.P., Picea glauca (Moench) Voss, Picea rubens (Sargent)] and found that the emission profiles from all host trees were drastically changed after herbivory. We then investigated whether some of the main elicitors (fatty acid conjugates [FACs], β-glucosidase, and glucose oxidase) studied were present in SBW OS. FACs (glutamine and glutamic acid) based on linolenic, linoleic, oleic, and stearic acids were all observed in varying relative quantities. Hydroxylated FACs, such as volicitin, were not observed. Enzyme activity for β-glucosidase was also measured and found present in SBW OS, whereas glucose oxidase activity was not found in the SBW labial glands. These results demonstrate that SBW L2 larvae have the ability to induce VOC emissions upon herbivory and that SBW OS contain potential elicitors to induce these defensive responses. These data will be useful to further evaluate whether these elicitors can separately induce the production of specific VOCs and to investigate whether and how these emissions benefit the plant.

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

“…Given that maltol is emitted from balsam fir during SBW herbivory (Leclair et al, ), and its potential role as a defensive secondary metabolite, we investigated the impact on SBW fitness by supplementing artificial diet with maltol. Using a range of maltol concentrations either attainable or higher than what is found in current‐year or 1‐year‐old foliage, significant mortality in SBW was observed but only at diet concentrations that were higher than foliar concentrations of maltol.…”

“…This suggests that acetophenones are likely not involved in defending balsam fir against herbivory. Many other secondary metabolites are similarly produced in conifers, with recent research demonstrating that various volatile organic compounds were produced upon herbivory by SBW second instars (L2) on spruce and fir host species (LeClair et al, ). Although some of the volatiles, including several monoterpenes, were common among the tree species tested, one compound, maltol, was only emitted from balsam fir, with the relative quantities of this compound increasing as herbivory progressed (LeClair et al, ).…”

“…In this pathway, maltol is listed as an ‘end product’ (Kanehisa et al, ). Given its emission as a volatile during SBW herbivory assays (LeClair et al, ) and its specificity to balsam fir relative to white, red, and black spruces, maltol was investigated for its potential role in direct chemical plant defense. Maltol can also be glucosylated to form maltol glucoside (maltol β‐D‐glucopyranoside), and this metabolite has been found in other plants, including the manuka tree ( Leptospermum scoparium JR Forst & G Forst; Adams et al, ) and the katsura tree ( Cercidiphyllum japonicum Sieb.…”